The Atacama telescope, by virtue of its large collecting area and excellent site can offer extraordinary new capabilities for detailed and extensive studies of the interstellar medium. Spectral line observations are critical for determining the kinematics of cold cloud cores, as well as of protostellar and protoplanetary disks. While the AT at submm wavelengths will have spatial resolution comparable with that of existing mm interferometric systems, it should have much higher sensitivity, allowing routine and extensive observations of a large variety of species, including those with low optical depth. The higher rotational transitions of CO and its isotopologues 13CO and C18O will undoubtedly be of great use. Both fine structure transitions of carbon will be available on a routine basis. These (especially together) will allow probing of interactions in regions of intense radiation intensity. As a definitive determination of which species will be of most value remains to be made, it is clear that relatively broadband high resolution (heterodyne) cameras will be critical for taking full advantage of the telescope.
An important new capability will allow us to disentangle the physics of cold interstellar cloud cores. By means of multi-frequency continuum imaging of the dust emission, it will be possible to obtain accurate temperature and density profile determinations for dense cores. While ALMA can do this in principle, its frequency range and sensitivity will be limited compared to that of the AT. In order to really determine the key physical parameters in cores that are collapsing or are potential sites of star formation, both the temperature and the density must be determined. The multi--frequency imaging between 300 and 1500 GHz (1 mm to 200 µm) is essential for disentangling these two key quantities. The independent determination of dust temperature and column density requires multiwavelength data, best resolved by observations in the submm regime.
Magnetic fields play a crucial role in the collapse of interstellar clouds, the redistribution of angular momentum, the generation of jets and outflows. They are responsible for the polarization of the FIR emission of interstellar dust, which results from the alignment of dust grains. The determination of the topology of magnetic fields in star forming regions requires both high angular resolution — in order to map the field structure on scales commensurate with the collapsing objects — a large field of view — in order to understand the connection of the field geometry in the protostar and in the surrounding molecular clouds — and high sensitivity — in order to effectively measure the degree of polarization, which is generally at a level of a few percent or less. With a resolution of 2" at 200 µm, of 3.5" at 350 µm and a FOV of 5' x 5', the Atacama Telescope would deliver exquisite images and fine structure detail of the magnetic field topology, in star-forming regions.