Polarization of Thermal Emission from Aligned Dust Grains under an Anisotropic Radiation Field

Abstract

If aspherical dust grains are immersed in an anisotropic radiation field, their temperature depends on the cross sections projected in the direction of the anisotropy. It was shown that the temperature difference produces polarized thermal emission even without alignment, if the observer looks at the grains from a direction different from the anisotropic radiation. When the dust grains are aligned, the anisotropy in the radiation produces various effects on the polarization of the thermal emission, depending on the relative angle between the anisotropy and alignment directions. If both directions are parallel, the anisotropy produces a steep increase in the degree of polarization at short wavelengths. If they are perpendicular, the polarization reversal occurs at a wavelength shorter than the emission peak. The effect of the anisotropic radiation will produce a change of more than a few percent in the degree of polarization for short wavelengths, and the effect must be taken into account in the interpretation of the polarization in the thermal emission. The anisotropy in the radiation field produces a strong spectral dependence of the degree of polarization and position angle, which is not seen under isotropic radiation. The dependence changes with the grain shape to a detectable level, and thus it will provide a new tool to investigate the shapes of dust grains. This paper presents examples of numerical calculations of the effects and demonstrates the importance of an anisotropic radiation field on the polarized thermal emission.