Polarization of Thermal Emission from Aspherical Dust Grains in an Anisotropic Radiation Field

If aspherical dust grains are immersed in an anisotropic radiation field, they have different temperatures depending on the cross section projected in the direction of the anisotropy. The temperature difference produces the polarized thermal emission without alignment, if the observer is looking at the dust grains from a different direction from the anisotropy. Model calculations are performed for oblate spheroidal grains in simple geometries of the anisotropic radiation. The results indicate that this process could produce polarization of a detectable level (approximately 1%) around the emission peak and that the polarization degree increases with the decrease of the wavelength. Consequences of this mechanism will be discussed in relation to observations.