Temperature Dependence of the Submillimeter Absorption Coefficient of Amorphous Silicate Grains

We have measured mass absorption coefficients of amorphous silicate materials for wavelengths between 100 (mu)m and 2 mm (5100 cm-1) and at temperatures between 300 and 10 K. For both interstellar analog MgSiO3 and simple silica SiO2, we find evidence for a strong temperature and frequency dependence. We define two distinct wavelength regimes, 500 (mu)m-1 mm and 100-250 (mu)m, for which the absorption coefficient presents different trends with frequency. To evaluate this frequency dependence, we fit our absorption coefficient using two power laws with spectral index (beta)that varies with temperature. We do not find a significant variation of (beta)with temperature between 100 and 250 (mu)m, whereas between 500 (mu)m and 1 mm a pronounced anticorrelation between T and (beta)exists. Globally, (beta)-values decrease from 2.5 to 1.5 between 10 and 300 K. This anticorrelation for interstellar analog grains has the same trend as the one observed using the balloon-borne experiment PRONAOS. We show that physisorbed water is not responsible for the observed temperature and frequency dependence and that OH groups could be at the origin of the submillimeter properties of the materials. As discussed in the literature, OH groups are often related to tunneling processes in two-level systems (TLS). In the case of the more complex MgSiO3 silicates, TLS could also be produced by the Mg+2 ions, which act as network modifiers, similar to how they act with OH groups.