Effects of Grain Magnetic Properties and Grain Growth on Synthetic Dust Polarization of MHD Simulations in Protostellar Environments

Abstract

Thermal dust polarization is a powerful tool to probe magnetic fields ($\text{B}$) and grain properties. However, a systematic study of the dependence of dust polarization on grain properties in protostellar environments is not yet available. In this paper, we post-process a non-ideal MHD simulation of a collapsing protostellar core with our updated POLARIS code to study in detail the effects of iron inclusions and grain growth on thermal dust polarization. We found that superparamagnetic (SPM) grains can produce high polarization degree of $p\sim 10-40\%$ beyond $\sim 500$ au from the protostar because of their efficient alignment by magnetically enhanced Radiative Torque mechanism. The magnetic field tangling by turbulence in the envelope causes the decrease in $p$ with increasing emission intensity $I$ as $p\propto I^{\alpha }$ with the slope $\alpha \sim -0.3$. But within 500 au, SPM grains tend to have inefficient internal alignment (IA) and be aligned with $\text{B}$ by RATs only, producing lower $p\sim 1\%$ and a steeper slope of $\alpha \sim -0.6$. For paramagnetic (PM) grains, the alignment loss of grains above $1\mu m$ in the inner $\sim 200$ au produces $p<<1\%$ and the polarization hole with $\alpha \sim -0.9$. Grain growth can increase $p$ in the envelope for SPM grains, but cause stronger depolarization for SPM grains in the inner $\sim 500$ au and for PM grains in the entire protostellar core. Finally, we found the increase of polarization angle dispersion function $S$ with iron inclusions and grain growth, implying the dependence of B-field strength measured using the DCF technique on grain alignment and grain properties.

Accepted to MNRAS. 31 pages, 23 figures, 13 pages in Appendix