This report covers the period July 1993 through June 1995.
The telescopes available for use by visitors include the 12 Meter Telescope at Kitt Peak, AZ; the 27 element Very Large Array (VLA) on the Plains of San Agustin, west of Socorro, NM; the Very Long Baseline Array (VLBA), with antenna sites at Mauna Kea, HI; Owens Valley, CA; Brewster, WA; Kitt Peak, AZ; Pie Town, NM; Los Alamos, NM; Ft. Davis, TX; North Liberty, IA; Hancock, NH; and St. Croix, VI; and the 140 Foot Telescope at Green Bank, WV. Scientists apply for observing time by writing to the Director, NRAO, 520 Edgemont Road, Charlottesville, VA 22903-2475. Proposals for observing time have deadlines of the 1st of January, July, and October for the spring, fall, and winter observing periods, respectively, on the 12 Meter; the 1st of February, June, and October for the next VLA configuration following proposal review; the dates for the VLA also apply to the VLBA; 140 Foot proposals may be submitted at any time.
The NRAO will send to any potential observer, on request, information sheets that give brief descriptions of each major telescope, receiver, feed, and computing system, provide a general familiarization with Observatory procedures, and describe how to prepare a proposal. Partial support of travel expenses to NRAO for observing and data reduction is available, as is partial support for page charges for publication of results obtained from observations using NRAO telescopes.
The NRAO is constructing the fully steerable 100 meter aperture Green Bank Telescope (GBT) at its West Virginia site. The Observatory has proposed to build the Millimeter Array (MMA), for which site tests are being conducted in Hawaii and in Chile.
NRAO is making most Observatory related information available on-line through the Internet. Information is available on all aspects of Observatory operations, including extensive information on the current status of the Observatory facilities, user policies and procedures, software updates and information, status of construction projects, and libraries of technical reports and memoranda. In particular, the instructions for electronic submission of proposals for observing time are available. The starting point for this information is the NRAO home page on the World Wide Web (WWW), at URL http://www.nrao.edu, which has connections to other NRAO pages. Readers interested in current NRAO related information should find what they need there.
Ten VLA antennas are now equipped with receivers for 7 mm operation. These receivers cover the 40-50 GHz frequency range, with a system temperature of about 100 K. The receivers were built by NRAO with funds supplied by the Institute of Astronomy of the Universidad Nacional Autonoma de Mexico (UNAM).
Three more receivers are under construction and will be installed before the winter observing season of 1995-96. A retrofit will also be started in 1996 to replace the first stage amplifiers in the older receivers with amplifiers being developed at the NRAO's Central Development Lab, using heterostructure field effect transistor (HFET) InP devices. The zenith system temperature at 7 mm should be reduced from 100 K to 70 K with this change.
Design and prototyping has begun for the next generation of wide bandwidth VLA receivers. The most challenging task is the design of a circular polarizer with a bandwidth ratio of 1.5:1. This will be needed if a single receiver is to operate from 18 to 26.5 GHz or 12 to 18 GHz as proposed for future VLA development. Design of this polarizer is currently underway. The goal is to install a prototype wideband receiver on the VLA in 1996.
The VLA local oscillator racks in the antenna vertex rooms are being shielded in an attempt to reduce self-generated radio frequency interference in the 327 MHz and 75 MHz bands. This project will be completed in July 1995.
A more complete description of the VLA is contained in the document "Very Large Array Observational Status Summary" available on request from W. M. Goss at NRAO, Socorro.
VLA Sensitivity --
System Temperatures and Antenna Efficiencies
Frequency System Temperature Antenna Efficiency
(GHz) (K) (Percent)
0.070 - 0.075 900-9000 20
0.30 - 0.34 150-180 40
1.34 - 1.75 30 51
4.5 - 5.0 60 65
8.0 - 8.5 33 63
14.4 - 15.4 120 52
22.0 - 24.0 150 42
40.0 - 50.0 100 20
NRAO is developing a plan to upgrade the VLA by replacing all receivers, except the 1.5, 8, and 45 GHz systems, with modern VLBA designs having state-of-the-art HFET amplifiers. In addition, the correlator will be replaced with one capable of wider bandwidths. This requires replacement of the waveguide with an optical fiber IF transmission system. Overall, continuum sensitivity and spectral line bandwidth coverage can be improved by roughly an order of magnitude.
The real-time system of Modcomp computers was upgraded. Prime motivation for this was to replace the obsolete Century drives, for which parts have not been available since 1992, and which therefore constituted an ever increasing threat to uninterrupted VLA operation.
Upgrades to and tests of the VLA hardware systems received continued support. In particular, much effort was expended in characterizing and improving the performance of the VLA at Q-band. Work was begun on plans for a new correlator control system. This has the potential to allow shorter minimum integration times and simultaneous line and continuum observations.
Significant changes and additions were made to the online system to allow for the 21 cm Sky Surveys. As a side effect, we now have an easily extendable mechanism to carry out mosaiced observations.
NRAO has two VLA surveys at 20 cm in progress. The NRAO VLA Sky Survey (NVSS) uses the D and DnC configurations and is an all-sky survey (+90 to -40 degrees). It is being carried out by J. Condon, W. Cotton, Q. Yin, R. Perley, and J. Broderick (Virginia Polytechnic Institute and State University). The emphasis of this survey is completeness. Polarization images are also being obtained. The NVSS limiting flux density is about 2.5 mJy, with a beam size of 45 arcsec. Two-thirds of this region has now been successfully observed. The survey observations should be completed in the summer of 1996. Reduced images of the first third of the sky are being made available by anonymous ftp on gibbon.cv.nrao.edu or through the WWW, starting on the NRAO home page, as the reductions are completed.
The Faint Images of the Radio Sky at Twenty Centimeters (FIRST) survey uses the B-array and is concentrating on the north galactic cap. The FIRST survey team includes R. Becker (UC-Davis), D. Helfand (Columbia), R. White (Space Telescope Science Institute [STScI]), M. Gregg (Lawrence Livermore National Lab [LLNL]), and R. Perley. This survey is emphasizing accurate radio positions for optical identifications and radio structure. The FIRST limiting flux density is about 1 mJy and the beam size is 5.4 arcsec.
So far about 1600 square degrees has been observed between 7h15m and 16h30m in R.A. and 28.5 and 43 degrees in declination (2000.0). Preliminary images and a catalog of the first 300 square degrees (declination 28.5 to 31) is available by anonymous ftp on baboon.cv.nrao.edu or through the WWW, starting on the NRAO home page.
During this reporting period there has been major progress in the status of the VLBA. It is now an operational instrument. The array is currently performing routine astronomical observing 60 percent of the time; 25 percent of the time is scheduled for test observations, calibration, and maintenance.
A large amount of effort has been expended by the VLBA staff to optimize the performance of the antennas, especially at high frequencies. The array is now well calibrated at 43 GHz and is routinely observing at that frequency. Plans are in place, and hardware is being produced, to extend the frequency range of the VLBA to 86 GHz. The following three tables describe the basic properties of the VLBA.
Antenna Locations and Codes Location North West Elev. Code Latitude Latitude (deg. ' ") (deg ' ") (m) Saint Croix, VI 17 45 30.57 64 35 02.61 16 SC Hancock, NH 42 56 00.96 71 59 11.69 309 HN North Liberty, IA 41 46 17.03 91 34 26.35 241 NL Fort Davis, TX 30 38 05.63 103 56 39.13 1615 FD Los Alamos, NM 35 46 30.33 106 14 42.01 1967 LA Pie Town, NM 34 18 03.61 108 07 07.24 2371 PT Kitt Peak, AZ 31 57 22.39 111 36 42.26 1916 KP Owens Valley, CA 37 13 54.19 118 16 33.98 1207 OV Brewster, WA 48 07 52.80 119 40 55.34 255 BR Mauna Kea, HI 19 48 15.85 155 27 28.95 3720 MK
Maximum Baseline Lengths (km) SC HN NL FD LA PT KP OV BR MK MK 8612 7503 6156 5135 4970 4796 4467 4015 4399 ... BR 5767 3658 2300 2346 1757 1806 1914 1214 ... 4399 OV 5461 3886 2328 1508 1088 973 845 ... 1214 4015 KP 4840 3623 2076 744 652 417 ... 845 1914 4467 PT 4580 3227 1664 565 237 ... 417 973 1806 4796 LA 4459 3007 1433 609 ... 237 652 1088 1757 4970 FD 4144 3106 1655 ... 609 565 744 1508 2346 5135 NL 3645 1611 ... 1655 1433 1664 2076 2328 2300 6156 HN 2853 ... 1611 3106 3007 3227 3623 3886 3658 7503 SC ... 2853 3645 4144 4459 4580 4840 5461 5767 8612
Receiver Bands and Typical Performance Frequency Range Typical Zenith SEFD+ Typical Zenith (GHz) (Jy) Gain(K Jy(-1)) 0.312 - 0.345 2256 0.092 0.600 - 0.630 2261 0.084 1.30 - 1.70 316 0.097 2.13 - 2.35 338 0.092 2.13 - 2.35* 425 0.078 4.50 - 5.14 309 0.131 7.88 - 8.93 323 0.117 7.88 - 8.93* 398 0.113 12.0 - 15.4 562 0.111 21.1 - 24.6 1001 0.103 42.3 - 43.5 1339 0.084 + System equivalent flux density * With 13/4 cm dichroic
The VLBA correlator is able to keep up with the large amount of data generated by both the VLBA and global networks and is also working steadily through the backlog of older global projects. There is no backlog for VLBA format projects, or for global projects observed since January 1995. There are a number of global projects that cannot, as yet, be correlated by the VLBA correlator for a variety of reasons; this is discussed in more detail below.
The FX correlator of the VLBA has exceeded all expectations as it has come into routine operation. The properties of the correlator have been studied extensively over the last two years, and the VLBA staff now understands it at a level that has enabled the production of noise-limited images of DA193 at a dynamic range of 120,000:1. These are the best very long baseline interferometry (VLBI) images, by a large margin, ever produced.
The correlator's software has undergone a major rewrite with the goal of producing a more robust and automated operation. The first stage of this rewrite was released in March 1995, and we have seen its beneficial effects already. The next major release of the software is scheduled for the fall of 1995. This will contain the software necessary to correlate sub-arrayed data, data with short scans, non-synchronized tape changes, and many other features. This will enable us to correlate almost all of the backlog of global projects, projects which were shelved because they contained some of these listed features.
Full automation has always been the plan of the VLBA project. When dealing with a major VLBI array and the vast amount of data that is generated, anything less than that would quickly result in a breakdown of the operation. The software plan to achieve full automation is being implemented, but it is a slow process as there are a large number of pieces that have to be connected to achieve our goal. Until that point is reached, the correlation of global VLBI projects remains a labor-intensive procedure requiring a large amount of human interaction. This is one of the major hurdles yet to be overcome.
A major enhancement of the VLBA occurred in early 1995 as the Effelsberg 100 Meter and the Green Bank 140 Foot developed the ability to write data to tape in VLBA format and were able to generate VLBA-style monitor data. This enables the VLBA to operate routinely with these antennas and avoid many of the problems that typically occur in correlating global, MKIII format, projects.
The maintenance of the VLA and VLBA involves a continuing program of preventive maintenance and repair as well as several major projects. With limited funds, our first priority remains the maintenance of the VLA rail system, followed by antenna painting, VLA waveguide access station repair, and smaller projects.
The VLA rail system maintenance program in 1994 concentrated on realigning the spur tracks that connect the main rail line to the antenna stations. Without proper alignment between the track and the concrete station, the transporter is unable to set the antenna down during an antenna move. In 1994 thirty spur lines were removed and reset. Also about 1600 meters of the mainline on the east arm was leveled and the gauge reset. In 1995 at least 24 spur lines will be realigned and six intersections will be rebuilt. The rail system will also be surveyed with new markers installed. In 1996 a large tie replacement and mainline leveling program is planned.
The project to replace the VLA underground power distribution system was completed in 1994.
The program to replace the VLA waveguide access stations using steel culvert is progressing. The existing concrete stations are deteriorating and collapsing. Twelve stations were rebuilt in 1994, which brings the total to 35 rebuilt out of 122 since the project started in 1991. Fourteen culverts already on hand will be installed in mid-1995. If funds allow, additional culverts will be purchased and installed later in the year.
The VLA antenna paint is showing signs of its age. In 1994 we repainted four antennas plus the quadrupods on nother four antennas. The VLBA antennas at Hancock, Los Alamos, and St. Croix were also repainted in limited areas in 1994. Owing to the salt water environment on St. Croix, we anticipate that maintenance painting will be needed every year on that antenna. In 1995 a painting program will be undertaken of similar scope to that done in 1994.
In 1991, the azimuth bearing in VLA Antenna 21 failed, putting the antenna out of service for almost a year. Monitoring of the metal content of the bearing grease and the vertical play of the azimuth bearings shows there are other antennas with worn or damaged bearings. In 1994, the azimuth bearing of antenna 9 was replaced. There are five other antennas that will need bearing replacement in the near future. The next bearing change will probably be done in 1996.
At several VLBA antennas, there is a growing problem with deterioration of the grout supporting the azimuth rail track. A repair procedure was developed and implemented at the Pie Town antenna in 1994. This is regarded as a critical situation, and four more antennas will be repaired in 1995.
In 1994, the last of the Convexes was turned off, causing Array Operations Center (AOC) general computing to rely exclusively on workstations. Due to budget constraints, there have been few changes on the hardware side at the AOC since the large workstation procurement of 1991 and 1992. In late 1993, we purchased our third high-end IBM 580; our 580s, all equipped with 14 gigabytes of disk space, are the machines of choice for projects requiring a lot of CPU time or disk space. In early 1995, we added a Sparc Station 20 to the group of visitor workstations; two more are on order.
The major current bottleneck is the number of very large VLBA projects, and the relatively small number of workstations that can handle these projects. We expect a noticeable improvement in this situation soon for two reasons. First, three IBM 580s and one 560 will be equipped with faster disks. This is expected to significantly reduce the total turn-around time of most of the major AIPS tasks. Second, we are in the process of upgrading 60 percent of the Sparc IPX workstations with new CPU chips which will boost the performance of each by 70-80 percent. All visitor stations and the most heavily used staff machines will receive this upgrade. We estimate that these measures will reduce the pressure on our high-end IBMs, where very large VLBA projects will experience less competition from medium-sized VLBA and VLA projects.
The possibility of near-realtime data filling was extended to all AOC workstations. Its operation was made more transparent to the user, and its general robustness was enhanced. As a result, this mode of data filling has become popular, as it gives the scientist instantaneous access to the data.
The VLA archiving project has made good progress, though poor quality tapes from the early 1980s caused significant delays. To date all observations from 1976 to 1983 and those from 1992 to present have been converted to one uniform format and transferred to exabyte tape. As a very useful by-product of this project, the new VLA archive catalog is populated by the header information of each observation the archiving process encounters. This database is accessible online through the WWW via the NRAO home page. It allows the user to interactively query the VLA database from anywhere in the world using various user supplied criteria.
The 12 Meter Telescope is a general purpose, millimeter-wave visitors facility. Using low-noise superonductor-insulator-superconductor (SIS) receivers, the 12 Meter offers near-continuous frequency coverage from 68 to 300 GHz, excluding atmospheric absorption bands. Both continuum and spectral line observing is supported, with a large number of observing options for each mode. Remote observing capabilities are supported.
All of the receivers are SIS mixers operating in closed-cycle, 4 K cryostats. Each receiver has two orthogonal, linear polarization channels with a 1.5 GHz intermediate frequency. All receivers can be tuned to be single sideband, with image rejection of 20 dB or more. The receivers are remotely tuned from the telescope control room using a computer-assisted system. The typical time to tune a receiver is about ten minutes. Details about individual receivers are given as follows:
68-116 GHz SIS Receiver. This receiver covers the 3 mm wavelength atmospheric window. It consists of two bands, 68-90 GHz and 90-116 GHz, in the same cryogenic package. SSB tuning is achieved through adjustment of the two tuning backshorts in the mixer. Receiver noise temperatures are <80 K (SSB). In good weather TR* system temperatures range from 130 K to 350 K, depending on the frequency and elevation angle.
130-180 GHz SIS Receiver. This receiver covers the 2 mm wavelength atmospheric window. It is also part of the same cryogenic package that contains the 3 mm receiver. SSB tuning is achieved through backshort adjustment. Receiver temperatures are <100 K (SSB) and system temperatures are typically 200-400 K.
200-300 GHz SIS Receiver. This receiver covers the 1.3 mm wavelength atmospheric window. It consists of two bands, 200-260 GHz and 260-300 GHz, in the same cryogenic package. Image sideband rejection is achieved through a quasi-optical interferometer, with the image terminated at 4 K inside the dewar. The 200-260 GHz and the 260-300 GHz receivers have two backshort tuners. Receiver noise temperatures are <200 K SSB and typical system temperatures are 450-700 K.
Eight-beam, 200-260 GHz Receiver. The completely rebuilt eight-beam receiver is once again available for use on the 12 Meter. This multi-beam receiver is now equipped with low-noise, tunerless SIS mixers. The receiver consists of a 2x4-element array with 87" beam spacing. The array can be rotated as a unit to any position angle and can track parallactic angle. The receivers will tune from at least 210 to 250 GHz.
