A statistical analysis of dust polarization properties in ALMA
  observations of Class 0 protostellar cores
  Recent observational progress has challenged the dust grain-alignment
theories used to explain the polarized dust emission routinely observed in
star-forming cores. In an effort to improve our understanding of the dust grain
alignment mechanism(s), we have gathered a dozen ALMA maps of
(sub)millimeter-wavelength polarized dust emission from Class 0 protostars, and
carried out a comprehensive statistical analysis of dust polarization
quantities. We analyze the statistical properties of the polarization fraction
P_frac and dispersion of polarization position angles S. More specifically, we
investigate the relationship between S and P_frac as well as the evolution of
the product S*P_frac as a function of the column density of the gas in the
protostellar envelopes. We find a significant correlation in the polarized dust
emission from protostellar envelopes seen with ALMA; the power-law index
differs significantly from the one observed by Planck in star-forming clouds.
The product S*P_frac, which is sensitive to the dust grain alignment
efficiency, is approximately constant across three orders of magnitude in
envelope column density. This suggests that the grain alignment mechanism
producing the bulk of the polarized dust emission in star-forming cores may not
depend systematically on the local conditions such as local gas density.
Ultimately, our results suggest dust alignment mechanism(s) are efficient at
producing dust polarized emission in the various 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. Further study will be needed to understand how more
efficient grain alignment via, e.g., different irradiation conditions, dust
grain characteristics, or additional grain alignment mechanisms can reproduce
the observations.
