Is the mm/submm dust polarization a robust tracer of the magnetic field topology in protostellar envelopes? A model exploration

TL;DR

This study evaluates how reliably (sub-)millimeter dust polarization traces magnetic field structures in protostellar envelopes, finding it generally effective but with limitations in disorganized or high-opacity regions.

Contribution

The paper provides a comprehensive analysis of the fidelity of dust polarization as a magnetic field tracer in protostellar environments using MHD models.

Findings

Dust polarization accurately traces B-field topology within ~100 au.

Orientation measurements are precise within 15 degrees in most cases.

Discrepancies occur in highly disorganized or high-opacity lines of sight.

Abstract

High resolution (sub-)millimeter polarization observations have opened a new era in the understanding of how B-fields are organized in star forming regions, unveiling an intricate interplay between the B-fields and the gas in protostellar cores. However, to assess the role of the B-field in the process of solar-type star formation, it is key to be able to understand to what extent these polarized dust emissions are good tracers of the B-field in the youngest protostellar objects. We present a thorough investigation of the fidelity and limitations of using dust polarized emission to map the B-field topologies in low-mass protostars. To assess the importance of these effects, we performed the analysis of B-field properties in 27 realizations of MHD models of star-forming cores. Assuming a uniform population of dust grains whose sizes follow the standard MRN, we analyze the synthetic polarized dust emission maps produced if these grains align with the local B-field thanks to B-RATs. We find that (sub-)millimeter polarized dust emission is a robust tracer of the B-field topologies in inner protostellar envelopes and is successful at capturing the details of the B-field spatial distribution down to radii ~100 au. Measurements of the los averaged B-field orientation using the polarized dust emission are precise to < 15{\deg} in about 75 - 95% of the independent lines of sight peering through protostellar envelopes. Large discrepancies between the integrated B-field mean orientation and the orientation reconstructed from the polarized dust emission are mostly observed in (i) lines of sight where the B-field is highly disorganized and (ii) lines of sight probing large column densities. Our analysis shows that high opacity of the thermal dust emission and low polarization fractions could be used to avoid utilizing the small fraction of measurements affected by large errors.