The submillimeter wave astronomy satellite: Mission science objectives

The Submillimeter Wave Astronomy Satellite (SWAS) mission is dedicated to the study of star formation and interstellar chemistry. To carry out this mission, SWAS will survey dense ( n H 2 > 10 3 cm −3 ) molecular clouds within our galaxy in either the ground-state or a low-lying transition of five astrophysically important species: H 2 O, H 2 18 O, O 2 , CI, and 13 CO. By observing these lines SWAS will: (1) test 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) supply heretofore missing information about the abundance of key species central to the chemical models of dense interstellar gas. SWAS will employ two independent Schottky barrier diode mixers, passively cooled to ∼150 K, coupled to a 54 × 68-cm off-axis Cassegrain antenna with an aggregate surface error ≤ 11 μm rms. During its three-year mission, SWAS will observe giant and dark cloud cores with the goal of detecting or setting an upper limit on the water abundance of 3 × 10 −6 (relative to H 2 ) and on the molecular oxygen abundance of 2 × 10 −6 (relative to H 2 ). In addition, advantage will be taken of SWAS's relatively large beamsize of 3.2 × 4.0 arcminutes at 551 GHz and 3.6 × 4.5 arcminutes at 492 GHz to obtain large-area (∼ 1° × 1°) maps of giant and dark clouds in the 13 CO and CI lines. With the use of a 1.4 GHz bandwidth acousto-optical spectrometer, SWAS will have the ability to simultaneously observe the H 2 O, O 2 , CI, and 13 CO lines. All measurements will be conducted with a velocity resolution of less than 1 km s −1 .