Torques on Spheroidal Silicate Grains Exposed to Anisotropic Interstellar Radiation Fields

TL;DR

This paper investigates the impact of recoil torques from photoelectric effects on the alignment of spheroidal interstellar grains, concluding they are minor compared to radiative torques and can be neglected in current models.

Contribution

It provides the first quantitative estimate of recoil torques' magnitude relative to radiative torques for spheroidal grains, guiding future modeling efforts.

Findings

Recoil torques are less than 10% of radiative torques.

Recoil torques can be neglected in current grain alignment models.

Further experimental data on grain surface properties is needed.

Abstract

Radiative torques, due to the absorption and scattering of starlight, are thought to play a major role in the alignment of grains with the interstellar magnetic field. The absorption of radiation also gives rise to recoil torques, associated with the photoelectric effect and photodesorption. The recoil torques are much more difficult to model and compute than the direct radiative torque. Here, we consider the relatively simple case of a spheroidal grain. Given our best estimates for the photoelectric yield and other relevant grain physical properties, we find that the recoil torques contribute at the 10% level or less compared with the direct radiative torque. We recommend that the recoil torques not be included in models of radiation-driven grain alignment at this time. However, additional experimental characterization of the surface properties and photoelectric yield for sub-micron grains is needed to better quantify the magnitude of these torques.