The Submillimeter Wave Astronomy Satellite: Science objectives and instrument description

The Submillimeter Wave Astronomy Satellite (SWAS), launched in 1998 Decembe r, is a NASA mission dedicated to the study of star formation through direc t measurements of (1) molecular cloud composition and chemistry, (2) the co oling mechanisms that facilitate cloud collapse, and (3) the large-scale st ructure of the UV-illuminated cloud surfaces. To achieve these goals, SWAS is conducting pointed observations of dense [n(H-2) > 10(3) cm(-3)] molecul ar clouds throughout our Galaxy in either the ground state or a low-lying t ransition 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-stand ing theories that predict that these species are the dominant coolants of m olecular clouds during the early stages of their collapse to form stars and planets and (2) supplying previously missing information about the abundan ce 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 an tenna with an aggregate surface error similar to 11 mu m rms. During its ba seline 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 oxyg en abundance of 3 x 10(-6) (relative to H-2). In addition, advantage is bei ng 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 degr ees) 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 abil ity 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 conducte d with a velocity resolution less than 1 km s(-1).