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

Abstract

Context. High resolution (sub-) millimeter polarization observations have opened a new era in the understanding of how magnetic fields are organized in star forming regions, unveiling an intricate interplay between the magnetic fields and the gas in protostellar cores. However, to assess the role of the magnetic field in the process of solar-type star formation, it is key to be able to understand to what extent these polarized dust emission are good tracers of the magnetic field in the youngest protostellar objects. Aims. In this paper, we present a thorough investigation of the fidelity and limitations of using dust polarized emission to map the magnetic field topologies in low-mass protostars. Methods. To assess the importance of these effects, we have performed the analysis of magnetic field properties in 27 realizations of MHD models following the evolution of physical properties in star-forming cores. Assuming a uniform population of dust grains which 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 Radiative Torques (B-RATs).Results. We find that (sub-)millimeter polarized dust emission is a robust tracer of the magnetic field topologies in inner protostellar envelopes and is successful at capturing the details of the magnetic field spatial distribution down to radii ∼ 100 au. Measurements of the line-of-sight averaged magnetic field line orientation using the polarized dust emission are precise to < 15 • (typical of the error on polarization angles obtained with observations from large mm polarimetric facilities such as ALMA) 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 magnetic 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.

Recent progress with observations and models to characterize the magnetic fields from star-forming cores to protostellar disks

“…When focusing on the smaller envelope radii < 500 au, where the magnetic field lines are more likely perturbed from the initially smooth configuration by infall and outflow and opacity effects kick in, 75% of the lines of sight still give robust results. Finally, Valdivia et al (2022) and Le find that the polarization fraction is correlated to the degree of organization of the magnetic field along the line-of-sight, confirming that the behavior observed at larger scales, in less dense gas, holds true down to the scales of the envelopes of protostars. Le find that the dust alignment efficiency does not significantly vary with local gas density in their observations, and that the synthetic maps only can reproduce the observed polarimetric data when significant irradiation from the central protostar is included.…”

“…Their work analyses result in synthetic polarized dust emission maps, which are compared to the true B-field properties in the models and to observations. Valdivia et al (2022) show that measurements of the line-of-sight averaged magnetic field line orientation using the polarized dust emission are precise enough to recover the mean field lines distribution with accuracy < 15deg (typical of the error on polarization angles obtained with observations from large mm polarimetric facilities such as ALMA) in about 95% of the independent lines of sight peering through protostellar envelopes at all radii. When focusing on the smaller envelope radii < 500 au, where the magnetic field lines are more likely perturbed from the initially smooth configuration by infall and outflow and opacity effects kick in, 75% of the lines of sight still give robust results.…”

“…Assessing whether the observations can be trusted to infer statistically robust constraints on the magnetic fields threading protostellar cores, and compared to B-fields in models, requires the analysis of synthetic observations from magnetized models. Le and Valdivia et al (2022) have post-processed outputs from non-ideal MHD models of protostellar evolution with the RAMSES code (Fromang et al, 2006) (see also the work of Kuffmeier et al, 2020 in Figure 1). Their work analyses result in synthetic polarized dust emission maps, which are compared to the true B-field properties in the models and to observations.…”

On Internal and External Alignment of Dust Grains in Protostellar Environments

“…Extensive efforts have been done to model and interpret submillimeter dust polarization from protostellar environments (see, e.g., Valdivia et al 2022). Nevertheless, previous studies usually make simplified assumptions on grain alignment physics for modeling dust polarization.…”