Two types of spectrometer are supported at the 12 Meter, a set of analog filter banks and a hybrid spectrometer that consists of a set of broad analog filters followed by a digital autocorrelation section. The filter banks are 256-channel units of 100, 250, 500, 1000, and 2000 kHz resolution, and a 128-channel unit of 30 kHz resolution. The hybrid spectrometer has 1536 spectral channels that can be distributed over eight independent intermediate frequency (IF) sections. In the typical 2-IF mode, bandwidths from 1200 to 37.5 MHz are available in steps of two, with corresponding channel spacings of 49 to 1562 kHz. A 24 kHz mode also exists. Data from the filter banks and hybrid spectrometer are recorded simultaneously.
An 8-channel, digital backend exists for continuum work. Analog voltages are converted to digital bits by a voltage-to-frequency converter and counter system.
The real-time control system is based on a modern, distributed processing network of Sun (Unix) workstations and microprocessors running the VxWorks operating system. System configuration and data transmission occur over an Ethernet system. Fast synchronization, timing, and error status conditions are handled by a custom bus. The interface to the control system is through either graphical screen displays or individual commands, issued on a workstation. The microprocessors control the data-taking backends and the telescope tracking servos. The control system is highly modular and loosely connected, which allows units to be tested independently and the system to be expanded nearly indefinitely.
The primary data analysis program at the 12 Meter for single-position integration and small-field mapping is UniPops, a much expanded and improved version of the POPS program in use at the NRAO for many years. UniPops runs on Sun workstations in a window display environment. For wide-field on-the-fly (OTF) mapping, classic AIPS is available. An initial AIPS++ OTF mapping package is available for real time viewing of the data. In addition to UniPops and AIPS, observers at the 12 Meter can also use packages developed at other observatories, such as the Max Planck Institute for Radioastronomy (MPIfR) NOD2, Institute for Millimeter Radio Astronomy (IRAM) CLASS, and Bell Laboratories COMB systems. Commercial packages such as PV-WAVE and SM are also available at the telescope.
For spectral line observing, a large number of switching modes are offered, including position, beam, and frequency switching, as well as total power observing. Several grid mapping procedures are available. Continuum observing can be performed by subreflector beam switching or position switching. Dual-beam continuum mapping with subsequent restoration analysis is available. OTF mapping is available for both spectral line and continuum modes. This observing mode is a highly efficient method for imaging large fields. This mode is now the preferred method of wide-field imaging at the 12 Meter.
Computers at the 12 Meter are on a T1 Internet node and are thus accessible to e-mail, ftp transfers, and remote logins. In addition, the 12 Meter offers extensive remote observing capabilities. From an X-windows workstation console on a remote Internet node, an observer can view the telescope status monitor, run the analysis programs, conduct computer "e-chat" sessions with the telescope operator, view automatic data displays scan by scan, receive weather and atmospheric transparency data, inspect digital chart records, and view selected video images of the site, the sky overhead, and the control room. Programmable observing scripts will be released during the coming observing season.
The antenna currently has an rms error of ~75 microns, an aperture efficiency at 230 GHz of about 30 percent. Surface adjustments are done using 38 GHz holography. A number of refinements to the holography procedure now make it possible to perform a cycle of holography, panel fitting, and panel resetting in a 24-hour period.
A polarimeter is now available for spectral line and continuum work in the 3 mm wavelength band. The polarimeter consists of a combination of a rotating wire grid and a plane mirror. The grid-to-mirror spacing can be adjusted to make the instrument sensitive to either linear or circular polarization. A linearly or circularly polarized noise source is available for calibration and system checks.
The 12 Meter staff is currently constructing a new 3 mm wavelength receiver with eight independent receiver channels configured in a 4-beam, dual-polarization cross. Two of the beam sets are separated by the subreflector beam throw so that it will be possible to perform dual-polarization, double-Dicke switched observations of point sources using the nutating subreflector. For wide field imaging, all four beams (eight receiver channels) will be used. Several other improvements will be made to the receiver optics and local oscillator injection to lower the noise temperature compared with the present generation of receivers. In all, the new receiver will be about 3.5 times more efficient for point source work and about seven times more efficient for wide field mapping compared with the present dual-polarization, single-beam 3 mm receiver. This receiver will be finished in about one year.
The staff is working to increase the IF frequency of 12 Meter receivers from 1.5 to ~3 GHz. This, together with some other modifications, will allow instantaneous observing bandwidths of 1.2 GHz. Larger bandwidths will be advantageous to extragalactic line work in the 1 mm band and to spectral line searches and surveys in all bands.
The Observatory is planning for the next generation multi-beam, millimeter-wave receiver having 32 beams or more. Spectrometer development for such a system is underway. A 32-channel spectrometer based on the Canaris chip is under consideration. An 8-channel, prototype spectrometer would comprise the first quarter of the system.
Major revisions have been made to the Cassegrain receiver system at the telescope. The maser-upconverter receivers have all been removed and replaced by the HFET receivers built for the GBT. This has produced an immediate improvement in performance and stability, and a reduced maintenance load. In addition, use of the receivers by observers gives us valuable experience that can be carried over to the GBT. The new receivers have good sensitivity and wide bandwidth. The K band receiver was measured to have a total system temperature of 40 K at 22 GHz during good weather. At X band, system temperatures are around 33 K. The new receivers have been installed in a way that makes it possible to switch between most of them in only a few minutes. Other parts of the 140 Foot electronics have been replaced or supplemented with equipment ultimately destined for the GBT. These include a fiber optic IF line and associated electronics, local oscillators, and a digital continuum detector. The spectral processor has been upgraded with additional memory and a faster control computer, and a Sparc 20 Workstation with a 9 Gbytes disk. It is now running under GBT control software. A VLBA data acquisition system was received and installed at the telescope and is used for routine VLBI observations. A site timing center has been established to transmit precise time and frequency over fiber optic cables around the Observatory with round-trip delay measurement. A weather station measuring wind direction and speed, barometric pressure and dew point has been installed and its output is being logged using GBT software.
Computing facilities have been enhanced by the purchase of a four processor Sparc 20 workstation as the site server. This machine also gives extra power needed for some GBT applications. Several additional workstations have been purchased for use of visitors, as well as a color laser printer and additional disk units. The overall computing environment is now adequate, and will remain so until the GBT comes into operation.
With the increasing use of the radio spectrum, protection of frequencies for radio astronomy takes on more importance than usual. Every year there are some 200 applications filed for new transmitters within 13,000 square miles which comprises the National Radio Quiet Zone (NRQZ) around Green Bank. These are evaluated for their potential influence on radioastronomical observations, and in some cases, we file objections with the Federal Communications Commission. Routine monitoring of frequencies of particular interest is now underway using the 140 Foot Telescope to build up an interference data base, and equally important, a list of relatively clear frequencies. More sophisticated propagation models are being employed using better terrain data to make our evaluations of potential interfering sources as accurate as possible. Work has begun on a monitoring station to be located at the GBT. Staff are routinely involved in identifying and suppressing local sources of interference. We have also provided technical input to legal proceedings which affect the management of the electromagnetic spectrum.
NRAO operates a VLBI station for the United States Naval Observatory (USNO) at Green Bank as part of a system to determine earth rotation parameters. The USNO had used one of the old 85 Foot telescopes for this purpose, but will soon move to a newly-constructed 20 meter that has a greater sky coverage and faster slew rate. The data derived from the VLBI observations are made available to the public, and are crucial in analyzing observations from synthesis instruments like the VLA and VLBA. Approximately 30 pulsars are observed with the pulsar monitoring telescope at frequencies of 327 MHz and 610 MHz.
The Naval Research Laboratory (NRL) is funding NRAO to operate two 85 foot telescopes as an interferometer for radio source monitoring, and another 85 foot telescope for monitoring of pulsar fluxes and arrival times. The interferometer operates at S- and X-band and has a 1-sigma noise limits of 4 and 7 mJy, respectively, for a ten minute integration. About 100 sources are routinely monitored; they range from stars to quasi-stellar objects ( QSOs).
For more than seven years we have operated a Science Teachers Institute in cooperation with West Virginia University, and funded by the NSF. This program has brought more than 350 pre-college teachers to Green Bank for two-week intensive sessions in astronomy, teaching methods, and real research, using the 40 foot transit telescope. Teachers routinely bring their students to the Observatory for hands-on experience with the 40 Foot. It is outfitted for continuum and some HI work at L-band, and is in use about 150 days of the year.
The NRAO is supporting the Japanese VLBI Space Observing Program (VSOP) and Russian Radioastron orbiting VLBI missions in various ways, at both the Green Bank and Socorro sites. The launch date of the VSOP satellite has slipped about one year during this reporting period and is now scheduled for August 1996. That of the Radioastron satellite has also slipped and is now scheduled for late 1977.
In Green Bank an earth station has been constructed that will communicate with each satellite. The station will uplink a precise timing reference signal to the orbiting telescope, and it will simultaneously downlink the digitized astronomical signal and record it on wideband tape for later correlation.
The project is funded by the NASA Office of Space Operations (Code O). It was started in 1990, and has continued during the present reporting period. The station utilizes a pre-existing 13.7 meter antenna belonging to the NRAO, but required in-house development of all the electronic instrumentation and control software. The construction phase was completed in September 1994, about six months later than originally planned but very close to the budgeted cost of $3.3M. A five-person technical team was responsible for the development over 4.5 years. Since then, the project has been in an interim operations phase pending launching of the satellites. With minimal staff, we are carrying out tests of the hardware and software, as well as developing the required data interfaces to and from the spacecraft operators, correlators, and orbit determination centers.
Several activities at NRAO Socorro are funded by NASA (Code S) in support of the international space VLBI missions. The project currently has three primary aspects: enhancement of the VLBA correlator and the AIPS data-analysis software to support VLBI observations involving an orbiting element, operation of the VLBA in conjunction with these missions, and a program of expert assistance in data analysis for users of the missions.
Up to 30 percent of the VLBA time scheduled for astronomical observing has been committed to observing programs conducted with the VSOP and Radioastron satellites. Both missions will have open access, peer reviewed scientific programs. The VSOP Announcement of Opportunity has been issued (see http://www.vsop.isas.ac.jp).
The Central Development Laboratory (CDL) in Charlottesville provides support for the Observatory in the areas of feeds, cooled front-end devices of both HFET and SIS-mixer types, and digital back-ends. Support is also provided in other areas as needs arise, for example, with some signal processing functions of the OVLBI earth station at Green Bank. During the past two years increasing attention has also been given to future requirements for the MMA, including both front-end and system design. In addition to equipment for NRAO facilities, the laboratory has provided HFET amplifiers, and in a few cases SIS mixers, to other observatories on a cost reimbursable basis when equivalent items were not available commercially.
Updating of test equipment and similar facilities is necessarily limited by available funds, but some important new items are as follows. A new cryogenic test station for HFET amplifiers, including computer control and monitoring, has been assembled and programmed. Construction of a similar facility for SIS mixers is also in progress. Sonnet em software for analyzing planar electromagnetic circuitry has been acquired and provided an important increase in accuracy over other programs available. A turbo-pump vacuum station and equipment to provide millimeter-wavelength spectrum analysis are also being obtained.
Development and production of cryogenically cooled HFET amplifiers has continued with units covering almost the entire frequency range from 300 MHz to 90 GHz. At the low frequency end, a balanced design is being developed to obtain good input match without the need for a circulator. Amplifiers of this type will be used on the GBT and will cover the range 290-1230 MHz in five bands. Of these, the design for 680-920 MHz is complete and amplifiers have been produced. At the highest frequencies an amplifier for the range 60-90 GHz is under development. A prototype which performed well up to 75 GHz was tested in 1994, and modification to extend the range to 90 GHz is under study. A front end covering the 70-90 GHz band, which will use the new amplifier, is under development and will be used for testing of the high frequency performance of VLBA antennas. There is also an ongoing program to evaluate noise characteristics of HFETs from leading manufacturers as part of a continuous effort to improve the performance of existing amplifier designs.
An important action has been the purchase from Hughes of a full wafer of InP HFETs, testing of which began in early 1995. Gate widths cover a range of 30 to 400 m, and are appropriate for the full frequency coverage of NRAO amplifier designs. The number of transistors on the wafer should be sufficient to cover all amplifier construction for several years. The noise performance is very good, for example, over the range 21-26 GHz a mean noise temperature of 12 K is achieved, which is 30 percent better than obtained with previously available production HFETs. The excellent high frequency performance requires that the circuit impedances be well controlled up to 100 GHz or more to prevent oscillations at such frequencies. As a result some difficulty has been experienced when the new HFETs were retrofitted into existing amplifiers for frequencies below about 18 GHz, and investigation of necessary modifications is in progress.
The purpose of the SIS mixer work is to provide front ends for the 12 Meter Telescope and to develop designs that can be used for the MMA. Tunerless mixers (i.e., those that do not require mechanical adjustment of a circuit element as the local oscillator (LO) is tuned) are preferred for simplicity, especially for the MMA. A study performed in 1993 led to the conclusion that about 36 percent bandwidth, comparable to that of rectangular waveguide, is feasible in tunerless designs. New tunerless designs for 200-300 GHz have been developed during the past year.
To increase the instantaneous bandwidth of SIS mixer receivers, it has been decided to change the IF amplifier response used from 1.2-1.8 GHz to 4-6 GHz. Better noise performance of recent HFET amplifiers allows the higher intermediate frequency to be used without serious loss in sensitivity.
A project to develop a fully integrated image separation mixer on a single quartz chip was started during the past year and the design phase is now well advanced.
In 1993 a problem arose in the device fabrication system at UVa, which has been the main source of supply of SIS mixers used at NRAO. A loss in performance of devices produced was traced to an unwanted edge deposit of Nb on circuit elements. This has now been eliminated and satisfactory fabrication was resumed in late 1994.
Work in this area mainly concerns design and measurement of feeds and related equipment, and analysis of antenna performance. During the past two years much of this work has been related to the GBT project, including feeds for all bands included in the initial outfitting. For example, a feed design for 3.95-5.85 GHz provides a calculated aperture efficiency of 71 percent, and an orthomode transition to separate polarizations for this frequency band has a return loss of better than 18 dB. A tertiary reflector system for beam switching and pointing adjustment has been designed for frequencies above 22 GHz. Calculations of the effects of gravitational distortion of the main reflector surface as a function of elevation show that aperture efficiency can be maintained within one percent by lateral adjustment of the subreflector position. Noise shields have been designed for the telescope that will intercept radiation from the feed that would otherwise be directed towards the ground and reflect it into the main reflector so that it terminates on the cold sky.
Some contributions to other telescopes are as follows. A preliminary design for wideband feeds covering 1.2-40 GHz has been developed for the VLA, to use with upgraded front ends. A feed for 40-52 GHz has also been designed and will be used with a two-feed receiver for testing VLA antennas in this band. The receiver will also be used as a prototype for a multi-feed system for the GBT and will provide an opportunity for refinement of the feed cluster design to find the best compromise between close spacing of beams and loss of aperture efficiency. A special feed for the 140 Foot Telescope has been designed for Zeeman effect observations. This covers the OH line band (1600-1730 MHz). The responses in the principal planes are equal within 0.4 dB, and the measured cross polarization level is -33 dB.
In 1993 work was just starting on a digital spectrometer for the GBT. As described in the last annual report, the total spectrometer bandwidth is 6.4 GHz and the number of correlation lag channels is 262,144. The instrument will incorporate 256 custom application specific integrated circuits (ASIC) designed by J. Canaris of the University of New Mexico for the Arecibo Telescope and other users as well as the NRAO. A feature added more recently is a pulsar observing mode with 1024 integrating time slots within the pulsar period. The first run of ASIC chips that were produced operated up to clock speeds of only 72 Mb/s, compared with the required 100 Mb/s. Detailed testing revealed that the low clock speed resulted from a small error in the chip circuitry. A run yielding satisfactory chips was obtained in the fourth quarter of 1994. In the meantime, work in Charlottesville had produced designs for the circuit cards and the sampler, and test fixtures had been completed. The sampler was tested at 2 Gb/s, and eight such units, running at 1.6 Gb/s, will be used. As a result, by mid-1995 the design of the spectrometer was complete and construction of the full system had been started. Completion of the project is expected before mid-1996.
There are three main elements to the NRAO's software strategy for supporting scientific data processing and analysis. First, AIPS supports the reduction and analysis of (primarily) radio interferometric data. AIPS has extremely broad and flexible capabilities and is used in many other areas of image processing and analysis outside of ordinary interferometric radio astronomy. Second, UniPOPS supports the reduction and analysis of single dish data, with a strong emphasis on NRAO's two current single dish instruments, the 140 Foot Telescope in Green Bank and the 12 Meter Telescope on Kitt Peak. Finally, the AIPS++ project is a development effort begun in 1992 aimed at producing an analysis package which will eventually replace both the AIPS and UniPOPS packages.
There have been two releases of Classic AIPS approximately every six months during the past two years. Typically 100 copies of each release are shipped, about half electronically. The total number of institutions actively using AIPS is over 250. AIPS is now available as binary executables for selected computer architectures (Sun, IBM, Linux, DEC, HP, and SGI). Currently, AIPS is shipped only to licensed sites. Beginning with the next release in July 1995, AIPS will be distributed as copyrighted code using the GNU general public license to allow wider distribution of AIPS. A simple site registration mechanism is being implemented to help track sites using AIPS or receiving support.
During the past two years, AIPS was ported to a number of new operating systems. These are (1) Solaris, Sun's new System V based OS; (2) Digital Unix (was OSF/1), used on DEC's Alpha series computers; (3) HP-UX, Hewlett-Packard's UNIX; (4) Linux, a public-domain UNIX OS for Intel x86 architecture personal computers; and (5) IRIX, Silicon Graphics' version of UNIX. The AIPS verification and performance package DDT was modernized and used to test these and numerous other computers.
