Two Problems in Interstellar Grain Alignment
Date
2011-03-02
Authors
Jordan, Margaret E
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Abstract
Starlight observed through the diffuse interstellar medium of the Galaxy is polarized and the polarization appears to track the Galactic magnetic field. This polarized starlight has been associated with non-spherical dust grains aligned with the magnetic field, and both grain aligning and disaligning mechanisms have been explored to explain the polarization. Radiative torques due to anisotropic stellar radiation incident on asymmetric grains appears to play a dominant role in alignment. Here, we investigate the importance relative to radiative torque alignment, of two possible effects on the grain’s alignment. The first is due to recoil torques as electrons and H atoms leave the grain surface and the second is due to a grains’s time-varying electric dipole moment. A grain irradiated by starlight may experience photoelectric emission, as well as photodesorption of adsorbed surface H atoms. Each ejection event imparts an angular impulse to the grain; integrated over the grain surface, they produce non-zero net recoil torques in the grain. The effects of these torques on grain alignment is the first subject of this dissertation. To evaluate these torques we constructed models of spheroidal grains irradiated by an anisotropic radiation field, and evaluated the resulting electric field intensities immediately below and immediately above the grain surface. The variation of the internal electric field intensity with surface position provides a measure of the photoelectric emission asymmetry. We used this asymmetry to estimate the photoelectric recoil torque on the grain. In the case of photodesorption, variation of the external electric field with surface position is used to estimate the recoil torque due to photodesorption. The maximum photoelectric torque was found to be ∼ 35% the radiative torque, and the maximum photodesorption torque was ∼ 30% of the photoemission torque. Because we set the conditions to maximize the torques in order to judge their relative importance, the torque contributions may be overestimated and actual torques may be on the order of 10% or less of radiative torques. Given this relatively small contribution to the total torque and that additional model uncertainties contribute at comparable or greater levels, we conclude that photoelectric and photodesorption torques need not be included in radiative torque alignment models. Charging processes, both the capture and emission of electrons from the grain, result in a net charge and an electric dipole moment that continually change for the grain. Recent analysis has shown that this changing dipole moment, in a grain drifting across the Galactic magnetic field lines, has the potential to disalign the grain. In the second half of this dissertation, the effects of the time-varying electric dipole on grain disalignment are explored. Previously, disalignment was presumed to be due principally to random collisions with interstellar gas atoms. Any successful alignment mechanism would have to align the grain on a timescale shorter than that of collisional disalignment. This is precisely what was found for radiative torque alignment. However, this new disalignment mechanism has the potential to act on a shorter timescale yet, and our investigation was designed to estimate this timescale. We considered four grain charge transport models, each subject to simulated individual charging events as a result of photoemission and capture of electrons, and tracked the effect of these events on the dipole magnitude and orientation for simulated times of up to 105 years. These models covered a range of internal charge transport types, from perfect insulator to perfect conductor, and a range of rotational speeds. Our results indicate disalignment timescales that are shorter than the timescales for radiative torque alignment, even for grains rotating suprathermally. This poses a potentially significant challenge for radiative torque alignment.
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Keywords
ISM, Dust, Grain, Alignment, Interstellar