Report of the Galactic Molecular Clouds and Astrochemistry
Working Group
The major question in Astrochemistry and Molecular Clouds is how molecules and dust grains cycle from the circumstellar envelopes of dying stars through diffuse and dense interstellar clouds, and how they are affected by the formation of new stars. The MMA is particularly well suited to study all phases of this chemical and physical cycle, especially those associated with the birth and death of stars. We describe here a number of projects which are in our view unique to the MMA. The set-up is different from the 1990 report, where the Astrochemistry was largely organized by type of reaction, rather than type of astrophysical object. It is now known that in general all processes (ion-molecule, neutral-neutral, grain chemistry) play a role at the same time, especially during the star-formation process.
Apart from the unique MMA projects, there are also many projects which are just barely possible with current instrumentation for a few objects, but which will benefit from the fact that the MMA can do them faster, deeper, and with better angular resolution. Thus, objects further away in the Galaxy or even extragalactic objects can be studied at the same linear resolution as nearby objects. The new science then comes largely from statistical studies on a more extensive data base. Such projects include statistical studies of the chemistry around young stellar objects with varying mass, luminosity, and age to investigate the response to different physical parameters and establish a chemical timeline. Also, the basic (photo-)chemical processes in circumstellar envelopes can be studied in many more evolved objects. The determination of the large-scale physical and dynamical structure of galactic molecular clouds is another example.
In summary, the conclusions of the working group are that:
The material in infall envelopes around low-mass young stellar objects forms the ``initial condition" for the chemical speciation prior to ``solar system" formation. Its study will require subarcsec imaging in many transitions at high spectral resolution and extremely high sensitivity to separate chemical and physical variations as functions of distance from the YSO. The MMA will be able to probe depletions up to a factor of 1000 compared with the surrounding cloud cores, and trace the nature of the gas-grain couplings in the envelopes.
We know from study of our own solar system that profound chemical
differences existed between the inner solar nebula (material now
within 5 AU) and the outer solar nebula. While it will not be
possible with the MMA to image gradients at 1-5 AU resolution even in
the nearest star-forming regions, large differences between the inner
and outer regions of disks around young stars will be apparent through
observations of highly (vibrationally) excited molecules. Such
measurements are essential to determine the major reservoirs of the
biogenic elements: carbon, oxygen, nitrogen and sulfur. For example,
in our own solar system, CH
is the carbon reservoir expected to
dominate in proto-Jupiter regions, and CO elsewhere because of kinetic
constraints. Similar statements can be made for N
/NH
. The MMA
will be the first instrument capable of examining fundamental
gas-phase partitioning in circumstellar disks. Comparison with MMA
images of molecules from cometary nuclei in our own solar system will
be particularly interesting to determine the similarities to the
cometary chemistry.
The MMA will be the most advanced radio instrument for the study of
the chemistry of large complex molecules formed in warm compact
regions. During the cloud collapse phase, many molecules will have
depleted onto interstellar grains. What happens to the molecules once
they are on the grain? How chemically active are the grain surfaces
and mantles? The MMA will be particularly powerful in separating the
gas-phase chemistry (ion-molecule, neutral-neutral chemistry)
occurring on larger scales from the dust chemistry (surface catalysis,
reactions in icy grain mantles), which takes place in warm compact
regions of 100-200 K where star formation is just starting. In
aperture synthesis mode, it will act as a powerful filter to help
``resolve out" any confusing spectra from the extended emission, so it
should be able to push past the confusion limits that hamper
single-dish telescopes. For example, at a spatial resolution of
1
, the MMA should be able to measure fractional abundances of
for large molecules like ethylcyanide and
glycine in 10 hrs of integration. This would be 
2 orders of
magnitude below current limits, and would greatly extend our
understanding of the nature of the grain-surface reactions. A
particularly interesting question is which molecules are coming
directly off the grains and which species are second generation
molecules produced in the subsequent complex gas-phase chemistry.
Observations of the H
O abundance through observations of the 183
GHz line will be important as well, since H
O is one of the major
oxygen containing molecules and the primary ice mantle
constituent. The high spatial resolution MMA observations will be a
valuable complement to the low spatial resolution space-based SWAS and
ISO data on other H
O lines.
