Galactic Molecular Clouds and Astrochemistry: Motivations for Submillimeter Capability Ewine van Dishoeck (chair), John Bally, John Black, Jeff Mangum, Joseph McMullin, Ed Churchwell I. Some of the main science drivers Molecular cloud structure Physical and dynamical structure at all spatial scales The magnitude and role of turbulence in determining the structure of clouds and control of star formation Cloud formation and destruction Cloud-cloud interactions Structure of ionization and shock fronts The origin, magnitude, and role of magnetic fields in cloud support, star formation, cloud structure, and internal dynamics Star formation Prestellar cloud properties and star formation environments Stellar accretion process and accretion disks Bipolar outflows (jets) and their interaction with the ISM Binary star formation Cluster formation Astrochemistry Molecular line probes of: quiescent clouds star formation regions PDRs shocks Chemical timescales Chemical composition in: star formation regions diffuse and translucent clouds shocks of various types solar nebula photon-dominated regions chemistry at high redshift The above list is not exhaustive and has mostly been covered in the initial report of the Tucson Workshop. We will not reiterate those arguments here. Our main intention here is to emphasize some of the arguments for including submillimeter capabilities for the MMA or the merged MMA/LSA. Aside from the fact that the Chilean site is probably one of the best sites in the world for ground-based submillimeter astronomy, there are also strong scientific drivers to incorporate submillimeter capabilites in the MMA design. The MMA will provide high sensitivity, sub-arcsecond angular resolution, imaging capability, and access to submillimeter wavelengths. The MMA will compliment NGST in its ability to detect YSOs (and other objects such as AGN, galaxies, etc.). While the NGST will study Class 1 YSOs and more evolved sources in the 1-30 micron range, the MMA will be able to investigate true protostars (Class 0 objects) at wavelengths ?300 microns. Below we give a few examples of innovative science that strongly support a submillimeter capability for the MMA out to at least the 650 GHz window and, if practical, to the 850 GHz window. II. Molecular cloud structure Absorption by local HI clouds toward several quasars, using VLBI techniques, have shown that structure on scale sizes of a few AU are common in neutral clouds in the Galaxy (Faison et al. 1998, ApJ, submitted). Although the MMA will not be able to resolve structures this small, the hierarchy of larger structures are of enormous interest and have not been investigated on the scales that the MMA will be able to provide. The MMA will have the resolution at submillimeter wavelengths to observe structure from about 30 mas at 0.35 mm (baseline of 3 km) up to ~30=B2 of arc at 7.5 mm (baseline 70m). The small end of this range of scalesizes (30 mas to a few arc seconds) are virtually unexplored at present. As we are finding out from the beautiful images provided by the JCMT with SUBA, the ability to observe large molecular cloud complexes with high spatial resolution is a powerful tool for understanding the physics, energetics, and structure of such clouds. The MMA will be able to provide even higher resolution and over large fields of view by fast OTF imaging techniques or mosaicing. Although many astronomers believe that much of the supersonic turbulence known to exist in molecular clouds may be driven by winds, jets, and outflows from star formation, a quantitative measure of this has not yet been assessed. The MMA, with its sub-arcsecond resolution and high sensitivity should be able to quantify this as well as shed light on the interaction regions of outflows with the ISM. The best probes and resolution will be at submm wavelengths. Surveys of mm-wave Galactic absorption lines toward low-latitude extragalactic radio sources (Lucas&Liszt, IAU 178, p.421, for example) have shown that the abundances of polyatomic molecules are much higher than suggested by visible and UV absorption lines in regions of low density and low shielding. The great value of extending such observations to submm wavelengths lies in the prospect of observing lines of additional chemically significant molecules, including light hydrides. The principle limitation so far has been the small number of continuum sources that are strong enough at submm bands. With the large increase in sensitivity of the MMA, the number of suitable sources will be greatly increased. III. The structure of ionization and shock fronts It has long been known from theoretical models that ionization fronts and shock fronts are generally only a few AU in thickness and have extremely rapid changes in density, temperature, velocity, and equation of state with position. Only in very rare cases has it been possible to study such regions with high enough resolution to constrain models in significant detail. With resolutions of 30-70 mas (0.35 - 0.87 mm and baseline of 3 km), it will be possible to resolve structures of 4 -9 AU at the distance of the nearest molecular clouds (~140 pc). Furthermore, many submillimeter lines lie in the appropriate energy range to probe both shock fronts (high excitation molecular lines) and ionization fronts (radio recombination lines of H, He, and C). The MMA will be the only radio telescope that will have the combined sensitivity, resolution, and appropriate energy range to do this. IV. Star formation A very strong case can be made for submillimeter observations of star formation regions. To understand star formation it will be necessary to understand the structure and dynamics of accretion disks, the origin and nature of bipolar outflows, and the role of magnetic fields on disks and outflows. This means that we have to study the dynamics, mass, temperature, ionization, density, etc. distributions of matter within a few AU of the central YSO. This will require both the highest possible spatial resolution for the MMA (i.e. the submm bands at 345, 650, and 850 GHz) and high excitation line probes which lie in the submillimeter bands. This is especially true of massive star formation whose distances are all 450 pc or greater and whose densities in the neighborhood of the central stars are so high that most of the traditional line probes are quite optically thick. In these objects, only high excitation molecular lines and/or recombination lines can probe the near environs of the central star. Traditional lower excitation molecular lines either do not exist in these regions or are so optically thick that they cannot provide information on the important physical quantities needed. At the distance of Orion (450pc) at 850 GHz with the baseline of 3 km, a resolution of 30 mas corresponds to a resolution of 13.5 AU. This is just adequate to study the accretion process and outflow structure of the Orion IRC2 outflow. The MMA will be able to resolve and study the structure of PROPLYDS in the Orion nebula at submm wavelengths in the continuum and high excitation molecular lines. The cometary structures in the Helix nebula detected by HST will be easily resolved by the MMA. It should be easy to study their dynamics via submm transitions of CO, CS, and other molecular transitions and their ionized shells via recombination lines. Hydrogen recombination lines can appear as strong maser lines in the mm, submm, and infrared of objects like MWC 349 and eta Carinae. In MWC 349, the masers provide valuable probes of the dense circumstellar disk surrounding the hot, young star. With the MMA imaging several transitions, it is possible to probe the density and temperature gradients as well as the kinematics of such a disk. With high sensitivity and resolution of the MMA, it is likely that a larger sample of YSOs can be observed with this technique. V. Astrochemisty A. Chemisty at high redshift The ability to detect both emission and absorption lines from gas at high redshift has been amply demonstrated at mm waves. The scope of such observations will be greatly enhanced at submm wavelengths, as some of the most significant atomic (e.g. [CII] 1.900 THz, [NII] 1.461 and 2.459 THz) and molecular (e.g. H2O 557 GHz and 1.113 THz) cooling transitions will come into view at reasonable redshifts. Not only are such transitions important probes of energy balance in the ISM of a high redshift galaxy, but they can also yield important limits on the early evolution of element abundances. B. HCl chemistry Despite its low abundance, chlorine has an interesting interstellar chemistry. The lowest rotational transition of HCl at 625.9 GHz has been detected in OMC1 and is likely to be detected in other hot cloud cores with the higher sensitivity provided by the MMA. Although at lower densities, the higher rotational transitions of CO are expected to provide the primary cooling of hot cores, above a critical density, molecular cooling is expected to become independent of abundance of the coolant so relatively rare molecules with high rotational constants such as HCl can contribute significantly to the cooling. C. Importance of the 400 GHz window H3O+ is vital to understanding interstellar oxygen chemistry. Its transition at 396 GHz will be paramount in this effort. Metal atoms in molecular clouds affect the ionization balance and hence the coupling to magnetic fields threading through the clouds. AlH J=3D1-0 has a frequency of 377.6 GHz and AlH+ have transitions at 393 and 395 GHz. D. Specific molecules 1) 650 GHz window: HCl, H2O masers, .... 2) 460 GHz window: CI, NH2, HDO, .... High excitaton lines to constrain physical conditions and cloud structure: 12CO, 13CO, C18O, C17O J=3D6-5-- mass of warm gas CS (10-9; 13-12) H2CO (various transitions)-- high density structure High angular resolution is important for hot cores, disks, structure of outflows and their interfaces with the ISM. Dust temperatures and emissivity variations across clouds: 450 m in combination with 1mm and 850 m.