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 similar to 0.1-1 mu m) are measured between 0.7 and 2.9 mm wavelength (3.5-15 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 usually adopted for interstellar silicate grains. The measured coefficients are found to depend on the powder production technique. Enstatite (MgSiO3) is part of pyroxene [(Mg, Fe)SiO3] and forsterite (Mg2SiO4) is part of oiivine [(Mg, Fe)(2)SiO4], both of which are thought to be principal constituents of interstellar dust particles. The frequency dependence of the absorption coefficient follows a power law with a temperature-dependent exponent for all three amorphous silicates. Depending 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 . SiO2. 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 over the entire temperature range; this value is significantly smaller than 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 amorphous grains first decreases with increasing temperature until about 20 K and then increases at higher temperatures. This unusual temperature-dependent property forms a significant part of the overall absorption 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 . 2SiO(2). A weaker temperature-dependent change is observed for the crystalline forsterite and enstatite powders. The observed temperature dependence of the far-IR absorption coefficient in the powders is well described by a two-level population effect previously found for the ubiquitous low-lying tunnelling states in bulk glasses.