A large amount of new or improved software was added to AIPS during the two years. Major areas of concern were (1) reading VLBA correlator data, correcting for correlator artifacts; (2) VLBI data processing, including fringe fitting, spectral-line polarization calibration, amplitude and bandpass calibration and fringe-rate mapping; (3) single-dish data processing, especially imaging from 12 Meter OTF observations; (4) automatic source finding and fitting; (5) wide-field imaging (for VLA surveys); (6) general imaging problems, including significantly enhanced interferometric imaging algorithms; and (7) improved access to information about the large volume of tasks available to the user. The major job of rewriting the AIPS Cookbook was begun, with introductory, calibration, display, spectral-line, VLBI, and site-specific chapters completed. Anyone who is interested in the details may consult the AIPSLetter which is issued with each release. Extensive documentation on AIPS is available on the WWW at URL http://www.cv.nrao.edu/aips/.
AIPS quarterly and annual reports, the AIPS memo series, AIPSLetters, Cookbook chapters, FAQs, software patches for recent releases, and even all current help files are available from this URL.
UniPOPS development has slowed at the NRAO, with the last major update in mid-1994 to version 3.3, followed by release 3.4 in May 1995. Version 3.4 is the default analysis system at the 140 Foot and 12 Meter Telescopes. In addition to installations at each NRAO site, UniPOPS has been distributed to over 24 sites. The latest versions have been installed at over 10 sites.
The most significant change in versions 3.3 and 3.4 from previous versions of UniPOPS is the new disk format of the basic data files. The 16-bit integer indexing was replaced with 32-bit integer indexing. This new disk format is also now the on-line disk format at the 12 Meter (replacing the VAX pdfl format). A single data file can now be made large enough to handle most observing programs. Versions 3.3 and 3.4 include the ability to access Green Bank spectral processor data as well as individual records from longer integrations. The SD-FITS writer now writes continuum as well as spectral line data. The most significant change for version 3.4 is the ability to run under the Solaris operating system. Version 3.4 is likely to be the last major release of UniPOPS; future single dish software development will move to the AIPS++ project. Future releases will be limited primarily to bug fixes, minor new verbs, and updates to the procedure library.
The NRAO's top priority for UniPOPS continues to be responding to user problems (either while observing or at their home installation).
NRAO is now providing on-line documentation for Internet users using the WWW. NRAO is working towards making most kinds of routine documentation available to users on-line, including facilities descriptions, proposal procedures, descriptions of major initiatives, and various NRAO memo series. Recent significant accomplishments are also documented. For example, users can access the first scientific results from the VLBA correlator, including copies of high resolution images. Plans are underway to provide a variety of documentation for the GBT, the MMA, the AIPS++ software project, and other initiatives. NRAO abstracts and preprints are also available through the Web. In late 1995 the Proceedings from IAU Symposium #170 will also be available electronically. The URL for the NRAO's master Home Page on the WWW is http://www.nrao.edu.
There were 211 workstations at NRAO at the start of 1995: 14 larger workstations for shared use on large problems, and 197 user workstations for moderate size problems. Most of the larger workstations (IBM RS/6000 560's and 580's, and Sun Sparcstation 20's) are reserved for users and visitors with large data reduction problems; many Sun IPX class machines are also reserved for visitor use. Two of the larger workstations (including one being delivered in mid 1995) are dedicated servers for software development. The larger workstations are typically equipped with 128-256 MBytes of physical memory and up to 8-10 GBytes of disk space, while the smaller user workstations typically have 1-2 GBytes of disk space and 40-64 MBytes of memory. There were 109 tape drives at the NRAO at the start of 1995, mostly DAT drives and Exabyte drives (including some high-density Exabyte systems), as well as a few remaining 9-track tape drives. A specialized system with a film recorder for producing high quality hard copy has also been installed at the NRAO, with procedures implemented to allow outside users access.
A computing hardware plan is under development to address the needs of the Observatory over the next ten years in computing hardware.
The AIPS++ Project is being conducted by an international consortium of radio observatories with the goal of producing a modern data analysis software system suitable for data from both interferometric arrays and single dishes. The consortium members are: Australia Telescope National Facility (R.Ekers), Berkeley-Illinois-Maryland Association (R. Crutcher), Herzberg Institute for Astronomy (D. Morton), National Center for Radio Physics (V. Kapahi), National Radio Astronomy Observatory (P. Vanden Bout), Netherlands Foundation for Research in Astronomy (H. Butcher), and Nuffield Radio Astronomy Laboratories (R. Davies).
The AIPS++ Project was reviewed by an external review panel in December 1994. Following the main recommendations of the review panel, a number of changes were made in early 1995. A full-time project manager, T. Cornwell, was appointed. The AIPS++ Consortium now oversees the work of the Project via an executive committee composed of directors from the principal partner observatories. Inside NRAO, AIPS++ is now treated as a construction project with dedicated staff and budget.
The immediate goals of the Project have been defined to be consolidation and testing of the AIPS++ library and development of a few key applications chosen to provide unique astronomical capabilities. The long-term goal of the Project has been defined to be the achievement of functional equivalence to AIPS by 2000. At that point, AIPS is expected to be a small subset of AIPS++, and most applications areas will look quite different from the corresponding areas in AIPS.
A development plan for the next 12-18 months has been instituted to provide a coherent overall picture of the direction of the Project in the intermediate term. Tracking of progress in AIPS++ is now performed using a Target Dates mechanism.
The NRAO AIPS++ group split into two principal groups: one in Charlottesville concerned mainly with support of single dish processing and another in Socorro concerned with project management and synthesis telescope support. In addition, NRAO plans to locate dedicated AIPS++ programmers at both the Green Bank and Tucson sites.
The following applications are now present and are being developed further: a tool for OTF mapping using the 12 Meter Telescope in rapid scanning mode, a self-calibration/deconvolution tool used principally on the Australia Telescope Compact Array data, and a tool for plotting and manipulating data from the GBT systems integration tests on the 140 Foot Telescope.
The following infrastructure library changes have occurred: a system for class documentation is now in place, numerous improvements have been made to the Glish system used for task control and command line interface, a very capable tool for visualization (AIPSView) has been developed by the Berkeley-Illinois-Maryland Association (BIMA) group at the National Center for Supercomputing Applications.
Intellectual developments are also crucial for the long-term success of AIPS++. A partial design for UV plane calibration and imaging was completed by a team drawn from the Australia Telscope National Facility (ATNF), the Netherlands Foundation for Research in Astronomy (NFRA), and the NRAO, and a collaboration of NFRA and ATNF personnel has developed a very general formalism for the calibration of synthesis polarimetric observations.
Documentation for both users and programmers is now available via the WWW at URL http://www.nrao.edu/aips++/docs/aips++.html. General project information, such as development plans and target dates, is also available via this URL.
The alidade is now complete. The electrical power system was successfully completed and the switch-over to permanent power was made. The structure has been rotated several times in azimuth using one of the four actual servo cabinets and two of the actual motors, along with their gearboxes, brakes, and tachometers. The elevation bearing support weldments, at approximately 80 tons each, were installed at the top level of the alidade towers, about 165 feet above the ground. The alidade elevator has been finished and now extends to this level as well.
The elevation shaft has been installed on the completed alidade. With the elevation shaft in place, the effort is focussed on the box girder subassemblies. All of the members of the box girder are at the site and the assembly of joints and beams has started. Installation of the elevation wheel, which was trial-erected at the plant in Mexia, TX, will follow. This phase of the construction will be completed in September, 1995 with the installation of the 22 counterweight boxes.
Preparation for the on site construction of the reflector backup structure (RBU) include the 150-foot by 150-foot reinforced concrete assembly pad with steel beam standoffs to support the RBU; the 90-foot measurement tower located at the vertex of the RBU; and a production aid (150' x 35') to be used for assembly and welding of the rib trusses in a horizontal position. Trial erection, i.e., construction on the ground of the RBU, is scheduled to be completed in Green Bank by the end of 1995.
After the design of the GBT was completed, a dynamic analysis was made of the structural model. It was found that a vibration could be excited in the feed arm when the antenna was switched in azimuth, but the magnitude of the effect could be kept small if an advanced servo preprocessor is used. An investigation will be made to see if the damping of the arm can be increased. Other systems to minimize the effects on telescope pointing from arm oscillations are being designed.
The NRAO is responsible for transforming the operational antenna delivered by the contractor into a functional radio telescope. This involves integration of the following systems that are being developed in house: the electronics system which includes a wide range of primary and secondary feeds, receivers, backends, LO, IF, and cryogenics, along with associated interconnecting cabling; and the monitor and control system that will be integrated with the antenna control system allowing the telescope to be pointed to acquire astronomical data. Interfaces with all NRAO systems will be readied; the data acquisition system will be put in place to acquire and process data from the telescope.
To enhance the performance characteristics of the telescope for operation at higher frequencies, additional systems will be implemented to provide the instrument with an active surface along with its associated measurement system, and a precision pointing system involving a laser ranging measurement capability to estimate antenna pointing in real-time.
Roger Norrod was appointed in October 1994 to coordinate the integration of the NRAO systems with each other and with the delivered antenna.
The 2004 surface panels of the GBT are mounted on actuators which enable the telescope surface to be readjusted. In the initial stages the motion of the 2209 actuators required to compensate for the changing gravitational deformations will be calculated from the structural model of the telescope. Ultimately the position of each panel will be measured with a laser ranging system and the panels will be adjusted on the basis of the measurement. The design goal of the system is to reach an rms surface accuracy of 225 micrometers (lambda/16 at 83 GHz). To observe at the higher frequencies which the GBT surface could support will require accurate pointing. The laser system will be used for this purpose as well.
All of the approximately 2400 active surface actuators have been received. These actuators incorporate the design changes developed in a series of tests and will enhance reliability and reduce radio frequency interference (RFI). The original goal for the actuators was 2000 hours of operation before failure. Two actuators now under test have operated for 8800 hours, and there has been only one failure, involving brush wear in the motor after 5500 hours.
The construction of the actuator control panels has been completed. Motor power supplies have been tested, and their performance is satisfactory and consistent from unit to unit. All cable interface boards, which include terminal strips to terminate the cables from the actuators, lightning suppressors, and connectors to interface to the control panels, have been manufactured. The actuator room, which will house actuator controls on the GBT, is now available to NRAO for installation of equipment. We have installed the mechanical and electrical equipment, but will wait until the building is on the telescope and properly climate controlled before installation of electronics.
The 20 laser range finder units are in production. The most intricate part of the laser range finder is the computer controlled mirror assembly. The assembly is being mass produced using numerically controlled machine tools in the Green Bank shop, and should be completed in 1995. In addition, all 25 of the oscillator assembly units have been acquired, and each has been fully tested and exceeds our specifications. Twenty transmitter modules and control electronics have been completed, and the bulk of the twenty receiver modules are finished.
The surface measurements will be made using retroreflectors mounted at the top of each of the actuators. All of the 2400 one-inch diameter retroreflectors have been acquired and meet all our specifications. The position of the antenna measured with respect to ground monuments using the laser rangers will also require the use of wide-angle retrospheres situated around the lip of the main reflector in order to connect the telescope metrology coordinate system and the ground based coordinates. The evaluation of the retrosphere prototypes has continued with good results. The final units will be produced by the Optical Sciences Center of the University of Arizona.
A crucial part of the precision pointing application of the rangefinders is the ability to track a moving target several hundred feet above the ground and generate its three dimensional position coordinates with a precision of 100 micrometers. An update rate of around twice per second is required. An experiment will be conducted to track several retrospheres mounted on the moving structure of the 140 Foot Telescope. Four ground-based laser rangefinders will be controlled via the Ethernet from the control building, and the range measurements to the various retrospheres will be used to establish the position of the retrospheres with respect to the ground monuments.
Three of the Gregorian focus front-ends have been completed and have been temporarily installed on the 140 Foot for user evaluation. The receiver for the frequency range 18-26.5 GHz incorporates four feeds, two operating at 18-22 GHz and two at 22-26.5 GHz. On the GBT, the beams will be separated by approximately 4 arcminutes, and beam switching following the cooled amplifiers will be supported. The eight channels are downconverted to 4 and 5.75 GHz center frequencies for transmission to the control room. For these trials only one beam in each band is available. The receiver for the range 12-15.4 GHz has two identical feeds, with cooled sloped-septum polarizers and HFET amplifiers to produce dual circular polarizations. The GBT beams will be separated by approximately 5 arcminutes, and beam switching following the cooled amplifiers will be supported. Finally, the range 8-10 GHz is covered by a single feed and cooled HFET amplifiers.
Progress continues on the three Gregorian receivers for the lower frequencies. The early results on the L-band (1.15-1.73 GHz) system are very promising, but tests showed that the large feed horn needed to be stiffened. This system, expected to be one of the workhorse receivers of the GBT, will be completed in 1995.
The prime focus front-end box is under construction. It will incorporate four bands: 680-920, 510-690, 385-520, and 290-395 MHz. The high band will utilize a corrugated horn feed, cooled waveguide ortho-mode transition, and cooled HFET amplifiers. The low bands will use dipole type feeds and cooled HFET amplifiers. Changing bands will require a manual feed change. The receiver may be configured to produce dual linear or circular polarizations in one or two bands simultaneously (NRAO hopes eventually to provide dual frequency feeds), or full Stoke's parameters at one band.
The receiver room LO rack is largely complete and tested. It provides two independent synthesizers covering 0.01-20 GHz with 1 Hz resolution, and electromechanical switches which will allow connection to any of the installed front-ends. The GBT LO reference distribution and round-trip phase monitor system will use optical fibers, and is similar to the system which has been built for the OVLBI Earth Station.
In the IF system, the IF Router will accept 64 IF inputs in the range 0.1-8 GHz; up to eight may be selected for transmission to the control room via optical fiber analog links. In the control room, broadband tunable converters will allow users to select one or more portions of a front-end's IF signal for input to any of several backends. The prototype version of the converter has been evaluated in the laboratory, and a similar system is in use with the GBT receivers on the 140 Foot Telescope.
A new digital continuum back end has been provided for the acquisition of the data needed for characterization of the antenna and for astronomical observations. Broadband and narrowband total power detectors will be included in the system IF chain. The detected total power signal will be converted to a pulse train using 10 MHz voltage to frequency converters, and will be transmitted to a continuum backend over multimode optical fibers.
Work on the new GBT spectrometer continues, both at NRAO and at the correlator chip design facility. Tests on the second wafer run correlator chips still indicate that the chip performance is acceptable for our application. However, the system clock rate has been decreased to 100 MHz from 125 MHz to achieve adequate performance margin. This decision means the spectrometer maximum bandwidth will be 800 MHz, which meets the original performance goal set by astronomical considerations. Progress has been made on the 2 GHz sampler, which now works well beyond 2 GHz. The prototype printed circuit boards have been tested.
Two types of software are being developed within the monitor and control system. One type, libraries, includes routines which implement the broad functionality required in several different applications. The development of the libraries has necessarily been an iterative process involving improvements and generalizations gleaned from their use in application software. The hardware driver libraries are in use. Design of enhancements to communication, control, and monitor libraries to ensure robustness and recovery is just beginning. The control and coordination library has passed testing and approximately half of it is in use. The other type is device-specific software. This type of GBT software is already in use for the site timing center, weather station, and the spectral processor.
Data analysis on the GBT will be made using the AIPS++ system. As a necessary precursor to this, spectral processor datasets from the 140 Foot have been converted into FITS binary tables. This is part of the 140 Foot Telescope experiment in which GBT software is being used to control and monitor pulsar observations made with the spectral processor.
A work definition and schedule has been developed for tests with integrated GBT systems at the 140 Foot Telescope. During the last half of 1995 and the first half of 1996, we will combine GBT electronics, monitor/control, and data analysis systems which will have been installed at the 140 Foot Telescope to perform astronomical observations of the types needed to calibrate and evaluate the GBT. Successful synchronized operation of the GBT systems on the 140 Foot Telescope as a test bed will give confidence for later integration on the GBT, and will provide a means to get observer and operator input during development of the user interfaces.
The completion of the GBT has been delayed beyond the original schedule, in part because the design of this complex structure required a greater effort than had been anticipated, and in part because the generation of the detailed fabrication drawings was more time-consuming. The contractor is now scheduled to complete the antenna in December 1996. It is anticipated that the commissioning phase, during which NRAO will complete the installation of the equipment and instruments for which it has the responsibility, will begin as soon as the contractor turns the antenna over to us, and will last about six months.
The MMA is a high resolution imaging telescope consisting of forty precision transportable antennas of diameter 8 meters to be located on a high elevation site suitable for millimeter/submillimeter astronomy. In conjunction with the U.S. community of millimeter-wave astronomers, design work on the MMA has been in progress at the NRAO for more than twelve years. The MMA is highlighted in the Bahcall Report as the second major initiative in ground-based astronomy (after the northern Gemini telescope) for the decade of the 1990s. Once constructed the MMA will combine the sensitivity provided by the collecting area of a telescope 50 meters in diameter with angular resolution < 01, superior to that of the design goals of the Hubble Space Telescope, and it will operate at frequencies 35-350 GHz at which thermal processes illuminate the sky. This unprecedented combination of sensitivity and angular resolution at short wavelengths will make available for astronomical investigation a wealth of unique opportunities and new science, including the ability to image the redshifted dust continuum emission and gas spectral line emission from evolving galaxies at epochs of formation as early as z = 10; reveal the kinematics of optically obscured galactic nuclei and QSOs on spatial scales smaller than 100 pc; and reveal the isotopic and chemical gradients within circumstellar shells that reflect the chronology of stellar nuclear processing.
