In addition to the complex ionized (and non-thermal) sources just
discussed, regions of massive star formation are rich in molecular
emissions. The NH
molecule is one of the most useful for the study of
high density molecular material, because of series of inversion
transitions just above 23 GHz. These transitions have hyperfine
components and, thus, allow direct estimates of density and clumping
(filling factor).
For example, the NH (2
2) inversion
transition has five components: a main line, two inner satellites
separated by 116.55 km/s, and two outer satellites
125.7 km/s from the main line. To resolve the lines, sometimes as
narrow as 0.25 km/s, a channel separation of 0.05 km/s is desirable.
To allow for enough channels outside the outer satellites in order to
measure the baseline, a total bandwidth of 100 km/s (8 MHz) needed
with about 2000 channels. If the source has gas motions which are of
order 16 km/s, every one of these channels could contain line signal.
(If the motions are more typically 5 km/s or less, some savings could
be had by positioning independent groups of channels on each of the
lines in which case only 1000 channels are necessary.) For greater
sensitivity, one would like to obtain the LCP and RCP data
simultaneously. Using the present correlator, most observers are
forced to ignore the outer satellites (although they can help
constrain LTE models and provide and independent column density
estimate). Typically only the main line and the inner satellites are
observed in a 3.125 MHz band with 16 or 32 channels. The main line
and the satellites are often blended in the channels which have only
1.25 km/s resolution.
A new broadband data transmission system and correlator are needed to
make a major advance and provide adequate spectral resolution and
capabilities to do multiple lines simultaneously. For example, in the
22.5 GHz band, one could make T
and density maps using the many
NH
transitions. For hot cores near H II regions one would like to use the
(11) and (22) transitions to get the extended, lower temperature
structures; use the (44) and (55)
transitions for moderate
temperatures, and higher metastable line to get the higher density,
hotter features; also the non-metastable lines such as the (21) can
be used to study the highest densities (N
); and finally the
(33) and (66) ortho lines often mase. Clearly at least 4, and
preferably 8, independent channels would be desirable. The frequency
range from (11) to (66) is 23-25 GHz and spectral channels of
width of about 8 MHz per line and 8 kHz spectral resolution. Also, a
simultaneously, broad-band continuum measurement would provide
valuable complementary information regarding the physical setting and
sources of excitation.
The CS molecule has its fundamental J=10 transition at 49 GHz,
in
the new 40-50 GHz system of the VLA. Recent observations
of the IRc2 region in Orion using the existing 45 GHz
system have approximately twice the spatial resolution of any existing
image made with a thermally excited molecular line. The outflow of
IRc2 is evident as well as a velocity gradient running along the
length of the Orion molecular ridge. The enhanced VLA could
produce an image with approximately 30 times better sensitivity,
making it possible to map the dynamics of the outflow as traced by the
weak emission in the line wings.
Maser observations, whether OH at 
18cm or CHOH at 7mm,
are some of
the most demanding spectral line projects. High spectral resolution
is required to measure the very narrow lines (
km/s), a wide
velocity range is needed because the masers in star forming regions
can be present over a wide range in velocity, wide fields may need to
be imaged because star formation is often highly clustered, and
polarization measurements are needed if one is trying to use the
Zeeman effect to measure the in situ magnetic field. One may wish to
cover 40-80 km/s with channels as narrow as 0.1 (even less for Zeeman
experiments). Thus 800-1000 channels are needed with circular
polarization information. Masers are important sign posts of new star
formation, they a valuable probes of the interstellar gas dynamics,
they are used in proper motions studies to determine the distances,
and they are valuable tools for self-calibration of continuum or other
line measurements because they are bright, unresolved point sources.
There are additional molecules of interest. Massive organic molecules (e.g., carbon chains and possible rings), are building blocks of molecules crucial to formation of living organisms. There are reports of mm-wavelength detection of amino acids. These observations were not possible with single dishes owing to line crowding, but were successful with an interferometer. Thus, large molecules with transitions at centimeter -wavelengths are probably best detected with the VLA, over (say) the Green Bank Telescope, because of the excellent spectral baselines achieved with an interferometer and the higher duty cycle on-source. Also, since the VLA antennas are much smaller than a large single dish such as the Green Bank Telescope, the increase in noise from the source (e.g., Sgr B2, Cas A) is much smaller for the VLA. The combination of all these advantages (for the enhanced VLA versus the Green Bank Telescope we estimate factors of 1.5 for collecting area, 1.5 for increased duty cycle, and a factor of order 3 (for a 100 Jy source) from system temperatures) yields a sensitivity increase of 7, or an integration time of 50-favoring the VLA! The wider field of view at the VLA might also be valuable if the position of the line emission/absorption is not precisely known.
Formaldehyde has been a staple molecule for centimeter-wavelength
telescopes for many years. Unfortunately, only two lines have been
available for study-at 6cm and at 2cm. These
lines are affected by the well known excitation anomaly which causes
them to be seen in absorption against the 2.7K background over much of
a cloud's extent. Fortunately, the VLA will make available several
very interesting new transitions with completion of the 45 GHz system.
Formaldehyde may turn out to be the ``molecule of choice" for probing
physical conditions in clouds with the VLA. In addition to the
6cm and 2cm lines, the 3(1,2)3(1,3)
line at 28.975 GHz, and the 4(1,3)4(1,4) line at 48.3 GHz
could be available on the VLA. These lines are very useful
densitometers. Mangum (1990) noted in his dissertation that these
K-doublet transitions have a spatial density filter built into their
excitation; for densities below 
the formaldehyde lines are in
absorption for J of 4 or below, while for densities above this value
all are in emission. At J 
5, all lines will be in emission
where the excitation is sufficiently robust. Mangum also showed that
formaldehyde is less susceptible to abundance peculiarities than some
other molecules, notably ammonia, which render interpretation of the
observations difficult. Formaldehyde observations at higher
frequencies will provide an excellent densitometer, free
from the complex cloud-envelope effects which plague interpretation of
lower-lying formaldehyde lines. Formaldehyde has been detected in
several external galaxies;
the additional higher frequency lines
should be detectable in several galaxies with the VLA, providing
valuable density information.