TEMPERATURE-DEPENDENCE OF THE ABSORPTION-COEFFICIENT OF COSMIC ANALOGGRAINS IN THE WAVELENGTH RANGE 20 MICRONS TO 2 MILLIMETERS

We have measured the absorption coefficient per unit mass of cosmic du st analog grains, crystalline fayalite and forsterite, amorphous fayal ite, and two kinds of disordered carbon grains, between 20 mu m and 2 mm over the temperature range 295-24 K. The results provide evidence o f a significant dependence on temperature. The opacity systematically decreases with decreasing temperature; at 1 mm, it varies by a factor of between 1.9 and 5.8, depending on the material, from room temperatu re to 24 Ii. The variations are more marked for the amorphous grains. The wavelength dependence of the absorption coefficient is well fitted by a power law with exponent beta that varies with temperature. For t he two amorphous carbons, beta(24 K) similar to 1.2 with increases of 24% and 50% with respect to the room-temperature values. A 50% increas e is found for amorphous fayalite, characterized by beta(24 K)= 2. A l ess pronounced change of beta with temperature, 14% and 10%, is observ ed for crystalline forsterite, beta(24 K)= 2.2, and fayalite, beta(24 K)= 2.3, respectively. For amorphous fayalite grains, the millimeter o pacity at 24 K is larger by a factor of similar to 4 than that of the crystalline counterpart. In addition, a temperature dependence of the infrared bands present in the spectrum of the two crystalline silicate s is found. The features become more intense, sharpen, and shift to sl ightly higher frequencies with decreasing temperature. The results are discussed in terms of intrinsic far-infrared-millimeter absorption me chanisms. The linear dependence of the millimeter absorption on temper ature suggests that two-phonon difference processes play a dominant ro le. The absorption coefficients reported in this work can be useful in obtaining a more realistic simulation of a variety of astronomical da ta concerning dust at low temperatures and give hints to better identi fy its actual properties. In particular, they are used to discuss the origin of the diffuse far-infrared-millimeter interstellar dust emissi on spectrum. It is proposed that composite particles formed of silicat e and amorphous carbon grains can reproduce the observations. The pres ence of these particles in the diffuse medium is consistent with the r ecent interstellar extinction model by Mathis.