At the recommendation of the NSF Astronomy Advisory Committee, the MMA project was formulated as an initial three-year phase of design and development, to be followed by actual construction of the array. In the design and development phase, prototype array hardware will be constructed, including fabrication of a prototype antenna, and facilities will be established to allow us to construct multiples of 40 of the essential parts of the array instrumentation with a high degree of reliability and reproducibility. The current MMA project schedule calls for the design and development phase to extend over the years 1997-1999, with the last development year overlapping with the first year of array construction. If this schedule is maintained, the array will be complete in 2004.
The challenging aspects of the MMA design include design of the antenna, definition of a phase calibration technique that will permit the array to operate as a coherent instrument on the longest baselines and at the highest frequencies for a maximum number of hours each year, and design of a versatile and easy to maintain receiving system. Because these design questions are so critical to the eventual performance of the array, we are attempting to formulate answers to them using not only the technical experience at the NRAO but also by relying on the advice of others who have experience with millimeter-wave synthesis telescopes at university-based observatories. The mechanism to accomplish these tasks is the Millimeter Array Development Consortium (MDC). Working groups of scientists and engineers at the NRAO, Owens Valley Radio Observatory (OVRO), and at the BIMA institutions have been established to analyze the pivotal MMA technical issues and to supply specific solutions. The four MDC working groups and their chairpersons are the following: antenna design, P. Napier; receivers, W. J. Welch; systems design, A. R. Thompson; and phase calibration, D. Woody. We expect that these working groups, augmented by an additional working group in array computing issues, will form the backbone of the MMA project throughout the design and construction years.
Management of the MMA project and the activities of the MDC working groups is reviewed annually by the MMA Advisory Committee (MAC), the membership of which is given in Section VII of this report. The MAC reports to the NRAO Director.
The evaluation of potential sites for the MMA is an activity on-going at the NRAO. Since the sensitivity of the MMA depends on the opacity and stability of the atmosphere, in turn, dependent on the precipitable atmospheric water content, it is important that the site be exceptionally dry. Two candidate sites are Mauna Kea in the northern hemisphere and the area near the Atacama desert in Chile in the southern hemisphere. The millimeter-wave opacity and atmospheric stability (the radio seeing) are being tested by remote instrumentation on both sites. On the basis of this information, we hope to make a site choice in the summer of 1996.
Progress in the MMA project, including a summary of technical memoranda issued and the site testing data, is made available on the WWW. Those interested in this information can find it in the MMA entries on the NRAO home page.
From 1 Janaury 1993 through 31 December 1994, the number of visiting scientists, students, and the institutions they represented is given in the following table.
1994 1995 Visiting Scientists 736 650 Students 133 147 Institutions 193 180
The distribution of telescope time among visiting scientists, students, NRAO staff, Jansky postdoctorals, maintenance and calibration time, and unscheduled time averaged over the two-year period 1993-1994 is given in the following table.
12 M 140 FT VLA VLBA Overall Visiting Scientists 43% 50% 52% 30% 44% Students 12% 18% 8% 3% 10% NRAO Staff 14% 12% 13% 5% 11% Jansky Postdoctorals 2% 5% 3% 1% 3% Test/Calib/Maint 28% 13% 23% 19% 21% Unscheduled 1% 2% 1% 42% 11%
The distribution of observing time over various research areas averaged over the period 1993-1994 is given in the following table.
12 M 140 FT VLA VLBA Overall Solar System 4% 2% 6% 1% 5% Stellar 7% 39% 27% 32% 26% Galactic 56% 37% 24% 16% 28% Extragalactic 33% 22% 43% 51% 41%
The Jansky Lectureship, named in honor of Karl G. Jansky who pioneered radio astronomy, was established by the AUI Board of Trustees in 1966. It is awarded annually to an outstanding scientist in astronomy or a related field. The 1993 lecture was given by Dr. David S. Heeschen, former Director of NRAO. He spoke on the subject "The Development of Radio Astronomy in the United States." In 1994 the Lectureship was held by Dr. Vera C. Rubin of the Department of Terrestrial Magnetism, Carnegie Institution of Washington. The title of her lecture was "What's the Matter in the Universe." Both lectures were given in Socorro and Charlottesville.
The Users Committee is made up of users and potential users of NRAO facilities from throughout the scientific community. It advises the Director and the Observatory staff on all aspects of Observatory activities that affect the users of the telescopes. This commitee, which is appointed by the Director, meets annually in May or June. The current membership of the Committee is:
The NRAO Visiting Committee is appointed by the AUI Board of Trustees to review the management and research programs of the Observatory. The Visiting Committee met in Tucson on April 27-29, 1994 and in Socorro on March 3-5, 1995. The current membership of the Committee is:
The Green Bank Telescope Advisory Committee is appointed by the Director and meets periodically to review plans and progress. It is composed of scientists and engineers representing the range of skills - structural, mechanical, electrical, computational, and scientific - needed for the telescope design and construction. The membership of the Committee is:
A Millimeter Array Advisory Committee has been appointed to provide continuing advice on all aspects of the project to the NRAO Director. The members of the MMA Advisory Committee are:
During this reporting period a variety of symposia, colloquia, and workshops were held at NRAO. IAU Colloquium No. 170 - CO: Twenty-five Years of Millimeter-wave Spectroscopy - was held in Tucson May 29-June 2, 1995. Proceedings will be published by Kluwer in the spring of 1996. A workshop on the source Cygnus A assembled researchers from many wavelength regions in Green Bank, May 1-4, 1995. The proceedings will be published by Cambridge University Press. A workshop on Multi-feed Systems for Radio Telescopes was held in Tucson, May 16-18, 1994. The proceedings appeared as Volume 75 of the ASP Conference Series. A workshop on Compact Extragalactic Radio Sources was held in Socorro, February 11-12, 1994. The proceedings were handed out at the conference and distributed to observatory libraries worldwide. Two summer schools were held in Socorro: Very Long Baseline Interferometry and the VLBA, June 23-30, 1993, and the Fifth Annual Synthesis Summer School, June 5-9, 1995.
Teacher Institutes were conducted in Green Bank in the summers of 1994 and 1995 for pre-college science teachers. The Green Bank site also hosted Chataqua Courses, Elderhostel, and annual meetings of the Society of Amateur Radio Astronomers.
The official opening of the VLBA took place on August 20, 1993 in Socorro. More than 400 people attended the ceremony and tours, which included demonstrations of VLBA operations and of the correlator. U.S. Senator Pete Domenici was the key speaker. Other speakers included U.S. Congressman Joe Skeen, Robert E. Hughes, president of Associated Universities, Inc., Hugh van Horn of the National Science Foundation, and Dan Lopez, president of New Mexico Tech.
During this reporting period, G. Croes retired as Assistant Director for Computing Systems; R. Simon was appointed to this position. T. Cornwell was appointed Assistant Director - AIPS++ Project; P. Diamond replaced him as Deputy Assistant Director - Socorro Operations. R. Burns retired; G. Hunt replaced him as Deputy Assistant Director - Computing Systems. Other retirements included C. Wade, R. Duquet, and B. Meredith (deceased). J. Horstkotte, J. Lamb , W. Junor, and D. Wood left the NRAO. S. Radford and A. Kemball joined the scientific staff as Assistant Scientists. A. Beasley, D. Frail, and M. McKinnon were promoted to Assistant Scientist. R. Perley was promoted to Scientist (tenure). T. Bastian and A. Zensus were promoted to Scientist. R. C. Walker and J. Wrobel took a leave of absence during this reporting period, as did A. Zensus. A. Zensus transferred from Socorro to Charlottesville, B. Glendenning from Charlottesville to Socorro, and G. Langston from Charlottesville to Green Bank.
The following Jansky postdoctorals either began, continued, or finished their appointments during the reporting period: D. Balser, C. Barnbaum, A. Beasley, B. Butler, C. Chandler, C. Carilli, J. Conway, K. Dwarakanath, G. Fuller, J. Higdon, W. Latter, M. McKinnon, J. Navarro, D. Nice , M. Rupen, E. Wilcots, E. Wollock, and J.-H. Zhao.
The following visitors made extended visits to the NRAO to work on the AIPS project: S. Bhatnager (Nat. Ctr. Radio Ast./Tata Inst. for Res.), M. Calabretta (ATNF), R. Martinez (UNAM), B. Sault (ATNF), D. Shone (Nuffield Radio Ast. Lab), and M. Wierenga (ATNF).
The following visitors were in residence at the Observatory for an extended period: T. Balonek (Colgate U.), S. Baum (STScI), W. B. Burton (Leiden U.), R. Davis (U. Manchester), R. Erickson (U. Massachusetts), N. Erickson (U. Massachusetts), D. Gallego (Obs. Yebes), Y. Gomez (UNAM), C. Heiles (U.C. Berkeley), M. Kestevan (ATNF), M. Kutner (Rensselaer Poly.), M. Leach (ATNF), L. Matveyenko (Space Res. Inst.) K. Mead (Union College), A. Novikov (Russian Academy of Sciences), C. O'Dea (STScI), P. Palmer (U. Chicago), L. Rodriguez (UNAM), A. R. Taylor (U. Calgary), J. van Gorkom (Columbia U.), H. van Langevelde (NFRA), A. Velazquez (UNAM), T. Wilson (MPIfR), and E. Wollman (Bates College).
The following graduate students were in residence at the NRAO for an extended period of time. Their research mentors are given in parentheses: L. Benaglia - La Plata/Argentina (W. M. Goss), D. Boboltz - Virginia Tech (P. Diamond), D. Briggs - New Mexico Tech, (T. Cornwell), A. Cox - U. Wisconsin (G. Van Moorsel), C. DePree - U. North Carolina (W. M. Goss), K. Desai - UC Santa Barbara (P. Diamond), D. Frayer - UVa (R. Brown), P. Gensheimer - U. Illinois (P. Jewell), G. Hoeppe - U. New Mexico (E. Brinks), B. Hufnagel - UC, Santa Cruz (E. Brinks), M. Ledlow - U. New Mexico (F. Owen), K. Leppanen - Metsahovi Radio Research Station/Finland (A. Zensus), A. Lobanov - Moscow (A. Zensus), K. Marvel - New Mexico State U. (P. Diamond), D. Mehringer - Chicago (W. M. Goss), D. Moffett - NMIMT (T. Hankins), G. Morrison - U. New Mexico (F. Owen), E. Murphy - UVa (F. J. Lockman), M. Swain - Rochester (A. Bridle), C. Taylor - U. Minnesota (E. Brinks), A.Vourlidas - NMIMT (T. Bastian), Q. Wang - NMIMT (F. Owen), L. Young - U. Illinois (W. M. Goss), F. Zhou - NMIMT (F. Owen), and J. Zhao - U. New Mexico (F. Owen).
The research interests of all staff members at the Observatory during the reporting period or currently present at NRAO are listed below.
D. S. Adler -- Interstellar medium; dynamics of spiral galaxies; clusters; star formation
D. S. Balser -- Galaxy: abundance, structure; ISM: HII regions, planetary nebulae; radio sources: continuum, line
C. S. Barnbaum -- Evolved stars: kinematics, isotopic abundances, circumstellar structure
T. S. Bastian -- Solar/stellar radio physics; solar/stellar activity; solar wind and heliosphere; scattering in the IPM
A. J. Beasley -- Radio interferometry; VLBI observing techniques
J. M. Benson -- Extragalactic radio sources; VLBI image processing
R. C. Bignell -- Polarization and imaging of extragalactic radio sources; planetary nebulae; supernovae remnants
A. H. Bridle -- Extragalactic radio sources
E. Brinks -- Interstellar medium in nearby galaxies; HI studies of galaxies; star-forming dwarf galaxies; interacting galaxies
R. L. Brown -- Theoretical astrophysics; interstellar medium; quasar absorption lines
B. J. Butler -- Using observations of the planets and their atmospheres at radio wavelengths to deduce information about them.
C. Chandler -- Radio and infrared studies of the formation of low-mass stars, circumstellar disks, and energetic winds and outflows
B. G. Clark -- VLBA control; software development
M. J. Claussen -- Masers; HII regions; molecular spectroscopy; spectropolarimetry; radio recombination lines
J. J. Condon -- QSOs; normal galaxies; extragalactic radio sources
T. J. Cornwell -- Interferometry; image reconstruction methods; coherence theory; radio source scintillation
W. D. Cotton -- Extragalactic radio sources; interferometry; computational techniques for data analysis
L. R. D'Addario -- Theory of synthesis telescopes; millimeter receivers; radio astronomy from space
V. Dhawan -- Extragalactic radio sources; VLBI; instrumentation
P. J. Diamond -- Spectral line interferometry; VLBI; software development
D. T. Emerson -- Nearby galaxies; star formation regions; millimeter wave instrumentation
J. R. Fisher -- Cosmology; signal processing; antenna design
C. Flatters -- VLBI polarization studies of extragalactic radio sources
E. B. Fomalont -- Interferometry; extragalactic radio sources; relativity tests
D. A. Frail -- Pulsars; supernova remnants; high energy transients; masers; HI absorption; interstellar scattering
G. A. Fuller -- Star formation; galactic molecular clouds
R. W. Garwood -- Galactic 21-cm line absorption; interstellar medium; high redshift 21 cm line absorption
F. D. Ghigo -- Interacting galaxies; extragalactic radio sources; interferometry
B. E. Glendenning -- Starburst galaxies; scientific visuali-zation
M. A. Gordon -- CO; galactic structure; gas-rich galaxies; interstellar medium
W. M. Goss -- Galactic line studies; pulsars; nearby galaxies
E. W. Greisen -- Structure of the interstellar medium; computer analysis of astronomical data
J. L. Higdon -- Multi-wavelength studies of star formation in interacting galaxies; primeval galaxies; galaxy kinematics
R. M. Hjellming -- Radio stars; radio and X-ray observations of X-ray binaries; interstellar medium
D. E. Hogg -- Radio stars and stellar winds; early-type galaxies
M. A. Holdaway -- Image reconstruction methods; VLBI polarimetry
P. R. Jewell -- Evolved stars and circumstellar envelopes; interstellar chemistry; millimeter-wave instruments and observing techniques
K. I. Kellermann -- Radio galaxies; quasars; VLBI
A. J. Kemball - VLBI data reduction techniques; masers; VLBI polarimetry
A. R. Kerr -- Millimeter-wave development
L. J. King -- Antenna structural/mechanical analysis and design; optimization methods for antenna structural performance
L. Kogan -- Maser radio sources; theory of interferometry; software for data reduction of VLBI
G. I. Langston -- Gravitational lenses; computational techniques for synthesis imaging
H. S. Liszt -- Molecular lines; galactic structure
F. J. Lockman -- Galactic structure; interstellar medium; HII regions
R. J. Maddalena -- Molecular clouds; galactic structure; interstellar medium
M. M. McKinnon -- Plasma astrophysics; pulsars; stellar radio emission; signal processing
P. J. Napier -- Antenna and instrumentation systems for radio astronomy
J. Navarro -- Pulsar astrophysics; instrumentation and techniques in radio astronomy
D. J. Nice -- Pulsars; binary evolution; gravitation
F. N. Owen -- Clusters of galaxies; QSOs; radio stars
J. M. Payne -- Telescope optics; millimeter-wave receivers; cryogenic systems
R. A. Perley -- Radio galaxies; QSOs; interferometer techniques
M. Pospieszalski -- Low noise front-ends and amplifiers; theory and measurement of noise in electronic devices and circuits
S.J.E. Radford -- Starburst galaxies, millimeter interferometry
M. S. Roberts -- Properties and kinematics of galaxies
J. D. Romney -- Active extragalactic radio sources; VLBI; interferometer imaging
M. P. Rupen -- Interstellar medium of early type galaxies; galaxy dynamics; supernovae; steep spectrum sources
R. A. Simon -- Theory of interferometry; computational imaging; VLBI
R. A. Sramek -- Normal galaxies; quasars; astrometry; supernovae
A. R. Thompson -- Interferometry; frequency coordination and atmospheric effects; distant extragalactic sources
B. E. Turner -- Galactic and extragalactic interstellar molecules; interstellar chemistry; galactic structure
J. M. Uson -- Clusters of galaxies; cosmology
P. A. Vanden Bout -- Interstellar medium; molecular clouds; star formation
G. A. Van Moorsel -- Dynamics of galaxies and groups of galaxies; techniques for astronomical image analysis
R. C. Walker -- Extragalactic radio sources; VLBI; VLBA development
D. C. Wells -- Digital image processing; extragalactic research
E. M. Wilcots -- Magellanic HII regions; HI in nearby galaxies; extragalactic star formation; irregular galaxies
E. J. Wollack -- Cosmic microwave background; in- strumentation for radio astronomy
D. S. Wood -- Star formation; HII regions and the interstellar medium; radio and infrared astronomy
H. A. Wootten -- Star formation; structure, and chemistry of the interstellar medium in galaxies; circumstellar material
J. M. Wrobel -- Normal galaxies; active galaxies; polarimetry
Q.-F. Yin -- Normal galaxies; imaging techniques
A. Zensus -- Active galactic nuclei; compact extragalactic radio sources; high resolution interferometry; space VLBI
A sampling of staff research highlights is given below.