A related, more speculative prospect for the MMA is that at submillimeter wavelengths, it may be capable of imaging transitions of low-lying vibrational modes of heavy species. This technique might give the best quantitative measure and identification of very complex species, such as PAHs or buckyballs. It may also give insight into the carriers of the diffuse interstellar bands -- the oldest unsolved question in molecular spectroscopy. The abundances quoted above are comparable to those inferred for these carriers in diffuse clouds. This would be a new type of astronomical spectroscopy which could be initiated by the high frequency MMA. Non-thermal (infrared) pumping of vibrational lines would greatly increase the detectability of these lines.
Figure 3. OVRO images of various molecular lines at 216-219 GHz in the Orion-KL star-forming region. The contours delineate the line emission, the color scale the millimeter continuum. Note that some lines such as SiO peak offset from the strong IRc2 continuum, whereas lines of vibrationally excited species are centered on the infrared continuum (Blake, Mundy et al. 1995).
Single-dish CO observations have shown that molecular clouds are permeated by outflows from young stellar objects over scales ranging from 0.01 pc to several pc. The mass loss from the YSO's appears to be highly time variable, resulting in pulsating jets with multiple internal working surfaces and bow shocks. Shocks can also be due to expanding H II regions, cloud-cloud collisions or supernova remnants running into the dense ISM.
Shocks have a profound effect on the chemistry of the cloud. With
increasing shock velocity, they will (i) release any icy grain mantles
back to the gas phase; (ii) drive endo-ergic reactions leading to
species like H
O and H
S ; (iii) dissociate molecules to atomic
form, with subsequent reformation in the cold post-shock gas; and (iv)
destroy refractory grain cores, returning Si back to the gas phase and
creating molecules like SiO. Current single dish observations cannot
resolve the shock structures themselves, which are typically only a
few hundred AU in size. In many cases, not even the type of shock --
J-type vs. C-type or both -- can be distinguished.
The MMA, with its subarcsec imaging capabilities, will for the first
time be able to quantitatively diagnose the shocks in the jets and
entrainment zones. Multi-species observations will be crucial to
determine the temperature, density and chemical conditions in the
primary outflows (the stellar jet), the cooling and molecular
reformation regions, and in the secondary lobes (entrained gas). The
ability of molecules like SiO to trace the outflow is demonstrated in
Figure 4. The advantage of the millimeter observations over more
traditional shock tracers, such as optical lines from Herbig-Haro
objects or H
near-infrared lines, is that these observations are
not affected by extinction but can probe the outflow physics deep
inside the cloud.
Figure 4. SiO emission tracing the outflow from the young stellar object NGC 1333 IRAS 2 (indicated by the star) as the jet punches into the surrounding molecular cloud, creating a trail of SiO molecules at high velocities. This image was obtained with the OVRO interferometer by Blake et al. (1995).
Translucent molecular clouds (A
few mag) display a
remarkable chemical complexity despite their low densities
(n
200-5000 cm
) and vulnerability to star light. They
afford the best laboratories for studying the basic physical and
chemical processes, such as heating and cooling of the gas, and
formation and (photo-)dissociation of simple molecules. Moreover, the
line shapes provide important clues to their origin and structure. The
MMA will be uniquely able to study these clouds through millimeter
absorption line spectroscopy against background continuum
sources. This will allow detection of species of much lower abundances
than in emission, such as triatomic or even polyatomic
species. Observations of a time series (
years) of these lines
will permit studies of the variations due to extremely small-scale
structures in the absorbing gas. Several hundred to thousands of
background sources are expected to be available to the MMA for such
studies, compared with only a handful possible with current
instrumentation.
This technique can also be used to search for a putative component of very cold molecular gas in the outer Galaxy, because the absorption lines are sensitive to gas with very low excitation temperatures, down to the cosmic background temperature of 2.7 K.
The neutral ISM in the vicinity of a host galaxy of an AGN is rarely
detectable in emission, but the presence of a strong radio continuum
makes it possible to use the same absorption line technique discussed
above to detect gas with <1 mag extinction. This neutral gas is the
ultimate reservoir fueling the nucleus. Multi-spectral studies will be
the primary means of determining its properties, amount, infall rate,
chemical composition etc. Some molecules not detectable in local
galactic gas due to telluric absorption can also be studied at various
redshifts. Particularly interesting candidates are O
and H
O .
