LABORATORY RESULTS ON MILLIMETER-WAVE ABSORPTION IN SILICATE GRAIN MATERIALS AT CRYOGENIC TEMPERATURES

Absorption spectra of crystalline enstatite and forsterite grains and amorphous silicate grains synthesized by sol-gel reaction (size simila r to 0.1-1 mu m) are measured between 0.7 and 2.9 mm wavelength (3.5-1 5 cm(-1)) at temperatures between 1.2 and 30 K. Some of the amorphous powders are precursors to forsterite (Mg2SiO4) and enstatite (MgSiO3). For the amorphous substances MgO . SiO2, 2MgO . SiO2, and MgO . 2SiO( 2) at 20 K, the millimeter-wave mass opacity coefficients are feued to be up to factors of 0.9, 3.5, and 11 times the Draine & Lee values us ually adopted for interstellar silicate grains. The measured coefficie nts are found to depend on the powder production technique. Enstatite (MgSiO3) is part of pyroxene [(Mg, Fe)SiO3] and forsterite (Mg2SiO4) i s part of oiivine [(Mg, Fe)(2)SiO4], both of which are thought to be p rincipal constituents of interstellar dust particles. The frequency de pendence of the absorption coefficient follows a power law with a temp erature-dependent exponent for all three amorphous silicates. Dependin g on the precise temperature, the power-law exponent ranges between a minimum value of 1.5 and a maximum of 2.5 for 2MgO . SiO2 and MgO . Si O2. At 20 K the index value is about 2. For the strongest absorber MgO . 2SiO(2), the power-law index has nearly a constant value of 1.2 ove r the entire temperature range; this value is significantly smaller th an 2, the value normally adopted for interstellar dust. The frequency- dependent absorption coefficients per unit mass for Mg2SiO4 and MgSiO3 are about 4 times larger for the amorphous precursor grains than for the crystalline ones. The millimeter-wave absorption coefficient for a morphous grains first decreases with increasing temperature until abou t 20 K and then increases at higher temperatures. This unusual tempera ture-dependent property forms a significant part of the overall absorp tion at long wavelengths: the relative change is as large as 50% at 1 mm wavelength for 2MgO . SiO2, 35% for MgO . SiO2, and 14% for MgO . 2 SiO(2). A weaker temperature-dependent change is observed for the crys talline forsterite and enstatite powders. The observed temperature dep endence of the far-IR absorption coefficient in the powders is well de scribed by a two-level population effect previously found for the ubiq uitous low-lying tunnelling states in bulk glasses.