Towards a circumstellar silicate mineralogy

Abstract

This paper reviews the formation, composition, and spectroscopy of circumstellar silicate grains. It is argued that 'clean' silicates (and oxides) are formed close to the stellar photosphere, but because of their low NIR absorptivity, these grains float with the gas in the pulsating photosphere until, at large distances from the stellar surface, enough Fe(2+) is incorporated (i.e., 'dirty' silicates) for radiation pressure to drive a dusty outflow. The silicate chemical condensation sequence is reviewed with emphasis on kinetics rather than thermodynamics, and likely condensates are identified. IRAS LRS spectra of tenuous dust shells show considerable variations in the shape of the 10-micron feature and at least 5 independent spectral components are indicated. These are compared with laboratory spectra and (preliminary) identifications are proposed.