SMA Seminar 02/12/2021

With the aim of characterizing the role played by magnetic fields in the formation of young protostars, several recent studies have revealed unprecedented features toward high angular resolution ALMA dust polarization observations of Class 0 protostellar cores. Especially, the dust polarization has been found to be enhanced along the cavity walls of bipolar outflows, which are subject to high irradiation from the reprocessed radiation field emanating from the central protostar. In addition, highly polarized dust thermal emission has been detected in regions most likely linked with the infalling envelope, in the form of filamentary structures, being potential magnetized accretion streamers. These observations allow us to investigate the physical processes involved in the Radiative Alignment Torques (RATs) acting on dust grains from the core to disk scales. Notably, we propose that the polarized emission we see at millimeter wavelengths along the irradiated cavity walls can be reconciled with the expectations of RAT theory if the aligned grains present in these cavities have grown larger than what is typically expected in young protostellar cores. To approach an estimation of the efficiency of dust alignment in protostars, we gathered a large sample of ALMA dust polarization observations of Class 0 protostars, to perform a statistical analysis examining the trend between the dispersion of polarization position angles and the fractional polarization. We report a significant correlation between these two quantities, whose power-law index differs significantly from the one observed by Planck in star-forming clouds, confirming the different nature for the disorganized component of magnetic fields at core scales. The grain alignment efficiency is surprisingly constant across three orders of magnitude in envelope column density. Synthetic observations of non-ideal magneto-hydrodynamic simulations of protostellar cores implementing RATs, show that the ALMA values of grain alignment efficiency lie among those predicted by a perfect alignment of grains, and are significantly higher than the ones obtained with RATs. Ultimately, our results suggest dust alignment mechanism(s) are efficient at producing polarized dust emission in the local conditions typical of Class 0 protostars. The grain alignment efficiency found in these objects seems to be higher than the efficiency produced by the standard RAT alignment of paramagnetic grains. We performed further detailed modelling of the protostellar inner envelope physical conditions, alongside tentative comparisons between ALMA molecular line observations of UV-sensitive chemical tracers and dust polarization observations. We found that indeed, super-paramagnetic grains, high irradiation conditions (comforted by the chemical observations), and large grains (>10μm) are necessary to reproduce the observed grain alignment efficiency. However, further study leading to a better characterization of dust grain characteristics, or additional grain alignment mechanisms, will be required to investigate the cause of strong polarized dust emission located in regions of the envelope where alignment conditions are not favorable.