D. Adler and D. Westpfahl (NMIMT) obtained a high resolution, high sensitivity HI image of the spiral galaxy M81. They found that while the northeast spiral arm showed characteristics indicative of the presence of a density wave, the southwest arm generally did not. It appears that the density wave in the southwest arm has been disrupted by interaction with one of the numerous companion galaxies or that the arms have evolved in a very different manner. They also found properties of model density waves in the HI distribution - the intensity distribution varies along the spiral arms, and the shape of the intensity profile reverses around co-rotation. Adler and Westpfahl have also investigated the small scale structure in the HI distribution in M81. Preliminary results showed that while the northeast arm has numerous HI holes, there are very few in the southwest arm. A similar trend was seen in the H-alpha distribution. This indicated that the presence of density waves (determined from HI streaming motions) correlates with the presence of star formation tracers (holes, HII regions). They also found very few of the holes were expanding shells, showing that the holes have "blown-out" of the HI disk. This indicated that the HI disk in M81 is thin, and that the holes are not recently formed entities.
B. Wakker (U. Ill.) and D. Adler studied the CO distribution in the inner 2' of the spiral galaxy NGC 628. They found that while the CO appeared to be aligned with the inner optical spiral arms, it did not show consistent correspondence with either the dust lanes or HII regions. They are currently obtaining data for four more fields around the nucleus. Adler, Wakker and Westpfahl are in the process of studying the HI distribution in the spiral galaxy NGC 628. While the spiral pattern in M81 was seen to be well defined in the HI, the HI structure in NGC 628 is dominated by holes, clumps, and filaments, making the spiral pattern (which is well defined in optical images) difficult to discern. It is likely that NGC 628 is a galaxy which is not dominated by a spiral density wave, with its current appearance influenced more by massive star formation events.
Adler, R. Allen (STScI), and F. Lo (U. Ill.) studied the CO distribution in the spiral galaxy M83. They found that while the integrated CO intensity fell off as an exponential between 3 and 11 kpc, the inner 3 kpc is not well modeled by an exponential. This suggested that the nuclear emission is a separate component from the disk emission. They also found that the CO is preferentially gathered near the spiral arms, indicating the presence of a density wave. Several factors, including the lack of a contrast between the arm and interarm velocity dispersion, indicate that the density wave is not very strong.
Adler, D. O. S. Wood (Kodak), and W. M. Goss studied the anomalies in the helium abundance and carbon ionization in the Galactic HII region W3. They confirmed previous observations of the anomolous helium abundance in W3A (Y+ = 20%), and detected strong C+ emission where no helium or hydrogen line emission exists in W3B and W3C. This C+ emission comes from the interface zone between the HII region and the surrounding molecular cloud. They showed that the C+ velocity is more typical of the surrounding molecular cloud while the H+ and He+ lines traced the ionized gas.
Dana Balser has made measurements of the cosmic abundance of 3He in the Galaxy in collaboration with T. Bania (Boston U.), R. Rood (UVa), and T. Wilson (MPIfR, Bonn). Observations of the 3He+ 8.7 GHz hyperfine line include a sample of HII regions using the 140 Foot Telescope and a sample of planetary nebulae using the MPIfR 100 meter telescope and the VLA. Together with numerical models based on radio continuum and radio recombination lines observations, these measurements provide constraints to stellar evolution, galactic evolution, and cosmology.
Cecilia Barnbaum, in collaboration with P. Keenan (Ohio State) and R. Stone (Lick Obs), has completed a low resolution, optical spectral atlas of carbon stars. This atlas will serve as a tool to help identification of these stars in external systems, such as the Magellanic Clouds and in nearby dwarf galaxies. An analysis of six years of high resolution spectra of the carbon star, V Hya, was completed this year with collaborators M. Morris (UCLA) and C. Kahane (Grenoble). The data strongly suggest that this star is rotating and has gained its excess angular momentum from engulfing a companion star.
Barnbaum, with A. Omont and C. Loup (Inst. D'Astrophysique de Paris), has recently finished classifying 72 cool IRAS sources that are apparently deficient in circumstellar gas and have an over-abundance of circumstellar dust. The goal of this project is to sort out these stars by luminosity type and to compare the circumstellar environments of giants and supergiants. Barnbaum, Omont, and Loup have also completed a preliminary analysis of the unusual star, U Equ, whose bizarre optical spectrum, discovered by them, shows an abundance of cool circumstellar gas at a large distance from the star and indicates the presence of a disk. H2O and OH masers, which they observed with the 140 Foot Telescope, show large changes in velocity and intensity on short time scales. They plan to continue monitoring this unusual star.
With K. Hinkle (KPNO), Barnbaum has recently completed an analysis of optical and infrared radial velocities of carbon stars. The results suggest that optical velocities of these stars are not good indicators of center-to-mass motion. Infrared light arising from more stable inner layers of the photosphere serves as much more reliable measure of motion.
T. Bastian, in collaboration with C. Lindsey (National Solar Obs.), K. Shibasaki (Nobeyama Radio Obs.), G. Dulk (Colorado/Obs. Paris), and Y. LeBlanc (Obs. Paris), has obtained a suite of images at 850 and 1300 microns (sampling above the temperature minimum region) with the James Clerk Maxwell Telescope, and at 1.3, 1.7, and 2 cm using the VLA and the Nobeyama Radioheliograph. The motivation for the present study is to understand the divergence between chromospheric models based on UV and EUV spectral lines and radio observations as one proceeds from the temperature minimum region up to the chromosphere.
Bastian has continued work on the structure and evolution of solar active regions with detailed modeling of the radiative transfer of gyroresonance emission from sunspots at coronal heights, a collaboration with A. Vourlidas, D. Gary (Caltech), and N. Nitta (Lockheed Palo Alto Res. Lab).
Bastian employed a new fast-snapshot imaging mode at the VLA to image microwave burst emission at a rate of 5 Hz during the previous maximum. The image sequences have revealed previously unobserved details of energy transport processes during the course of solar flares. All flare observations have been closely coordinated with the Japanese Solar-A mission (Yohkoh), the Compton GRO, and numerous ground-based observatories. Collaborations include N. Nitta, A. Kiplinger (NOAA), M. Aschwanden (NASA/GSFC), D. Gary (Caltech), H. Hudson (Inst. for Astronomy).
A collaborative effort has continued with the Swiss group at ETH/Zurich, using a fast, broadband, digital spectrometer with the VLA to, for example, directly image solar millisecond spike bursts and Type I radio bursts, a collaboration with S. Krucker (ETH), A. Benz (ETH), and M. Aschwanden (Maryland).
Bastian has also explored the issue of turbulent inhomogeneities in the solar corona and has recently shown that problems associated with the interpretation of the polarization properties of certain meter and decimeter wavelength bursts can be understood in terms of strong mode coupling resulting from scattering in the solar corona. Experimental work has been directed toward exploiting scintillation phenomena to constrain the spatial spectrum of electron density inhomogeneities in the outer corona.
Work in stellar radiophysics has included a survey of magnetic cataclysmic variables, with A. Beasley (NRAO), L. Ball (U. Sydney) and K. Wu (U. Sydney); and a search for coherent radio pulsations from the magnetic cataclysmic variable AE Aqr, with Beasley and J. Bookbinder (Center for Astrophys.). With Beasley, Bastian has initiated a program of time resolved imaging of radio flare outbursts on active binaries (the RS CVn binaries) with the Very Long Baseline Array.
A. Beasley, with collaborators E. Fomalont, R. C. Walker, J. Wrobel, V.Dhawan (NRAO), M. Eubanks (USNO), and A. Readhead (Caltech), has observed three (of fourteen) epochs of geodetic-mode snapshot observations of ~3000 flat-spectrum sources selected from the MERLIN calibrator list. Apart from its fundamental role in defining the VLBA astrometric reference frame and enabling routine VLBA phase referencing to occur, when complete these data will also be an invaluable resource for studies of compact extragalactic radio sources. The project will be producing 2.7 and 8.4 GHz images for the entire sample, which will be made available to the community on an ongoing basis. The typical positional accuracy achieved by their survey observations is better than 1 milliarcsec.
On August 29, 1994, simultaneous VLA/VLBA/ASCA observations of the active star UX Ari were made to synthesize the coronal structure of this star by examining the two main indicators, radio/X-ray emission, with high spatial and spectral resolution. Using phase referencing, a quiescent halo radio source twice the size of the binary system was detected, with a peak flux of 1.1 mJy. Further RSCVn observations in February 1995 confirmed the existence of these extended halos around UX Ari and HR1099. This work was done with J. Linsky (JILA) and M. Guedel (ETH Zentrum).
With F. Owen and G. Morrison (New Mexico), Beasley has obtained 24 hours of VLA A-array observations of 43 intermediate redshift Abell clusters (z = 0.2). These data will be used to search for lensing of more distant background sources, and to examine the evolution of the radio luminosity function for cluster sources.
In July 1994, Beasley and S. Ellingson (U. Tasmania) used the Australia Telescope to perform a survey of 70 IRAS-selected ultra-compact HII regions of the Magellanic Clouds for 6.6 GHz methanol maser. The purpose of this survey was to examine the effects of low metallicity on the existence of methanol masers (an effect previously seen for OH/H2O masers), and to identify a sample of masers for subsequent VLBI studies; in particular, to measure the transverse motion of the LMC/SMC relative to nearby extragalactic sources. The ultimate goal in this project is to identify a suitable sample of masers in all large members of the Local Group (LMC/SMC/M31/M33) and measure their transverse velocities using differential astrometry. These velocities are strongly predicted in various Local Group models, and depend on estimates of M/L and dark-matter fractions. They successfully detected a strong methanol maser in the LMC, to date the strongest known extragalactic methanol maser, towards IRAS 05011-6815.
E. Brinks, in collaboration with B. Elmegreen (IBM Res. Div.), D. Elmegreen (Vassar College Obs), M. Kaufman (Ohio State), and M. Sundin (Chalmers, Sweden), has conducted an in depth study of the ocular galaxy IC 2163. Ocular galaxies are interacting systems which are caught at a very particular time, when one of the objects displays a strong double arm which gives it the appearance of an eye. Hence the name ocular galaxy. IC 2163, which is interacting with NGC 2207, is probably the best example. Because the ocular structure is short lived, the parameter space of possible orbits is restricted and in the case of the IC 2163/NGC 2207 pair a unique orbit could be determined.
Brinks has been working with I. F. Mirabel and P.-A. Duc (Saclay, France) on the far-infrared luminous object Arp105. This object is a magnificent example of a spiral galaxy in the process of merging with an elliptical. Huge gas tails stretch 100 kpc to either side of the spiral. All HI is found in these tails, with major concentrations near the tips; no HI is detected in the merging objects. The HI clumps near the tips contain newly formed stars. These clumps could become self-gravitating and detach from the tails, making them dwarf galaxies in their own right rather than tidal debris. Strong CO emission is found exclusively at the center of the merging activity and is absent from the tails.
With C. Mundell and A. Pedlar (Jodrell Bank, UK), and S. Baum, C. O'Dea, and J. Gallimore (STScI), Brinks has been involved in high resolution (0.15 arcsec) observations with MERLIN of the prototypical Seyfert 1 galaxy NGC 4151. Using HI in absorption, the inner 100 parsec was probed. Absorption was seen against one component of the complex nuclear source only, setting limits to the thickness of the putative hydrogen disk of 50 parsec. Based on the absorption measurements, the geometry of the inner 100 parsec of NGC 4151 is derived.
C. Chandler has concentrated on understanding the origin of the CO rotation-vibration overtone bandhead emission at 2.3 m in young stellar objects. The excitation conditions required for the CO overtone bands at 2.3 m to be in emission are extreme: temperatures of ~4000 K and hydrogen number densities of 10(11) cm(-3). The bandheads therefore trace material very close to young stars, probably located in a disk or a wind within a few stellar radii. The CO emission is a powerful probe of the circumstellar environment of young stars, and it is important to know precisely where the CO is originating. To determine whether circumstellar accretion disks or neutral protostellar winds can account for the observed bandhead fluxes and line profiles, Chandler and collaborators J. Carlstrom and N. Scoville (Caltech) have modeled the predicted CO emission from disks and winds. Disks most easily reproduce the observed CO bandhead spectra, with accretion rates of ~10(-8)-10(-7) M(sun) yr(-1), similar to those derived from modeling the spectra energy distributions of young stellar objects. Winds are less attractive as the origin of the CO emission because unreasonably high mass-loss rates are needed to reproduce the CO spectra.
Of particular interest currently in the field of star formation is understanding how molecular outflows are driven. It is believed that all young stars pass through a phase of energetic mass loss before reaching the main sequence, most commonly observed as high velocity molecular emission. Understanding the origin of the driving stellar or disk wind is therefore essential to understanding star formation. However, it is still not clear what drives the outflows, nor how the molecular gas is accelerated. With S. Terebey (Caltech/ IPAC), M. Barsony (Calif., Riverside), and T. Moore (ADFA), Chandler has been working on combining sensitive near-infrared images of two young, embedded sources in the Taurus star forming complex, with high-resolution mapping of the molecular gas using the Owens Valley millimeter array and the James Clerk Maxwell Telescope. The results have been compared with recent models of the morphology and dynamics of outflows, and suggest that the molecular outflows must be driven by jets imparting momentum to the ambient gas through shocks, rather than by a radial stellar wind sweeping up the molecular material.
Circumstellar disks appear to be a natural by-product of star formation, probably forming at a very early state from infalling high angular momentum material from the surrounding molecular cloud. While disks have been inferred to exist in the youngest embedded sources, they are difficult to study directly in the presence of a surrounding envelope of gas and dust. Chandler, A. Sargent (Caltech), D. Koerner (JPL), and D. Wood have used interferometric measurements from the Owens Valley millimeter array and the VLA to separate the disk from the envelope in the young protostar HH24MMS in Orion. They find that the dust properties on its disk are similar to those around more evolved T Tauri stars.
M. Claussen, with R. Gaume (USNO), W. M. Goss, and C. DePree (North Carolina), high resolution and high sensitivity VLA observations were made of the important star-forming region Sgr B2 (in the Galactic Center) in the radio continuum and radio recombination lines. Many new, small HII regions were detected; nearly half of all the regions have diameter < 5000 AU. Recombination line emission was detected from many HII regions, but some of the most compact were not detected. Kinematic and dynamical information is obtained from the recombination line emission.
With Gaume and A. Fey (NRL), high resolution, high sensitivity VLA observations were made of the radio continuum, recombination line and water maser emission toward the cometary HII region G34.3+0.2. Models of ultracompact HII regions now in vogue fail to explain these observations, so doubt is cast on these models, in particular the moving star bow shock model. Water masers are found associated with the continuum region, but also are found far away from the main region, where no radio continuum emission exists. These masers probably mark young, lower mass star formation regions.
Claussen, with R. Gaume, K. Johnston (USNO), and T. Wilson (MPIfR), reported on high angular resolution and high sensitivity VLA observations of the radio continuum and water masers associated with the very young, luminous protostar, W3 IRS5. At least seven distinct radio continuum sources were discovered, which are likely a cluster of B0 stars with ionized stellar winds. This contrasts with earlier interpretations which suggested that IRS5 is a single massive star of high luminosity. The distribution of water masers roughly follows the radio continuum and the masers probably trace outflows around the several stars.
With B. Wilking, J. Benson, P. Myers (CFA), H. A. Wootten, and S. Terebey (Caltech), Claussen reported the results of a 13-month monitoring survey of water masers in low-mass, young stellar objects. Three sources of water maser emission have bolometric luminosities of less than 3 L(sun). All sources with L > 25 L(sun) displayed some maser activity. Maser activity occupies about one-third the duration of the embedded state of evolution.
J. Condon, W. Cotton, E. Greisen, R. Perley, Q. Yin, and J. Broderick (VPI&SU) have observed almost 7 sr of sky for the 1.4 GHz NRAO VLA Sky Survey (NVSS), which will cover the 10 sr north of delta = - 40 deg. by the fourth quarter of 1996. The principal data products will be a set of 2326, 4 deg. x 4 deg. continuum map "cubes" with three planes containing Stokes I, Q, and U images, plus catalogues of discrete sources. Over 500 maps and their associated catalogs are already available electronically (see NRAO master home page on the WWW, http://www.nrao.edu.
Condon, Broderick, G. Seielstad (U. North Dakota), K. Douglas and P. Gregory (U. British Columbia) produced the final 4.86 GHz Green Bank sky maps covering 0 deg. < delta < +75 deg. by combining the epoch 1986 and 1987 maps. These images contain 75,000 radio sources stronger than S 18 mJy.
Condon, E. Anderson (UVa), and Broderick identified extragalactic sources in the IRAS Faint Source Catalog, Version 2 with radio sources stronger than 25 mJy at 4.85 GHz in the 6 sr declination band 0 deg. < delta < +75 deg. and with sources stronger than 80 mJy in the 3.4 sr area 0h < alpha < 20h, -40 deg. < delta < 0 deg. VLA maps were used to confirm a sample of 354 radio identifications. Most are normal or starburst galaxies, but statistically useful numbers of radio galaxies and quasars were found. Nearly all blazars become optically thin at 100 GHz. Nonthermal sources with steep far infrared/optical spectra and dust embedded sources visible only at far infrared and radio wavelengths must be very rare.
