MASER MAPPING AND DUST PROPERTIES OF RED SUPERGIANT WINDS

Maser emission from the circumstellar envelopes of four late-type red supergiants has been mapped with milli-arcsecond resolution using MERL IN1. The wind is driven by radiation pressure on dust and the structur e and kinematics of the masing regions reflect the dust properties. Th e unbeamed radius of water maser blobs, similar to 10(12) m, has been measured for the first time. The velocity gradient is used to derive t he dust absorption coefficient which increases with radius from less t han or equal to 0.1 to less than or equal to 1.0 m(2) kg(-1) Compariso n with laboratory studies suggests that small crystalline grains are f ormed near the star and are annealed into astronomical silicates at la rger distances.