The SOMA-POL Survey. I. Polarization and magnetic field properties of massive protostars

TL;DR

This study uses sub-millimeter polarimetric observations to analyze magnetic field orientations and dust properties in 29 massive protostars, revealing a bimodal magnetic field alignment and an evolution-related change in dust polarization.

Contribution

It provides new insights into the magnetic field configurations and dust properties in massive star-forming regions using polarimetric data from JCMT-POL2.

Findings

Magnetic fields show a bimodal orientation distribution relative to core elongation.

Polarized fraction anti-correlates with the luminosity-to-mass ratio, indicating dust property evolution.

Grain alignment persists at high intensities, supporting magnetic influence in core dynamics.

Abstract

The role of magnetic fields in regulating the formation of massive stars remains much debated. Here we present sub-millimeter polarimetric observations with JCMT-POL2 at $850\:\mu$m of 13 regions of massive star formation selected from the SOFIA Massive (SOMA) star formation survey, yielding a total of 29 massive protostars. Our investigation of the $p'-I$ relationship suggests that grain alignment persists up to the highest intensities. We examine the relative orientations between polarization-inferred magnetic field direction and source column density elongation direction on small and large scales. On small scales, we find a bimodal distribution of these relative orientations, i.e., with an excess of near-parallel and near-perpendicular orientations. By applying a one-sample Kuiper test and Monte Carlo simulations to compare to a relative orientation distribution drawn from a uniform distribution, we statistically confirm this bimodal distribution, independent of the methods to measure structural orientation. This bimodal distribution suggests that magnetic fields are dynamically important on the local scales ($\lesssim 0.6\:$pc) of massive protostellar cores. We also examine how basic polarization properties of overall degree of polarization and local dispersion in polarization vector orientations depend on intrinsic protostellar properties inferred from spectral energy distribution (SED) modeling. We find a statistically significant anti-correlation between the debiased polarized fraction and the luminosity to mass ratio, $L_{\rm bol}/M_{\rm env}$, which hints at a change in the dust properties for protostellar objects at different evolutionary stages.