T. Cornwell and D. Briggs (NMIMT) developed a new form of visibility weighting, called robust weighting, that is intermediate in properties between uniform and natural weightings. Robust weighting allows an explicit tradeoff between resolution and noise level, allowing, for example, the construction of a dirty image with resolution close to that of the uniformly weighted image while losing little sensitivity compared to the naturally weighted image.
Briggs and Cornwell have worked on improving deconvolution of moderately resolved sources. They have developed a new deconvolution algorithm, the Non-Negative-Least-Squares algorithm, that allows very high dynamic range imaging of compact objects. The commonly used CLEAN algorithm produces artifacts that can be confused with calibration errors when applied to such objects. Cornwell, Kemball, and Benson used this algorithm to process VLBA data on DA193, thus producing an image with dynamic range of 120,000 (peak/rms), a record for VLBI that represents more than an order of magnitude improvement over the previous dynamic range record for VLBI.
Cornwell, in collaboration with K. Anantharamaiah (Raman Res. Inst.) and P. Gothoskar (NCRA), used the VLA to investigate anisotropic angular broadening of radio sources when observed close to the sun (2-16 solar radii). Axial ratios in the range 2-16 were seen, with the large values closest to the sun. The direction of elongation shows that the magnetic field lines are non-radial even at the largest distances measured. Comparison with earlier results suggests that the turbulence in the solar wind is higher during solar maximum.
W. D. Cotton has been heavily involved in the NRAO/VLA Sky Survey, redesigning image processing alogrithms, processing data, and constructing a catalog of sources from the images. With D. Tody (NOAO) and W. Pence (GSFC), he published a binary table extension to FITS which has been adopted as an official IAU standard. With F. Yusef-Zadah (Northwestern U. ), M. Wardle (U. Rochester), F. Melia (U. Alabama), and D. Roberts (U. Illinois), a study of the anisotropy in the angular broadening of Sgr A* was completed. A VLBI polarization study of 3C 138 was published with D. Dallacasa (NFRA), C. and R. Fanti (Bologna), R. Spencer (Jodrell Bank), R. Schilizzi (NFRA), and T. Foley (NFRA). He continues to participate in VLBI studies of a complete sample of radio galaxies with G. Giovannini (Bologna) and collaborators.
P. Diamond, with M. Miyoshi (Japan), J. Moran (CFA), J. Herrnstein (CFA), L. Greenhill (CFA), and N. Nakai (Nobeyama Radio Observatory), performed VLBA observa-tions of the H2O masers towards the galaxy NGC 4258. They made the surprising discovery that the masers lay in a Keplerian disk around the center of the galaxy. From the parameters of the disk, they were able to deduce the presence of a super-massive (4 x 10(7) M(sun)) black-hole. Monitoring observations continue.
With C. Lonsdale (Haystack), C. Lonsdale (Caltech/ IPAC), and C. Smith (UCSD), Diamond analyzed VLBI data on Arp220, producing evidence that OH megamasers (long thought to be produced in large molecular clouds along the line of sight to AGN) probably originate in the molecular torii close to the galactic nuclei themselves. Observations to confirm this hypothesis await correlation.
With J. Marcaide (U. Valencia), A. Alberdi (IAU-Andalucia), and E. Ros (U. Valencia), Diamond was involved in the first observations to demonstrate the shell-like radio structure of SN1993J. Further observations have demonstrated that the shell is expanding at a uniform rate, with a possible deceleration visible in September 1994. When combined with optical emission line data, they have determined the distance to M81 to be 3.8 0.8 Mpc.
With A. Kemball, W. Junor (New Mexico U.), A. Zensus, J. Benson, and V. Dhawan, Diamond produced the first VLBI synthesis images of SiO masers around M-giant stars. The observations produced the surprising discovery that the masers lay in coherent ring-like structures rather than the chaotic picture that was expected. The first images were modeled as ellipsoids of expanding, possibly decelerating, gas. Further observations of several sources have confirmed the ubiquity of such structures, raising the possibility of determining distances to the sources through maser proper motion studies.
Diamond, Kemball, and Cotton have also been involved in the development of techniques for polarization calibration of spectral line VLBI data, and Diamond (with K. Leppanen [Metsahovi Radio Res. Station, Finland] and A. Zensus) has worked on polarization calibration techniques at 22 GHz. Both techniques have demonstrated that much new science can be done with VLBI polarization observations.
V. Dhawan, J. Romney, J. Benson, K. Kellermann, R. C. Vermeulen, and R. C. Walker are deciphering the nucleus of 3C 84. The first 7 mm VLBA map of 3C 84 shows a jet with two bends within 0.5 pc, and a new component emerging in the inner 0.1 pc of the nucleus. New observations are planned for late 1995 to monitor proper motions in the jet and counter-jet, and the evolution of the nucleus at 100 arcsec resolution. A 6 wavelength campaign covering 13 cm to 7 mm is on to measure the opacity gradient and temporal changes in the disk obscuring the counter-jet, as well as the sub-luminal proper motions in the jet and possibly in the counter-jet.
Dhawan is working on millimeter/space Very Long Baseline Interferometry. The most compact sources exhibit a variety of quirky phenomena, including radio to X-ray variability on a time scale down to days, superliminal motion, and possibly evolution with redshift. This project combines VLA and VLBA snapshots of about 120 sources, to provide uniform and nearly simultaneous measurements of flux at 7 mm and 2 cm, and VLBI structure at 7 mm. The goals are to identify phase reference calibrators for the VLBA at 7 mm, and eventually, 3 mm wavelength; to identify sources with the highest brightness temperature, in time for the space VLBI missions, Radioastron and VSOP; and to increase, by almost an order of magnitude, the sources available for mapping with 100 uas resolution with ground-based millimeter VLBI.
The 7 mm data were correlated towards the end of 1994. Over 90 percent of the sources were detected and are in the process of being mapped; 3mm data on about 25 of the strongest sources has been taken on the new coordinated mm-VLBI Array (CMVA) and are awaiting correlation at Haystack. These will form the core sample for a space VLBI proposal.
Also important has been the gathering of experience with new capabilities at 7 mm, e.g., referenced pointing at the VLA, opacity correction, calibration, and fringe fitting on short coherence times for the VLBA.
Superluminal motion, familiar in distant quasars and active nuclei, has been discovered in the galactic hard X-ray source GRS1915, just 12 kpc away (F. Mirabel, L. Rodriguez, Nature, 371, 46 1994).
VLBI resolution is a few AU at the source, and two attempts were made in late 1994 to follow the evolution of the jets, within a day of a flux increase recorded at the Green Bank Interferometer (monitored by E. Waltman, NRL). Unfortunately, the bursts were neither long lived or strong, and only the weak core was detected.
The proposal, with L. Rodriguez (U. Mexico), W. M. Goss, and D. Frail, is pending, with the addition of the phased VLA to the VLBA in spectral-line mode, to follow the changing HI opacity against the moving continuum, when the source cooperates with a sufficiently strong outburst.
A VLBA calibrator survey project (with A. Beasley), a collaboration between NRAO and Jodrell, uses the full astrometric capabilities of the VLBA on 3000 sources, to provide an all-sky catalogue of phase reference calibrators, with 1-2 mas astrometric positions, for the VLBI community.
To date three of 14 epochs have been observed, with the structural information for each source available at 2.2 and 8.4 GHz for one epoch, while the astrometric position accuracy achieved is typically 1 mas.
Dhawan is collaborating with P. Diamond and A. Kemball on a long-term VLBA project on the dynamics of the of stellar wind close to the star, as probed by SiO maser emission at 7 mm. The emission is well ordered, tangentially amplified, and located in the extended atmosphere at 2-4 stellar radii (~50 mas), where the dynamics are complex and dust-grain formation occurs. There is the potential for astrometric positions to tie the radio and optical reference frames. More epochs are scheduled in 1995.
D. Emerson, with A. Rogers and S. Doeleman (Haystack Obs), M. Wright, G. Bowers, and D. Backer (UC Berkeley), S. Padin and J. Phillips (Caltech), L. Greenhill and J. Moran (CFA), and K. Kellermann, recently completed a study of small scale structure and position of Sgr A* from VLBI at 3 mm wavelength. Some observing and data analysis algorithm development has also been published or is in press.
J. R. Fisher and G. Langston searched for highly redshifted HI in several gravitational lens objects with negative results. Fisher and D. Westpfahl (NMIMT) are finishing a theoretical paper on turbulent viscosity in dwarf galaxies and its implication for the dark matter problem. Fisher will submit a paper on the fundamental limits and design parameters for phased array feeds. This is the first step in a project to develop fully sampled multi-beam arrays with the ability to correct for telescope aberrations.
D. Frail, N. Kassim (NRL), T. Cornwell, and W. M. Goss have used new deconvolution methods to do a deep search for a radio continuum shell around the Crab nebula. Such a shell is predicted from models of Type II supernova, and its detection would solve the mystery of the missing mass in the Crab. Frail and S. Kulkarni (Caltech) made extensive observations looking for a radio counterpart of the March 1, 1994 gamma-ray burster. Daily observations were made from 3 to 15 days after the initial gamma-ray burster as well as 25, 50, and 100 days. No time variable radio source was detected to a 5 sigma limit of 3.5 mJy. Frail, Goss, and J. Whiteoak (Sydney) have imaged the fields of three young pulsars and found supernova remnants in association with them. Together with other associations found with Kulkarni and those in the literature, important conclusions can be reached concerning the lifetime of supernova remnants and the distribution of pulsar velocities at birth. Work continues with J. Weisberg (Carlton College) and others to measure HI absorption toward bright pulsars with the goal of providing fundamental distances for use in distance models for all pulsars. In a related study, Frail, Weisberg, J. Cordes (Cornell), and C. Mathers (Carleton College) monitored seven pulsars over three years, looking for time variations in the HI absorption spectra as the pulsar moved through the interstellar medium. These observations provide a way to study cold neutral gas on spatial scales of tens of AU. Working with Kulkarni, Frail postulated that the galactic supernova remnant G10.0-0.3 was associated with the soft gamma ray repeater SGR1806-20. The subsequent confirmation of this hypothesis has lead to progress in our understanding the origin and nature of these objects. Scattering measurements, using angular broadening, of OH/IR stars in the Galactic center were continued with H. Van Langevelde (Leiden), Cordes, and P. Diamond. Using the VLA, scattering diameters at 1.6 GHz were measured and found to be among the largest scattering known in our Galaxy. Non-circular scattering disks were measured, evidence that the electron density variations are distributed anisotropically.
G. Fuller, in collaboration with E. Ladd (FCRAO), F. Adams (U. Michgan), R. Padman (MRAO), and P. Myers (CFA), have recently completed two detailed studies of the continuum and spectral line emission from the circumstellar material of the young embedded source L1551 IRS-5. The first study of the infrared, submillimeter continuum, and molecular line emission from a new Class 0 embedded young source has also been completed in collaboration with E. Lada (Maryland), and C. Masson and P. Myers (CFA). This work confirms the young age of this source and indicates that it is an example of an outflow driven by a jet from the star. A study of the molecular line emission from this source and another very young source in Taurus by Myers, P. Caselli, D. Mardones, M. Tafalla, and D. Wilner (CFA), R. Bachiller (IRAM), and Fuller has provided evidence that the molecular envelopes around these sources are gravitationally collapsing. A detailed study of the circumstellar disk associated with the young binary GW Ori has been published in collaboration with R. Mathieu and E. Jensen (U. Wisconsin), F. Adams (U. Michigan), and D. Koerner and A. Sargent (Caltech). Also, in collaboration with Mathieu and Jensen, a survey of submillimeter continuum emission from the disks around young binary stars in nearby molecular clouds has been undertaken. This survey has demonstrated that binary companions significantly alter the structure of the circumstellar disks around young stars. A study of the sulphur monoxide emission from protostellar outflows has also been completed in collaboration with L. Chernin and C. Masson (CFA).
F. Ghigo, in collaboration with P. Appleton (Iowa State) continued studies of ring galaxies. Radio continuum images made with the VLA are expected to allow them to identify regions of thermal and non-thermal emission in star forming regions for comparison with optical and infrared images. This should allow scenarios of density wave induced star formation to be tested. So far, a snapshot survey of 13 galaxies at L, C, and X bands was completed. Six of the galaxies have adequate flux to warrant further study with more detailed mapping. Subsequent mapping was done on Arp 10 and VII Zw 466, for which there are good optical and infrared images for comparison. Analysis of the VLA maps is continuing.
Studies of Cygnus X-3, using monitoring data from the Green Bank Interferometer (GBI), have been done in collaboration with E. Waltman, R. Fiedler, and K. Johnston (NRL), H. Wantanabe, S. Kitamoto, S. Miyamoto (Osaka U.). It was discovered that major radio outbursts are preceded by several days of unusually low quiescent activity (< 30 mJy). The GBI data was compared with X-ray data by Watanabe, Kitamoto, and Miyamoto. This showed that the active radio phase corresponded to increased X-ray flux and increased X-ray absorption. Increased mass loss from a WR-type secondary onto an accretion disk could explain these correlations. Initially, the increased accretion could quench the radio emission for a few days prior to a major outburst. Analysis of the February 1994 outburst sequence showed that successive flares display progressively less opacity, an effect seen in previous major outbursts.
M. Gordon re-examined the accuracy with which astronomers can estimate column densities of interstellar dust from the radiation of heated dust grains. This widely used technique relies principally on assumptions that dust grains and gas are uniformly mixed on size scales of star forming regions and larger and that the emissivity of dust grains as a function of temperature is everywhere well-known. His study (in press A&A 1995) suggests that this technique may yield gas column densities in dense star forming regions accurate within a factor of 20, comparable to accuracies intrinsic to analyses using emission from interstellar molecules in diffuse and translucent regions.
D. Hogg, with collaborators H. Kobulnicky and J. Dickey (Minnesota), A. Sargent (Caltech), and P. Conti (Colorado), have mapped the distribution of the atomic and molecular gas in the Wolf-Rayet starburst galaxy Heinze 2-10. The peak of the molecular gas distribution is offset by about 100 pc from the optical peak, which contains some 400 Wolf-Rayet stars. The optical knots have masses comparable with those of globular clusters and comprise up to half of the dynamical mass in the inner region. It appears that we are viewing a moderately advanced merger between two dwarf galaxies.
J. Bregman (Michigan), D. Hogg, and M. Roberts completed a study of the X-ray galaxy NGC 1291. This SO galaxy shows a striking anticorrelation between the hot, X-ray emitting gas which is confined to the optical bulge and the atomic hydrogen which was known to lie in a ring outside the nucleus. Modeling of the X-ray spectrum requires a component of temperature seven million degrees, similar to what is expected from the velocity dispersion of the bulge stars, and a second component whose temperature is greater by a factor of two, and whose origin remains unclear. The morphology is consistent with the suggestion that the hot gas is a bulge phenomenon and the atomic component is a disk phenomenon.
P. Jewell, working with J. M. Hollis and F. Lovas (GSFC), detected interstellar CH2 (methylene). CH2 is a structural constituent of a number of important interstellar molecules, including formaldehyde, formic acid, acetaldehyde, methyl formate, and formamide. In addition, it is formed in one branch of the dissociative recombination reaction of CH3+ that also produces the common interstellar molecule CH. Although the molecule is important to interstellar chemistry, it is difficult to detect. The primary transition frequency accessible from the ground arises from an energy level 215 K above ground state and is split into several fine structure and hyperfine components. Furthermore, the transitions are between 68 and 70 GHz where atmospheric absorption is significant. Nonetheless, using the new generation of SIS receiver at the 12 Meter Telescope, several hyperfine components were detected in the Orion and W51 molecular clouds, securing the identification. The branching ratios of the reactions leading to CH2 and CH3 were found to be 0.9 and 0.1, respectively.
Working with P. Gensheimer and L. Snyder (Illinois), Jewell completed work on the study of a number of circumstellar envelopes using J = 3-2 (348 GHz) CO emission. Line profiles were modeled to derive the stellar mass loss rates. Of particular note, the envelope of the M supergiant star alpha Ori (Betelgeuse) was observed in three CO transitions and mapped in the J = 2-1 transition. The alpha Ori emission was modeled using a detailed radiative transfer calculation and also using the simple analytical method of line intensity as a function of energy in the optically thin limit. The two methods agree extremely well and suggest a mass loss rate of ~7x10(-6) M(sun) yr(-1).
K. Kellermann and collaborators A. Zensus, M. Cohen (Caltech), and R. Vermeulen (Caltech) are using the VLBA at 15 GHz to study the morphology of compact radio sources. In this first phase, images of about 120 of the brightest AGN and quasars are being made with an angular resolution better than 1 mas.
In other VLBI and VLBA studies, Kellermann, Zensus, and Cohen have observed the nucleus of the nearby radio galaxy NGC 5128 (Cen A) at 7 mm and obtained an unprecedented linear resolution for any AGN to give a measured diameter of 0.01 parsecs for the central radio core. Observations at 7 mm of Sgr A*, together with D. Backer (Berkeley) et al. at 7 mm and 3 mm, indicate that interstellar scattering is important even at these short wavelengths and indicate a linear size less than 1 AU with no evidence of any symmetry. With V. Dhawan and J. Romney, VLBA images were obtained of the radio galaxy NGC 1275 (3C 84) with an angular resolution up to 0.3 mas, which show a bright resolved core and two sided extended jet-like structure. 3C 390.3 was studied with the VLA, the VLBA, and global VLBI observations together with W. Alef and E. Preuss (MPIfR, Bonn) at 1.3, 6, and 18 cm. The 6 cm observations continue to show evidence for superluminal motion. Further VLA and VLBA observations of high redshift quasars were made with L. Gurvits (JIVE) as part of their program to explore the use of parsec scale radio sources as cosmological probes.
