The Submillimeter Wave Astronomy Satellite: Science objectives and instrument description

The Submillimeter Wave Astronomy Satellite (SWAS), launched in 1998 December, is a NASA mission dedicated to the study of star formation through direct measurements of (1) molecular cloud composition and chemistry, (2) the cooling mechanisms that facilitate cloud collapse, and (3) the large-scale structure of the UV-illuminated cloud surfaces. To achieve these goals, SWAS is conducting pointed observations of dense [n(H-2) > 10(3) cm(-3)] molecular clouds throughout our Galaxy in either the ground state or a low-lying transition of five astrophysically important species: H2O, (H2O)-O-18, O-2, C I, and (CO)-C-13. By observing these lines SWAS is (1) testing long-standing theories that predict that these species are the dominant coolants of molecular clouds during the early stages of their collapse to form stars and planets and (2) supplying previously missing information about the abundance of key species central to the chemical models of dense interstellar gas. SI TIAS carries two independent Schottky barrier diode mixers - passively cooled to similar to 175 K - coupled to a 54 x 68 cm off-axis Cassegrain antenna with an aggregate surface error similar to 11 mu m rms. During its baseline 3 yr mission, SWAS is observing giant and dark cloud cores with the goal of detecting or setting an upper limit on the water and molecular oxygen abundance of 3 x 10(-6) (relative to H-2). In addition, advantage is being taken of SWAS's relatively large beam size of 3'.3 x 4'.5 at 553 GHz and 3'.5 x 5'.0 at 490 GHz to obtain large-area (similar to 1 degrees x 1 degrees) maps of giant and dark clouds in the (CO)-C-13 and C I lines. With the use of a 1.4 GHz bandwidth acousto-optical spectrometer, SWAS has the ability to simultaneously observe either the H2O, O-2, C I, and (CO)-C-13 lines or the (H2O)-O-18, O-2, and C I lines. All measurements are being conducted with a velocity resolution less than 1 km s(-1).