Grain alignment and disruption by radiative torques in dense molecular clouds and implication for polarization holes

Abstract

Dust polarization induced by aligned grains is widely used to study magnetic fields in various environments, including star-forming regions. However, the question of to what optical depth grain alignment still exists in a dense molecular cloud (MC) is unclear. In this paper, we aim to achieve analytical formulae for the minimum size of aligned grains ($a_{\rm align}$) and rotational disruption ($a_{\rm disr}$) by RAdiative Torques (RATs) as a function of the local physical parameters within dense MCs. We first find the analytical approximations for the radiation strength and the mean wavelength of the attenuated radiation field in a dense MC without and with embedded stars and then derive analytical formulae for $a_{\rm align}$ and $a_{\rm disr}$ as functions of the visual extinction $A_{V}$ and the gas density. We find that within a starless core of density $n_{\rm H}\sim 10^{5}\rm cm^{-3}$, grains of size $a<0.25μm$ can be aligned up to $A_{V}\sim 5$ by RATs, whereas micron-sized grains can still be aligned at $A_{V}\sim 50$. The increase in the alignment size with $A_{V}$ can explain the presence of polarization holes observed toward starless cores. For MCs with an embedded protostar, we find that 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, which reproduces 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 discuss its potential for constraining grain growth.