Kellermann, E. Fomalont, E. Richards (UVa), B. Partridge (Haverford), and R. Windhorst (Arizona State) have made the deepest radio source survey yet made, reaching an rms noise level of only 1.5 mJy with combined D and C configuration VLA observations. Approximately 28 sources brighter than 9 mJy have been catalogued in a region about 2 arcminutes in extent, coincident with an HST MDS image. All but one of the brightest 16 radio sources have optical counterparts, mostly with faint disk galaxies, many of which occur in pairs or groups or appear distorted. Observed radio and optical luminosities suggest that these are low luminosity radio sources with the radio emission confined to the galactic disk with active star forming regions.
Kellermann, P. Shaver (ESO) and J. Wall (RGO) have almost completed a radio/optical study of a complete sample of 896 flat spectrum quasars. Identifications, which are almost complete, show no evidence of any radio quasars with redshifts greater than 5, indicating that the previously reported cutoff in the space density of quasars at high-z is real and is not due to absorption by intervening dust. One result of this study was the identification of PKS 1251-407 with a QSO at a redshift of 4.46, making this the most distant known radio source. VLBA observations of 1251-407 show that it is barely resolved with an angular size about 1 milliarcseconds.
A. Kemball, in collaboration with P. Diamond and W. Cotton (Kemball, Diamond, and Cotton, 1995, A&A Supp. Series 110, 383), made an investigation of data reduction techniques for spectral line polarization VLBI observations. VLBA observations of SiO maser emission towards the late-type stars TX Cam and U Her were made in collaboration with Diamond, W. Junor, A. Zensus, J. Benson, and V. Dhawan (Diamond et al, 1994, Astrophysical Journal Letters, 430, L61).
An intra-day variability VLBI monitoring project is being carried out by Kemball and collaborators G. Taylor, R. Vermeulen, T. Pearson, A. Readhead (Caltech), and F. Banagnoff (UCLA). Kemball, R. Porcas and A. Patnaik (MPIfR) are carrying out observations of the gravitational lens 0218+357.
L. Kogan has completed the theory and application into the Astronomical Image Processing System (AIPS) of the digital correction for an FX correlator.
G. Langston has continued the search for gravitational lens systems. This research has been done with collaborators at MIT and has resulted in discovery of another system, MG 1549+307. Langston is designing systems for control of the Orbiting VLBI tracking station in Green Bank.
H. Liszt and R. Lucas (IRAM) studied the centimeter and millimeter wave specta near 0-velocity toward a small sample (typically 12 to 30) of compact extragalactic continuum sources. These sources are occulted by diffuse/translucent (AV 2 mag) clouds in the Galaxy. In Lucas and Liszt 1993, A&A, 276, L33 and 1994, A&A, 280, L5, they showed that simple molecules like CO, HCO+, HCN, C2H, etc., can form with dark cloud relative abundances even when AV < 1, and that molecular absorption lines are more common, complex, and broader than those of CO emission. The idea that the gas is diffuse is reinforced by Liszt and Lucas (ApJ, 431, L131, 1994), who showed that HCO+ emission is visible in the diffuse gas heavily studied around Zeta Oph.
In a survey of H2CO emission (Liszt and Lucas, 1995, A&A, in press), Liszt and Lucas showed that formaldehyde turns on abruptly when N(HCO+) > 1-2x10(12) cm(-2); borrowing from work in progress, they also show that 13CO turns on even more rapidly than the H2CO. They then conducted a survey of 30 lines of sight in HCO+ absorption (Lucas and Liszt, 1995, A&A, in press). In contrast to CO emission, HCO+ absorption is present along nearly every line of sight at b < 15-20 degrees. OH and HCO+ absorption lines vary in lock-step, as expected from conventional diffuse cloud chemistry, but there is far too much of the HCO+.
Liszt and Lucas have also addressed the problem of derivation of molecular abundances toward Cas A from absorption lines observed at millimeter wavelengths with single dishes. Having seen some species (HCO+ and HCN) in absorption, but CS (J = 2-1) in emission, they concluded that the background source brightness is not high enough to provide reliable absorption gas columns, and that a variety of millimeter-wave studies in the literature should be ignored (Liszt and Lucas, A&A, 294, 811, 1995).
With W. B. Burton (Leiden), Liszt mapped gas at -190 km/s toward and around Sgr A, concluding (Liszt and Burton, ApJ, 407, L25, 1993) that it is a foreground feature. In a more detailed study (Liszt and Burton, ApJS, 98, 679, 1995), they found no association at small scales (15") mapped with the JCMT. This latter work also shows that Sgr A West is embedded in neutral gas at +50 km/s and highlights the non-circular motion in the circumnuclear gas disk.
In collaboration with Robert Spiker (UVa), Liszt mapped the GMC associated with the Sgr C complex (Liszt and Spiker, ApJS, 98, 259, 1995). This cloud sits at the base of non-thermal filament, perhaps interruping it. The PDR recently mapped in the FIR arises where the HII region interacts with one edge of the cloud.
Liszt derived the mean pressure in galactic plane GMC's from a small survey of 13CO J = 2-1 emission from the inner galaxy (Liszt, ApJ 411, 720, 1993) and discovered that nearly every cloud seen in 13CO can now be detected in many common molecules (HCO+, HCN, C2H, CS; Liszt, ApJ, 442, 163, 1995). The conditions implied by these observations are not extreme; the emission arises not from hot, dense cores but from ambient GMC material. Nonetheless, the matter must be clumped within the beam because the implied density (a few thousand per cc) is far higher than the mean density of GMC's.
Liszt and J. Dickey (Minnesota) (1995, AJ, in press) describe HI in the disk of two Seyfert galaxies (NGC 4051 and NGC 6814) using VLA data. In each case the HI is doughnut shaped, with CO filling in the hole. There is no sign that the AGN has influenced the galaxian disk on the kiloparsec scales mapped.
F. J. Lockman and collaborator B. Savage (Wisconsin) have completed a survey of the Galactic HI emission toward the Hubble Space Telescope Key Project Quasars. The HI data allow the observations of quasars to be corrected for Galactic reddening and extinction and allow a more accurate calibration of the velocity scale of ultraviolet spectra. With collaborators from Wisconsin and GSFC, Lockman completed a detailed correlation of the soft X-ray flux against the HI column density in Ursa Major. The results are perplexing in that a general anti-correlation exists, but deviations from it suggest that there is substantial angular structure in the hot gas of the Galactic halo. Lockman, with M. Elvis and C. Fassnacht (CFA), produced an HI map of the North Ecliptic Pole (NEP) region. The NEP has special significance since it is often a heavily observed direction for space observatories. These data will be useful in evaluating the intrinsic X-ray spectrum of extragalactic objects. A recent very deep search for faint Galactic high velocity HI clouds (with Savage and E. Murphy [UVa]) yielded many new detections, yet every one is associated with a previously known high velocity cloud complex. This implies that there is no independent population of small clouds raining down onto the Galaxy - the low NH clouds are merely the outlying regions of the large complexes.
R. Maddalena and J. Williams (Berkeley) investigated the evolutionary state of the unusual molecular cloud discovered by Maddalena and P. Thaddeus (1985). A recently mapped HI cloud, which is probably photo-dissociated molecular material, apparently connects the molecular cloud with two regions of OB star formation, each about 50 pc from the molecular cloud. The HI observations do not contradict the original suggestion that the molecular cloud is young and has not yet formed massive stars. However, the observations add weight to the possibility that the molecular cloud is an old remnant of a star forming complex. The reason why such a large molecular cloud has no closely associated star formation remains an enigma.
I. de Pater, C. Heiles (Berkeley), Maddalena, and about 25 others have combined their observations with eleven telescopes of the radio continuum emission from Jupiter prior to and after the impact of comet Shoemaker-Levy/SL9. The non-thermal emission from Jupiter at all frequencies increased dramatically during the impacts (e.g., ~10% at 70-90 cm and ~45 % at 6 cm). The spectrum also hardened considerably during the impacts. In the three months that followed the impacts, the spectrum continued to harden and the flux density began to subside. The changes in radio emission could have been the result of an energization or spatial redistribution of the emitting particles.
M. McKinnon's earlier analysis of polar cap geometry showed that polar caps are elliptical in cross section and quantified how the geometry evolved as the pulsar ages. Polarization observations of known mode changing pulsars were made with the VLA, and the resulting data were used to test different explanations for the mode changing phenomenon. Before these pulsar observations could be conducted at the VLA, a new signal processor had to be constructed for the high time resolution required by the observations. McKinnon was responsible for the integration of the filters, quadrature phase-shift networks, detectors, and data acqusition computer which comprised the new signal processor. He also developed a polariation calibration procedure to compensate for the instrumentatl effects which were present in the polarization data.
McKinnon made observations at millimeter wavelengths to measure the Sunyaev-Zeldovich effect in radio jet lobes and the atmospheric opacity due to water vapor. The Sunyaev-Zeldovich effect measurements were made with the 12 Meter Telescope at 90 GHz and were used to set upper limits on the thermal pressures confining the jets in selected extragalactic radio sources. The 225 GHz atmospheric opacity measurements, produced by radiometer tipping scans, were part of the NRAO Millimeter Array site survey. The survey monitored atmospheric phase stability in addition to the opacity of potential array sites. With F. Owen, he developed and implemented the radiometer observing techniques and data analysis procedure for both the phase stability and opacity measurements.
P. Napier is carrying out research in the area of instrumentation and antennas for radio telescopes. He is studying how focal plane array feed systems can be used to correct for errors in large reflector telescopes and is analyzing novel antenna designs suitable for the Millimeter Array. He is also studying enhancements to the feed systems on the VLA antennas to extend the frequency range of this instrument.
D. Nice, in collaboration with R. Sayer and J. Taylor (Princeton), developed a pulsar search system for use with the 140 Foot Telescope. The system has been used in searches towards gamma ray sources and runaway OB stars (in collaboration with V. Kaspi [Caltech]) as well as for a survey of the northern celestial sphere for millisecond pulsars. Ten new pulsars have been discovered, including PSR J1518+4904, a pulsar with a 40 ms in a moderately eccentric, nine day orbit. Long term observations of this source have detected relativistic precession of its orbit, from which a total system mass of 2.6 solar masses was derived, strongly suggesting the pulsar's companion is also a neutron star.
Nice is undertaking long term monitoring of pulsars in two separate projects. Several dozen pulsars are observed bi-monthly with the 140 Foot Telescope, in collaboration with Z. Arzoumanian (Cornell). Thirty pulsars are observed nearly daily with telescopes 85-3, in collaboration with D. Stinebring (Oberlin), V. Kaspi (Caltech), D. Backer (Berkeley), and R. Foster (NRL). Pulsar rotation ephemerides are made available from these projects for use by high energy observatories such as the Compton Gamma Ray Observatory.
In collaboration with J. Taylor and F. Camilo (Jodrell Bank), Nice has also made pulsar observations with the 300 meter Arecibo telescope. Recent results include measurement of the fluxes, spin-down rates, and dispersion measures of 29 newly discovered slow pulsars; long term monitoring of several newly discovered millisecond pulsars; and measurement of millisecond pulsar proper motions and analysis of the kinematics of the milllisecond pulsar population.
F. Owen, G. Morrison (UNM), and K. Dwarakanath (Raman Institute) are in the process of studying radio galaxies found over supercluster scales associated with very rich clusters. Results from VLA surveys of Abell 2125 and Abell 2645 show that these two clusters of the same richness (4) and distance (z = 0.25) have very different numbers of radio galaxies. Abell 2125 has a much larger blue fraction and also a much larger number of radio galaxies spread over at least 5 Mpc in projection on the sky. Owen and Morrison are also studying a sample of very rich clusters both nearer by (z < 0.15) and more distant z > 0.35 to look for an epoch dependence in a number of radio galaxies in similar rich clusters.
Owen, J. Burns (NMSU), M. Ledlow (NMSU), and W. Voges (MPE) continue to study the X-ray properties of radio galaxies. In a study with C. Smith (NMIMT), they find that nearby radio galaxies not catalogued to be in rich clusters still have a cluster-like X-ray source extending beyond the radio structure in about 85 percent of the cases studied. The median X-ray luminosity of these sources is consistent with clusters of about 30 percent lower richness than Abell's richness Class 0. Owen, J. Eilek (NMIMT), and F. Zhou (NMIMT) are studying the relationship of the X-ray and radio structures of Abell 2199. In the inner 30 kpc of the cluster, both the X-ray and radio sources are complex and seem to be related, suggesting the interaction of two. With C. Loken (NMSU) and Eilek, Owen is studying the ends of FR I radio sources at 90 cm with the VLA. They hope to distinguish supersonic, transonic, and subsonic models for these sources. Owen and Ledlow are continuing to study the optical properties of galaxies near the FR I/II break. It is hoped that the CCD imaging of a sample of 3CR and B2 radio galaxies will be completed this year. Together with Q. Wang (NMIMT) and Eilek, Owen is continuing their study of the radio and X-ray properties of Abell 400 and Abell 2634.
S.-K. Pan, R. Bradley, and J. Hesler (UVa) have successfully developed a submillimeter (475-700 GHz) planar Schottky diode waveguide mixer. This mixer uses no mechanical tuners, has a very broad RF bandwidth, and requires very moderate LO power (~0.5 mW) to operate. Therefore, it is an ideal building block of a MMA site survey submillimeter radiometer. At room temperature, the best mixer conversion loss and noise temperature measured to date are Lc = 10 dB (DSB) and TM = 3090 K (DSB) at 585 GHz and Lc = 10.2 dB (DSB) and TM = 3610 K (DSB) at 690 GHz.
Pan and A. Kerr have developed a new 270-300 GHz mixer. The mixer uses a 230 GHz device fabricated by UVa, has two adjustable tuners and, therefore, can be operated in either double or single sideband mode. Typical values of mixer conversion loss and mixer noise temperature are Lc = 2.5 dB (DSB) and TM = 23 K (DSB) at 270 GHz and Lc = 7.0 dB (DCB) and TM = 68 K (DSB) at 300 GHz. The mixer performance can be further improved by a factor of two to three by using a properly tuned SIS device.
M. Pospieszalski has performed evaluations of d.c., signal and noise properties of AlGaAs/InGaAs/InP heterostructure field-effect transistors (InP HFET's). Devices from all leading laboratories (Hughes, TRW, Martin/Lockheed) have been evaluated. Close collaboration with Hughes Research Labs was established which led to the development of devices, amplifiers, and receiver with world record noise performance from 26 to 80 GHz. These receivers match or exceed the specifications for HFET receivers for the MMA (MMA Design and Development Plan, Sept. 1992). Technical and scientific support was provided for the world's first integration of an SIS mixer with a wideband HFET amplifier by S. Padin et al. (OVRO/JPL). Also, in a collaborative effort with a TRW team, a number of millimeter wave monolithic microwave amplifiers were evaluated.
M. Roberts, collaborating with J. Bregman (Michigan), D. Hogg, and A. Sandage (OCI), conducted a variety of radio, optical, UV, and X-ray studies of galaxies, all complementary.
The global properties of several thousand galaxies were analyzed in a collaborative review with M. Haynes (Cornell). A number of unexpected results as a function of type were found. As an example, galaxies of types S0-Ir have, in the mean, the same mass-to-luminosity ratio. This ratio is distance dependent and yet shows essentially no Malmquist bias. For the catalogue galaxies studied here the mass carefully adjusts the light output of a galaxy.
The strong lever arm of UV light (measured from above our atmosphere in a shuttle experiment) was used to study the hot stellar population of galaxies and of globular clusters. In both instances the confusing signature-red light of the more common cool stars was suppressed. Valuable insight into initial mass functions, interstellar matter, spiral arm-interarm contrast, and the blue stellar population of globulars was obtained.
R. Sramek, K. Weiler (NRL), S. Van Dyk (UC Berkeley), M. Rupen, and N. Panagia (STScI) have continued their study of radio supernovae. Two bright and nearby supernovae have been discovered and extensively monitored in the last three years. SN1993J is a bright Type II supernova which appeared in M81 in March 1993. Radio emission was detected within a few days of the optical discovery and over three hundred observations were made at the VLA at several wavelengths during the first year. The rising portion of the light curve is well defined, and SN1993J will be the best studied radio supernova known. This large number of early and high quality observations shows that the normal form of the shock/circumstellar interaction model does not adequately fit the data. The circumstellar density profile is flatter than that expected from a constant mass-loss rate, constant velocity stellar wind. This can be interpreted as a decrease in the pre-SN mass-loss rate just prior to the SN explosion. The supernova SN1994I in M51 is the second recent and nearby supernova. This Type I b/c supernovae is now being monitored through its decay phase.
