Silicate Grains in Space: Laboratory Results

Abstract

Since their discovery in the spectra of some strong IR sources (Stein et al., 1969; Woolf and Ney, 1969; Forrest et al., 1979), the bands at around 10 µm and 20 µm have been respectively attributed to typical Si-O stretching and O-Si-0 bending modes in the SiO₄ tetrahedra of the silicate structure. The presence of silicate grains around “O” stars ([O]/[C]≫1) and in the interstellar medium (ISM) is also supported by theoretical computations on the grain condensation mechanism in space conditions. In fact, in the “O” stars envelopes CO is very likely to exist in the gas phase (Salpeter, 1977), as it is also confirmed by astronomical observations (Knapp and Morris, 1985; Zuckerman and Dyck, 1986). The oxygen in excess takes part to the condensation in solid grains which involve also Si, Fe, Al, ecc. (see, for instance, Hackwell, 1971).