Laboratory Results on Millimeter-Wave Absorption in Silicate Grain Materials at Cryogenic Temperatures

Details Laboratory Results on Millimeter-Wave Absorption in Silicate Grain Materials at Cryogenic Temperatures Laboratory Results on Millimeter-Wave Absorption in Silicate Grain Materials at Cryogenic Temperatures

May 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...462.1026a&link_type=abstract

Astrophysical Journal v.462, p.1026

Astronomy and Astrophysics

Astronomy

108

Infrared: Ism: Continuum, Ism: Dust, Extinction, Methods: Laboratory

Scientific paper

Absorption spectra of crystalline enstatite and forsterite grains and amorphous silicate grains synthesized by sol-gel reaction (size 0.1-1 μm) 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 2SiO2 at 20 K, the millimeter-wave mass opacity coefficients are found 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 olivine [(Mg, Fe)2SiO4], 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 2SiO2, 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 2SiO2. 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.

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