Grain Alignment and Disruption by Radiative Torques in Dense Molecular Clouds and Implication for Polarization Holes

Abstract

Abstract Dust polarization induced by aligned grains is widely used to study magnetic fields in various astrophysical environments. However, the question of to what optical depth grain alignment still exists in a dense molecular cloud (MC) is unclear. In this paper, we derive analytical formulae for the minimum size of aligned grains (a align) and rotational disruption (a disr) by RAdiative Torques (RATs) as a function of the local physical parameters within MCs. We first find the analytical approximations for the radiation strength and mean wavelength of the attenuated radiation field in a dense MC with and without embedded stars, and then derive analytical formulae for a align and a disr as functions of the visual extinction A V and gas density. We find that, within a starless core of density , grains of size can be aligned at A V ∼ 5 by RATs, whereas micron-sized grains can still be aligned at . The increase in a align with A V can explain the presence of polarization holes observed toward starless cores. For MCs with an embedded protostar, the efficiency of both alignment and rotational disruption increases toward the protostar due to the increasing radiation strength. Such a disruption effect results in the decrease of the polarization degree with A V or emission intensity, reproducing the popular polarization holes observed toward the location of protostars. Finally, we derive the formula for the maximum A V where grain alignment still exists in a starless core, and we discuss its potential for constraining grain growth.