Numerical modeling of thermal dust polarization from aligned grains in the envelope of evolved stars with updated POLARIS

TL;DR

This study models thermal dust polarization in the envelope of an evolved star using an updated POLARIS code, revealing how magnetic field strength, grain properties, and disruption processes influence polarization signals.

Contribution

It introduces a detailed numerical model of dust polarization considering grain alignment, magnetic fields, and grain disruption effects in evolved star envelopes.

Findings

Minimum grain size for alignment is 0.007-0.05 μm.

Polarization degree reaches 10% with ordinary grains, up to 40% with superparamagnetic grains.

Grain disruption reduces maximum grain size and polarization degree.

Abstract

Magnetic fields are thought to influence the formation and evolution of circumstellar envelopes around evolved stars. Thermal dust polarization from aligned grains is a promising tool for probing magnetic fields and dust properties in these environments; however, a quantitative study on the dependence of thermal dust polarization on the physical properties of dust and magnetic fields for these circumstellar environments is still lacking. In this paper, we first perform the numerical modeling of thermal dust polarization in the IK Tau envelope using the magnetically enhanced radiative torque (MRAT) alignment mechanism implemented in our updated POLARIS code, accounting for the effect of grain drift relative to the gas. Despite experiencing grain drift and high gas density $n_{\rm gas} > 10^6\,\rm cm^{-3}$, the minimum grain size required for efficient MRAT alignment of silicate grains is $\sim 0.007 - 0.05\,\rm\mu m$ due to strong stellar radiation fields. Ordinary paramagnetic grains can achieve perfect alignment by MRAT in the inner envelope of $r < 500\,\rm au$ due to stronger magnetic fields of $B\sim10$ mG - 1G, producing the polarization degree of $\sim10\%$. The polarization degree can be enhanced to $\sim20-40\%$ for superparamagnetic grains with embedded iron inclusions. The magnetic field geometry affects the resulting polarization degree due to the projection effect. We investigate the effect of rotational disruption by RATs (RAT-D) and find that the RAT-D effect decreases the dust polarization degree due to the decrease in the maximum grain size. Our modeling results motivate further observational studies at far-infrared/sub-millimeter to constrain the properties of magnetic fields and dust in evolved star's envelopes.