B. Turner continued the study of physical and chemical conditions in small translucent molecule clouds (both high latitude cirrus cores and low latitude Clemens-Barvainis objects). Physically self-consistent models (hydrostatic equilibrium polytropes) have been fit to extensive observations of C18O and 13CO, which include the effects of external radiation fields and provide the radial distributions of all physical quantities including electrons. Once these conditions are known, the chemistry of other molecules can be derived. Many molecules are detectable in these objects (2-4 mag extinction) and extensive observations in more than one line have been made in an ensemble of ~35 objects of the species H2CO, NH3, HCO+, N2H+, SO, SO2, SO+, HCS+, CS, C2S, HCN, HNC, C3H2, HC3N, SiO. The models have now been to all of these species up to CS in the list. Ion-molecule chemistry is found to describe all of the species except H2CO, which requires photocatalysis on grains. All species exhibit a distinct transition from diffuse cloud to dense cloud chemistry, that is, their fractional abundances increase sharply between ~0.6 magn and 3 or 4 magn extinction. HCO+ is shown to be a precursor to CO for extinctions 0.6 magn, while CO becomes the precursor at higher extinctions. Ion-molecule chemistry is shown to describe sulfur chemistry through the entire diffuse-dense cloud range, hence the traiditional shock chemistry is not required. With the possible exception of SO+ (for which one or two reaction rates may need revision, the detailed variation of abundance with extinction is well predicted for all species by the models and by ion-molecule processes. That the dense cloud abundances are reached for all analysed species at 5 magn or less suggests UV radiation penetrates dense clouds efficiently, presumably because they are clumped.
Turner and collaborators T. Steimle (Arizona State) and L. Meerts (Katholiek U., Netherlands) detected the first sodium bearing molecule, sodium cyanide (NaCN), in the outer envelope of IRC10216. The fraction of cosmic Na in the form of NaCN is probably several percent, so that significant amounts of Na escape incorporation into grains, unlike the fraction (<0.002) that applies for Si. Together with MgNC, NaCN suggests that cyanides are formed efficiently by ion-molecule chemistry of refractory elements in IRC10216, expected because of the high HCN abundance and the ionized form of the refractory atoms.
In collaboration with L. Avery (Herzberg) and T. Amano (Ibaraki University, Japan), Turner has completed a highly sensitive spectral survey of the 2 mm band (132-170 GHz) with the 12 Meter Telescope. The survey is of seven objects: SgrB2OH, SgrB2N, W51M, Ori(KL), Ori-S, W3(IRS5), and IRC10216. Five-sigma noise level is typically 30 mK, spectral resolution is 768 kHz (1.5 km/s at mid-band), and beam size (FWHP) is 42" at midband. The survey was conducted over two years and required 21 days of observing time. It used the dual-channel SIS junction receiver and the hybrid spectrometer at 600 MHz bandwidth. It is the most sensitive millimeter wavelength survey yet made and comprises the most objects. The density of lines in most of the objects is unprecedented, and major programs will be required to identify most of them, and to extract the differences in chemistry and excitation that characterize each object.
J. Uson has continued the search for proto-clusters of galaxies through the observation of their (redshifted) 21 cm emission at a frequency in the range of 305 MHz to 335 MHz. The survey contains a dozen fields at redshifts between 3.1 and 3.7. The VLA P-band system has been pushed to a sensitivity of better than 0.8 mJy/synthesized-beam for spectral channels of width 100 kHz. This sensitivity corresponds to a one-sigma value of about 4 x 10(12) M(sun), somewhat dependent on the values of key cosmological parameters. At this level, systematic errors are important. Large errors in the system delay files have recently been found and corrected. They gave rise to non-linearities in the calibration process which affected the statistical properties of the noise in the images and have been found to have produced a spurious detection of emission at z = 3.4 which had been tentatively reported (Uson, Bagri and Cornwell, Phys. Rev. Lett. 67, 3328, 1991). The data on all the fields are now being recalibrated and reanalyzed. Absorption at a redshift of Z = 3.4 is seen towards the radio galaxy 0902+343, which is at about the same redshift. Higher resolution data have confirmed this absorption and are being used in order to study the degree of fragmentation and structure of this system. The data suggest a young cluster of galaxies forming around the radio galaxy, perhaps the progenitor of a cD galaxy.
Uson, S. Boughn (Haverford) and J. Kuhn (NOAO-NSO and Michigan State) have continued their measurements of diffuse B, V, and R band light in dense clusters of galaxies using multiple overlapping CCD exposures and a tesselating algorithm to make mosaic images with good fidelity, limited thus far only by photon noise. they have developed an analysis technique that uses pixel intensity histograms to determine faint light levels (ApJ, 369, 46, 1991). Data taken in different observing runs produce mosaic images that agree (in a statistical sense) at the level of 5 parts in 10(5). They have also obtained data on several other cD and B clusters. The results so far are consistent with halos of diffuse light being present only in the cD clusters but not in the binary ones. They have obtained as well higher resolution data (~ 1") in order to better discriminate a truly diffuse component from a blend of faint cluster galaxies.
Uson and Boughn have concluded their search for a stellar component to the dark matter in the Milky Way, other galaxies and dense clusters of galaxies through the search for a near-IR excess in multiband photometry over large regions. They have found that no more than one percent of the dark matter can be in the form of the faintest main-sequence (M8) stars. Their results constrain the possible shape of the mass component that has been invoked to explain the announced microlensing events, whose mass-function must be sharply peaked just below 0.1 M(sun), the mass marking the onset of hydrogen burning (Phys. Rev. Lett. ,74, 216, 1995; with the correct figures in Phys. Rev. Lett. ,74, 3304, 1995).
The determination of the abundance of 3He in the interstellar medium has important implications for theories of cosmology, stellar evolution and the chemical evolution of the Galaxy. Previous determinations of the 3He abundance have been reported by Bania, Rood, and Wilson (ApJ,. 323, 30, 1987) who claimed nine detections based on observations of the hyperfine transition of 3He+ at 8665.65 MHz. They determined line-to-continuum ratios in the range 10(-3) to 10(-4) and the derived abundance ratios 3He to H are in the range of 1 to 15 x10(-5) by number. Uson has used the VLA in an attempt to either confirm or reject the strongest of these detections by observing W43 with enough spatial resolution to study as well the clumpiness of the region. Whereas all the hydrogen signal observed with the single dishes by Rood and collaborators was detected, no signal was detected from 3He+ even though the VLA spectral line system was pushed to a dynamic range of 20000:1. The upper limit is compatible with the single-dish signal if one assumes that the 3He is smoothly distributed. However, this changes the derived ratio of 3He to H which has to be at least one order of magnitude larger than what Rood and collaborators derived, thus changing the cosmological interpretation of the single-dish measurements.
G. Van Moorsel and S. Joersaeter (Stockholm Obs) completed the study of NGC 1365, in which they investigate the dynamics of this large barred spiral with high spatial resolution. A paper has been submitted for publication. In July, 1994, Van Moorsel worked with colloborators (L. Sparke [Wisconsin] and U. Schwarz [Groningen]) on the NGC 3718 project in Groningen. During this period they reduced and analyzed the NGC 3718 data. NGC 3718 is unique in that the plane of the disk is warped to a high degree, allowing them to view widely different parts of the disk in the line of sight. The data allows them to study this galaxy with greatly improved spatial and velocity resolution compared to previous data, and with higher sensitivity.
R. C. Walker, J. Romney, and J. Benson found a counterjet in 3C 84 (NGC 1275) in VLBA first science observations (ApJ 430, L45, 1944) at 8.4 GHz. Vermuelen, Readhead, and Backer (ApJ, 430, L41, 1994) independently found the counterjet in Network observations at 22 GHz. At 22 GHz, a simple relativistic jet model explains much of the appearance and evolution of the source. At lower frequencies, the counterjet is much weaker than the model would suggest. It appears that the counterjet is free-free absorbed, perhaps in the accretion disk. If so, VLBA observations of the source can give strong constraints on the physical conditions in the accretion region.
E. Wollack is making a determination of the cosmic microwave background anisotropy angular power spectrum and polarization properties at intermediate angular scales, in collaboration with the Princeton University gravitational research group (D. Wilkinson, L. Page, M. Devlin, N. Jarosik, and B. Netterfield). Wollack's current laboratory efforts are directed toward improving the noise performance and baseline stability of continuum receivers. The results of these efforts will be used in a multi-purpose Q-band array for the Green Bank Telescope. A two channel prototype receiver will be fabricated and characterized over the course of the next year.
H. A. Wootten and collaborators J. Mangum (Arizona) and G. Fuller continue a project to obtain high resolution formaldehyde and ammonia maps toward the youngest protostars. Ten objects have now been surveyed at high resollution, demonstrating that formaldehyde emission probes physical conditions on the smallest scales, while ammonia ceases to be an effective probe at distances closer than about 1000 AU from protostars. A study of water maser emission, using the Haystack telescope, drew to a close. The study showed that water maser emission occurs during about one-third of a young star's embedded evolutionary phases, and fixed CO mass loss rates, 6 cm radio luminosity, and IRAS luminosity above which water maser emission is always detected.
J. Wrobel , P. Papadopoulos, E. Seaquist (U. Toronto), and L. Binette (ESO) report (ApJ, 446, 150, 1995) the principal results of a VLA search for extended 6cm continuum emission from 12 radio quiet QSOs. They find evidence for extended emission in QSO 1821+643 and the tentative detection of similar structure in three others. They suspect that a low-power AGN resides in the center of the QSO 1821+643 host galaxy, ejecting plasma into a dust and gas rich environment and creating extended (~110 kpc) low brightness radio emission. However, star formation may also be a contributor to the radio as well as to the far infrared luminosity of this QSO.
The BL Lac object Mrk501 belongs to a class of core dominated radio sources with orthogonal misalignments between the position angles of their radio jets on parsec and kiloparsec scales. Wrobel and J. Conway (Onsala) present new VLBA and MERLIN images of Mrk501, and analyse them in terms of the geometrical models for such orthogonal jets proposed by Conway and Murphy (1993). Recent hydrodynamical studies by Hardee, Cooper, and Clarke (1994) of the effects of driving helical distortions into a jet by motion of the central engine provide a physical mechanism for generating helically distorted jets with just the properties required by the geometrical model. Application of the hydrodynamical models to Mrk501 shows that driving periods of a few times 10(4) yr are required. Both this period and the jet morphology are consistent with the helical structure being excited by the orbital motion of a binary black hole system. This result, combined with the fact that orthogonally misaligned radio sources comprise approximately half of the total population of core dominated sources, suggests that binary black holes may exist within 50 percent of core dominated sources. For Mrk501, analysis of the observed jet path, of the VLBA intensity distribution, and of synchrotron/inverse Compton models for the integrated radio to gamma-ray spectrum provide strong constraints on the physical properties of the jet. Most importantly, the Lorentz factor of the bulk flow must satisfy gamma > 2.1, and the half opening angle of the helix cone xi < 12 deg. These properties and others derived for Mrk501 are consistent with this BL Lac object being a radio galaxy of Fanaroff and Riley (1974) class I (FR I), whose innermost VLBI regions are viewed at a small angle theta c < xi 3 to the line of sight. For the gamma and zeta limits mentioned above, the simplest relativistic beaming models, with their assumption of a single bulk flow velocity, are inconsistent with the presence of a putative counterjet feature seen on VLBA scales. Such difficulties are avoided by a model, such as that proposed by Laing (1993) for FR I radio galaxies, which invokes a slower moving shear layer on the surface of the jet.
Q. Yin has pursued three primary areas of research: supernova in starburst galaxies,VLA observations of a complete sample of 244 FIR/X-ray position coincidences, and the NRAO VLA Sky Survey. From 24 VLA observations of the compact radio source Mkn 297A (SN1982AA) at four wavelengths, lambda-lambda 20, 6, 3.6, and 2 cm during 1980 January to 1993 June, it was found that this was a single supernova. It still remains the highest peak luminosity known for supernova. Yin has calculated the mass-loss rate of the progenitor. The rate is quite high. This high mass-loss rate indicates that its progenitor was a high mass star, possibly a massive (13 M(sun)) red supergiant. Short period fluctuations in the observations are significant and are consistent with refractive interstellar scintillation. Yin, Z. Huang, T.Thuan, and R. Chevalier (UVa), and J. Condon have obtained an 8.4 GHz VLA A-array map of the starburst galaxy M82 with a resolution O."182. About 50 compact sources in the central region were detected. Comparison with previous observations shows that most source are declining in flux. The birth rate of new source was estimated. The compact radio sources obey a sigma (radio surface brightness)-D(diameter) relation which is remarkably similar to that followed by supernova remnants in the Galaxy and the Magellanic Clouds.
Yin, Huang, W. Saslaw (UVa), and D. Heeschen have observed the peculiar pair of galaxies NGC 3395 and NGC 3396 at four frequencies with the VLA. The two galaxies have a common striking feature - the radio emission is not centered on the optical but is much stronger in the half of each galaxy that is nearer its companion. This contradicts models in which tidal interactions cause gas to flow into the center of the galaxy and produce starbursts only there. A radio bridge is found connecting the two galaxies and following the intergalactic optical emission fairly closely. Condon, Yin, and Thuan have continued their research on a complete sample of 244 RIR/X-ray position coincidences. This represents the largest homogeneous sample of FIR-selected galaxies yet detected in X-rays. They have made optical identifications for 88 objects of the 244 FIR/X-ray positions.
Observations for 1.4 MHz NRAO VLA Sky Survey (NVSS) began in 1993 September and should cover the sky north delta = -40 percent before the end of 1996. The VLA maps will be a powerful tool for astronomical research because of their sensitivity and position accuracy. J. Condon, W. Cotton, E. Greisen, R. Perley, Q.Yin and J. Broderick (VPI&SU), have observed almost 7 sr of sky for 1.4 GHz NVSS. A number of maps are already available electronically, and the catalogues associated with these maps will be released soon.
A. Zensus led an effort to study the properties of parsec-scale jets found in compact active galactic nuclei, aiming at understanding the underlying physics of such sources and at testing predictions of detailed physical models. This work includes long-term monitoring of structural changes and spectral variations with VLBI, episodic VLBI imaging at mm-wavelengths, imaging of polarized parsec-scale emission, and theoretical modeling of the jet trajectories and jet emission properties.
With M. Cohen and S. Unwin (Caltech), Zensus published a paper (ApJ, 443,35, 199 ) on the jet of the quasar 3C 345, archetype of a core dominated superluminal radio source. They analyzed component motions using polynomial fits to the position offsets relative to the stationary core, in a form suitable for comparison with three-dimensional jet models. Apparent superluminal motion is present over the full jet length, and measured speeds increase monotonically with time or distance from the core, consistent with a jet of constant Lorentz factor (about 10) bending away from the line of sight near the core the jet ridge line, as traced by the superluminal features, is strongly curved. Trajectories of successive components differ substantially within 2 mas from the core. The outer component show monotonic curvature, whereas the inner components show repeated bends or wiggles. The three-dimensional path of an inner component was reconstructed and found to be consistent with modest intrinsic bends amplified by projection effects.
A model for the radio-through-X-ray emission from 3C 345 was developed which self-consistently combines the inhomogeneous jet model of Konigl with homogeneous spheres for the superluminal components. This (constant Lorentz factor) model accounts for the flat radio spectrum of the nucleus and the steep spectra of the superluminal components as well as for kinematic constraints from the proper motions. The results require a small opening angle (about 0.5 degrees) of the core, again amplified by projection. Inverse Compton emission from the nucleus is the likely origin of the observed soft X-ray flux, and it probably dominates over X-rays from the superluminal components. The model suggests that the jet axis lies within 5.4 degrees to the line of sight, consistent with the kinematic results. The kinematic luminosity of the parsec-scale jet is sufficient to power the outer radio lobe, but barely if the jet is light, which is taken as evidence in favor of a heavy, proton-dominated jet. Zensus recently extended the studies of this and other AGN, focussing on the jet's spectral properties (with A. Lobanov [Lebedev Phys. Inst]) and the polarized jet emission (with K. Leppanen [Metsahovi Radio Res. Station], and P. Diamond, NRAO). Leppanen, Zensus, and Diamond (AJ, 1995, in press) developed a novel calibration method for polarization-sensitive observations with the Very Long Baseline Array, particularly useful for observations at high frequencies, where typically no suitable calibration sources are available. They obtained the first 22 GHz polarization-VLBI images of the quasars 3C 345, 3C 273, and 3C 279. They found that in 3C 345 and 3C 279, the magnetic field is predominantly aligned perpendicular to the jet, whereas in 3C 273 the filed is longitudinal, at least in the vicinity of the compact core. The core polarization in all three quasars is low (< 3%), whereas the jet component are up to 30 percent polarized. The polarization properties of 3C 345 and 3C 279 are largely consistent with a general model based on strong plane-perpendicular shocks in the jet plasma.
Lobanov and Zensus studied the long-term spectral evolution of the synchrotron emission from the jet in 3C 345. They derive luminosity variations of core and moving jet components that suggest a variable pattern speed along a curved jet, and may require intrinsic acceleration of components. The spectral properties of one jet component can be explained by a strong shock, albeit requiring a variable Doppler factor along the jet. In a general sense, shock emission may be the dominant physical process at separations from the core of up to about 1.2-1.5 mas (4-6 pc), whereas at larger distances other mechanisms must be relevant.
In related efforts, W. Steffen (MPIfR, Bonn), Zensus, T. Krichbaum and A. Witzel (MPIfR, Bonn), and S. Qian (Beijing Observatory) (A&A, in press) developed a general model that explains the curved trajectories frequently observed in AGN radio jets by motion along a helical jet structure.
An extensive multipage table categorizing all the visitor proposals which are carried out during the calendar year using the NRAO observing facilities is prepared annually. Tables for each calendar year since 1986 are available. Entries include NRAO proposal number, names of co-investigators and affiliation, telescope(s) used, and a brief proposal title. Requests for 1994 Visitor Research Programs, or the same for any previous year, should be addressed to J. Nance, 520 Edgemont Road, Charlottesville, VA 22903-2475.
Paul A. Vanden Bout, Director