The proposed system will be capable of performing high spectral
resolution studies of a number of important atomic and molecular
species, including [SiI], [OI], [CII], CO, CH, OH, and [NII]. Of
particular interest are the [CII] 158 
m and [NII] 205 m
lines, and a variety of lines from simple hydrides such as OH, CH, and
CH 
.
COBE found the [CII] 158 m line to be the dominant cooling mechanism of the
interstellar medium (Wright et al. 1991; Bennet et al.\
1994). Knowledge of the origin of the [CII] emission is essential to
our understanding of the interstellar medium both in the Milky Way and
external galaxies. Three sources for [CII] emission have been proposed:
the galactic HI, PDRs, and the Warm Ionized Medium (WIM). Emission in
the [CII] line is the dominant cooling mechanism in diffuse HI clouds and
may contribute significantly to the galactic [CII] luminosity.
KAO observations have shown [CII] emission to be at
least as extensive as [CI] in several giant molecular clouds
(Stacey et al. 1993; Stutzki et al. 1988). Such extended [CII] emission
probably reflects the clumpy structure of molecular clouds, permitting
C-ionizing photons to penetrate through much of the cloud. Small
PDRs are produced on the clump surfaces and give rise to the [CII]\
emission. Alternatively, the emission could arise from the surface of
a more homogeneous cloud if the FUV radiation field is several times
larger than the local interstellar value (Stacey et al.). Because of
the importance of the [CII] line as a coolant, detailed understanding
of the emission mechanism is a prerequisite for understanding the
structure, energy balance, and evolution of molecular clouds in the
Milky Way and external galaxies. To determine which model for [CII]\
emission is most appropriate for an individual cloud requires
knowledge of the spatial relationships between the ionized and neutral
components. One of the best methods of disentangling the relationships
between different atomic and molecular species is comparison of their
line profiles. By taking advantage of the velocity information
contained in the profiles, we can obtain a direct probe of conditions
at different cloud depths. The high sensitivity, spatial coverage, and
spectral resolution of the proposed heterodyne array will for the
first time permit large numbers of high-quality, high-resolution [CII]\
spectra to be obtained. Our proposed instrument will be 
times more sensitive per pixel and have 16 times the spatial coverage
of earlier heterodyne instruments flown on the KAO.
The potential power of STAR in probing conditions in and around
molecular clouds is illustrated in Figure 1. In the middle of the figure we
show an image of NGC 6334 made in CO J=4 
from AST/RO with a
receiver system constructed in the PI's lab. The small square of emission
in the top right hand corner is the size of the region that could be
mapped in CII at full beam spacings with a single receiver channel in one
flight ( 
hrs). The second square shows the region that could
be mapped with STAR in the same period of time, essentially the whole
cloud. The CII spectra at the bottom left were taken by Borieko and Betz
(1995). The width and complexity of the line profiles are well matched
to the 40 km/s velocity coverage and 0.125 km/s velocity resolution
available in each STAR pixel.
Figure 1: Imaging Capabilities of STAR. Simulated performance advantage of STAR over a single pixel receiver in mapping large scale molecular cloud cores. Each pixel of STAR was assumed to have the same receiver noise temperature as the single pixel receiver. We assumed a conservative receiver noise temperature of 1000 K, and an atmospheric opacity of 0.125. The SNR in the calculations was 10.
The Warm Ionized Medium (WIM) may also contribute significantly to the
[CII] luminosity of the Galaxy. Since the ionization potential of N is
slightly higher than that of H, the [NII] line at 205 m must arise
in regions where hydrogen is largely ionized, and thus can serve as an
effective probe of both the WIM and low-density HII regions (Tielens
1995). The [CII]/[NII] intensity ratio can be used to investigate how much
of the Galaxy's [CII] emission arises in these ionized regions. COBE
mapped the [NII] 205 m line at low spatial and spectral
resolution. With the proposed instrument, it will be possible to probe
much lower density regions in both transitions. By mapping a variety
of GMCs, isolated globules, and planetary nebulae, we will gain a
deeper understanding of the relative importance of the WIM, PDRs, and
diffuse HI in the production of [CII].
In consequence of their low molecular weights, the rotational lines of
many astrophysically abundant simple hydrides fall into the 
- 2.4
THz band accessible to the proposed spectrometer. The OH and CH
radicals have long been known to be present in a variety of sources,
from their lambda-doubling transitions at cm-wavelengths. These
transitions are typically masers, however, and so are very difficult
to interpret in terms of molecular abundances. The far-IR lines of OH
are well-known diagnostics of interstellar molecular shocks and are
also a valuable probe of oxygen-rich circumstellar envelopes. Other
diatomic hydrides (e.g., SiH) have rotational transitions in the same
frequency region. In contrast to the cm-wavelength transitions, the
rotational transitions accessible to the proposed array receiver
should provide much more direct information about physical conditions
in the emitting regions.
In circumstellar envelopes and planetary nebulae, the CH and OH
radicals are predicted to be the products of photochemical processes
which depend on a variety of factors, and since CH and OH contain
three of the most abundant elements in the stellar ejecta, their
transitions near 2 THz will be critical indicators for these
processes. In particular, CH has strong transitions at 180.6, 149.5,
149.2, and 125.0 m with low-lying energy levels. The OH molecule
has its 
rotational transition at 163.5 m with an
upper state energy about 270 K above ground, providing a good
diagnostic of warm gas and/or excitation through IR photons in
vibrational modes.
A startling result from ISO has been the discovery of pure rotational
lines of CH in NGC 7027 (Cernicharo et al. 1997). This is one of a
small class of highly reactive molecular ions (including CO ),
which are destroyed in virtually every collision with the most
abundant interstellar species (H, H 
, and e) in molecular clouds
and nebulae. Since CH can evidently be formed only through
high-temperature chemical reactions, its presence (like that of CO )
in detectable amounts is an important signature of energetic processes
in the very boundary layers of molecular gas exposed to high fluxes of
UV starlight and/or powerful shock waves. Two of the strongest of the
CH emission lines are at 119.9 and 179.6 m. The unprecedented
sensitivity and resolution of the proposed instrument will make it
possible to use such transient chemical species as CH+ as specific
probes of the short-lived, energetic transition zones in star-forming
regions of much lower surface brightness than the bright planetary
nebula NGC 7027.
The proposed receiver will also dramatically open up the study of
low-lying rotational transitions of simple hydrides of other
relatively abundant elements. Rotational lines of HCl at 159.9 m;
of KH at 186.6, 166.1, and 149.7 m; and of CaH at, e.g.,
197.9 m, can be searched for to examine formation of metallic
hydrides in a variety of astrophysical contexts, including the wide
range of conditions encountered in star-forming regions and the
circumstellar ejecta of post-AGB stars, supergiants, and PNs.
The proposed instrument will also allow studies of the molecular
emission from warm gas associated with shocks or PDRs. This includes
hydrides (CH, CH , OH), as discussed above, as well as high
rotational levels of CO. KAO studies have shown that these lines are
powerful probes of the physical conditions (n and T) of the
emitting gas. The high spatial and spectral resolution will for
example allow detailed studies of the interrelationship between
various molecular and atomic tracers of edge-on PDRs (i.e., the
Orion Bar) and molecular outflow regions (i.e., Orion IRC 2) and
thus provide more stringent tests of the physics and chemistry of such
regions (cf., Tielens
et al. 1993).