The MMA will be particularly well suited to obtain totally new information about the physical and chemical structure of evolved stars. Part of this science is also discussed in the report of the working group on ``Star formation and Stellar Evolution"
The most exciting new astrochemical aspect is that the MMA will, for the first time, be able to measure the sizes of many stars and spatially resolve their atmospheric regions in which equilibrium chemistry prevails. Specifically, this will allow study of the dust formation at a few stellar radii and the mechanism by which it is pushed away from the star. Imaging of the abundance profiles of refractory molecules as functions of distance from the star will provide information on the processes of grain nucleation, gas-grain interactions and grain growth when molecules condense into grains.
Interstellar clouds are inhomogeneous on almost all scales and this structure results from complex dynamical processes whose precise mechanisms are poorly understood, but are governed by several forces including gravity, magnetic fields, rotation, thermal pressure, and turbulent pressure. In addition discrete sources inject energy in the form of winds and radiation producing systematic motions and shocks. The motions, structure and redistribution of energy and matter in clouds requires measurements of density and velocity on all scales in order to reveal the underlying forces at work in cloud evolution and star formation.
Our current understanding of clouds is hampered by the lack of images that cover all relevant spatial scales in a sufficient number of spectral lines. The scale of density fluctuations in clouds is well below the resolution of current filled aperture antennas. Since the physical structure, dynamics and chemistry are all intertwined, it is essential to obtain images of clouds in a large number of molecular species and transitions to deduce the temperature, density and chemical state of the gas.
The MMA will provide a major advancement over single-dish large-scale
surveys in at least three areas. First, the MMA will be able to image
rapidly large fields with unrivalled clarity over scales from 1
to 0
1 probing masses from 
to 
M
. Second, the images will not be hampered by the
instrumental signature of the telescope, which limits single-dish
statistical analyses. Third, the MMA will be able to obtain
multi-species, multi-transition observations at the same angular
resolution, whereas single dish studies are complicated by different
beam-sizes and/or use of different telescopes at the different
frequencies.
The MMA observations can be used to investigate several critical aspects of molecular clouds. One aspect is that they are expected to provide new insight into the role of turbulence in shaping the cloud's structure. How is turbulence generated, how does the turbulent energy cascade up or down with scale size, and how does it affect cloud dynamics? The small-scale turbulent fluctuations may have chemical signatures, and thus images in key molecules may be a sensitive means to probe the non-linear dynamics of the gas.
The MMA will also provide a better understanding of the fragmentation process by which large scale filamentary structures cascade into smaller gravitationally bound structures. Such processes are thought to be a critical part of the star formation process and may determine the initial mass function. The culmination of fragmentation is a critical and poorly understood stage where the high resolution and full coverage of the MMA will be essential.
If the MMA were located on a Southern site, the Magellanic Clouds will become as valuable for comparative study of Galactic molecular clouds, star-formation, and circumstellar matter as they have been in optical investigations of stellar populations and stellar evolution for many decades. The Clouds provide unique laboratories for investigating how astrochemistry and molecular cloud structures respond to different conditions of element abundances, stellar populations, external radiation field, mean pressure of the ISM, ambient large-scale gravitational potential, etc., in comparison with the quite different conditions in the Milky Way.
In its continuum (total power) mode, the MMA can make unique high resolution images of the large scale dust emission. The dust properties are expected to vary across the cloud, with small grains abundant in the more diffuse (outer) regions and larger grains dominant in the denser (inner) parts. It will be particularly exciting to correlate these dust properties with chemical variations: can we see molecules going onto and from the grains? Can we test the grain chemistry?
In addition, polarization measurements of the dust emission hold the most promise for studying the magnetic fields within the dense gas of molecular clouds. Background starlight polarimetry at both optical and near-infrared wavelengths has not proven to be a reliable probe of the magnetic field in cold dense gas, unlike the polarization of thermal dust emission which arises predominantly from the densest regions of molecular clouds. High angular resolution polarization measurements with the MMA will probe magnetic field geometry, both on the scales of the bound cores within clouds as well as on the scales of newly forming protostars, to assess the role the magnetic fields play in the formation of dense cores and stars.
