Water in Comets: A Target of Opportunity Proposal to Observe
Cometary OH

A. Wootten,(NRAO), L. Woodney (NRAO, U. Md.), J. McMullin (NRAO)
and REU Students

Water is the primary parent constituent of cometary atmospheres within 3 AU of the sun, but is normally unobservable or difficult to observe using earthbound telescopes. In the inner solar system, water escaping from the nucleus photodissociates, resulting in the production of the observable daughter molecule OH.

We propose to measure the water production rate in three well-placed comets. The first, C/1998 J1 (SOHO), is a rich and productive comet, which is of interest because it is one of the few recently discovered sun approaching comets which is not a Kreutz family comet. The Kreutz comets being fragments of a large comet which is believed to have broken up over 1000 years ago. Though many Kreutz comets have been observed, little is known about the rest of the family of sun approaching comets. Since comets are some of the most pristine objects in the solar system, by studying them and looking for significant groupings in composition and size distributions we can find clues to the conditions in the solar nebula. A measurement of the OH production rate will give us a lower limit on the size of all three of these comets.

Most sun approachers are bright only near perihelion and thus are difficult to discover or observe due to their proximity to the sun. C/SOHO is unique in that its high rate of activity allows us to observe it. C/SOHO displayed an optical outburst in early June. Crovisier (IAUC 6934) has reported some OH observations during that period from Nancay. Unfortunately, the comet has rapidly moved into southern skies since then. While it is still available to the 43m telescope, it will soon be inaccessible to Nancay. The line should be quite bright (we estimate 0.4 K) and within the 43m declination limits until the second week of July, though it is receding rapidly from Earth. We propose to obtain a few observations, as scheduling permits, during the next few weeks. The comet is near 8H -37d the week of 21 June.

A second comet, C/1998 K5 (LINEAR), is distinguished by being one of the smallest comets ever detected. A lower limit on its size could help determine whether this is the first comet which has been seen with a radius of less than 1 km. It also passes close to Earth this week, with closest approach on 21 June. Reaching perihelion in mid-July, it will be well-placed for observation through the summer. Owing to its small size and thus low production rate, it will be much harder to observe. From its apparent magnitude, we estimate a line strength of a few mK at best. As it recedes from Earth through July, it will become a more difficult object, but probably still detectable into early August. If scheduling permits, it would be optimal to observe this comet during late June, but if a full transit is unavailable we would like to try to detect OH emission later in the summer. During August, it lies near 3H +34d.

A third comet, 21P/Giacobini-Zinner (G-Z), passes close to a radio source on 1998 July 31. Bryan Butler and Pat Palmer will observe this near-miss with the VLA, to establish a pencil beam profile of the OH distribution through the inner coma. They have asked if we could obtain a profile from the 43m near the same time; the line should be 3 to 6 mK and should require about 5 hours to obtain a 5-sigma detection.

In addition to providing what may be scientifically interesting size limits and supporting data for VLA observations in the case of G-Z, these observations will provide the summer students with a chance to gain observing and data reduction experience, as well as an introduction to some cometary science.

Scheduling Parameters

C/SOHO - Comet SOHO is moving south quickly. It is therefore best to schedule it sooner in the observing window rather than later.

21P/Giacobini-Zinner - In order to observe G-Z as closely in time to the VLA observations as possible, we prefer the early August time slot over the later one.

C/1998 K5 (LINEAR) - Comet LINEAR is intrinsically faint and will be difficult to detect. Though it would be scientifically interesting to determine an OH production rate for this comet, we prefer to observe 21P/Giacobini-Zinner if we have time to observe only one comet in August. G-Z will be much easier to detect and the data are needed to support the VLA observations.

Al Wootten