The Temperature of Nonspherical Circumstellar Dust Grains

The temperatures of prolate and oblate spheroidal dust grains in the envelopes of stars of various spectral types are calculated. Homogeneous particles with aspect ratios a/b ≤ 10 composed of amorphous carbon, iron, dirty ice, various silicates, and other materials are considered. The temperatures of spherical and spheroidal particles were found to vary similarly with particle size, distance to the star, and stellar temperature. The temperature ratio Td(spheroid)/Td(sphere) depends most strongly on the grain chemical composition and shape. Spheroidal grains are generally colder than spherical particles of the same volume; only iron spheroids can be slightly hotter than iron spheres. At a/b ≈ 2, the temperature differences do not exceed 10 %. If a/b >∼ 4, the temperatures can differ by 30–40 %. For a fixed dust mass in the medium, the fluxes at wavelengths λ >∼ 100 μm are higher if the grains are nonspherical, which gives over estimated dust masses from millimeter observations. The effect of grain shape should also be taken into account when modeling Galactic-dust emission properties, which are calculated when searching for fluctuations of the cosmic microwave background radiation in its Wien wing.