The absorption line technique discussed in Section 2.4. for translucent clouds will also be the prime method for searching for a putative component of very cold molecular gas in the outer Galaxy, and for tracing its structure. This is because the absorption lines are very sensitive to gas with very low excitation temperatures, down to the cosmic background temperature of 2.7 K. The MMA will allow such absorption line studies to be extended to thousands of background continuum sources, allowing statistical studies throughout the Galaxy.
The correlator set-up should be as flexible as possible, since most projects require observations of multiple lines in multiple species and multiple bands. The correlator bandwidth and number of channels is adequate, but the observing efficiency could be significantly enhanced if more spectral windows were available (8 instead of 4). Many projects, especially at the highest spatial resolution, require long integration times (20 hrs or more). The number of lines to be observed in a 16 GHz bandwidth is readily 10-15, including isotopes to constrain optical depth.
Simultaneous multi-band observations are not essential, except perhaps at the longest baselines, where a H2O maser line may be used in another band as phase calibrator. We recommend to keep this option open as an "insurance policy" until it is certain that other atmospheric phase correction methods work.
The coldest (T=10 K), moderate density (
cm
)
circumstellar envelopes are best studied at the lower frequencies,
because high-frequency transitions of important heavy rotors like CCS,
HC3N, CH3C2H etc., are not detectably excited.
The 36-54 GHz range is also important for diffuse/translucent cloud absorption line studies, both in our Galaxy and at higher redshifts. Such studies could also be done with the (upgraded) VLA, but with lower efficiency.
The 650 GHz window is a very good window to study high excitation lines of various molecules in "hot core" type regions. Particularly useful lines are CO 6-5 and its isotopes to constrain the warm gas, and to derive abundances. This window also some unique molecules, such as HCl, and perhaps vibrational lines of large molecules.
In PDRs, the atomic carbon [C I] line at 492 GHz will be a useful tracer, but the conclusion of the working group was that the 490 GHz window has lower priority than the 650 GHz window. The puzzle of atomic carbon in PDRs is expected to be largely "solved" by single dish/ISO observations in the next decade, and interferometer data may not add much in our Galaxy. High resolution [C I] interferometer observations of galaxy nuclei will still be useful.
Baselines as large as 3 km at 230 GHz are definitely warranted in
order to study high-mass star-forming cores at large distances at the
same linear scale as Orion-KL. The rms sensitivity of 
6-12 K in
a 1 MHz bandwidth in 1 hr integration at 0.08" resolution is
sufficient to detect many of the stronger lines, which typically have
K.
There are also strong scientific arguments to push for baselines larger than 3 km. Particularly exciting is the possibility to probe the chemical composition of the inner 5 AU of solar nebulae and the opportunity to observe maser lines with VLBA resolution and perform proper motion and dynamics studies with them.
For Galactic studies, the southern hemisphere is preferred because of the larger part of the Milky Way that is accessible, as well as the presence of many nearby low-mass star-forming regions and the spectacular high-mass eta Carina region. The Magellanic Clouds also provide a strong scientific argument for a southern site (see Section 3.).
The working group members concluded that there are compelling
scientific reasons for large scale OTF mosaics (see
Section 3.), but that it should not be driving the
design. Most of the members felt that the emphasis will lie on imaging
areas of 10'-30' at 
1" in the weaker 13CO, C18O, CS etc. lines,
rather than the strong 12CO line over degree scales. Typical
integration times are therefore likely to be >1 sec. For example,
the integration time to obtain an rms of 0.3 K at 100 GHz at 
2"
resolution is 1 min at 1 MHz resolution. These demands are such that
the large scale mosaicing is not pushing the design.
Continuum (total power) mapping is definitely warranted.
Large scale polarization mapping in the dust continuum emission would be useful to constrain magnetic field structure. See star-formation working group for requirements.
The working group identified a major problem area in the data reduction and analysis effort. Any large-scale OTF mosaics with correlator dump times of 1 s involve an order-of-magnitude increase in data rates over what has been possible today at any wavelength. Such large data sets and data rates will require new techniques both in computation and in scientific analysis.