Physical modelling of dust polarization from magnetically enhanced radiative torque alignment in protostellar cores with polaris

ABSTRACT Magnetic fields (B) are an important factor controlling the star-formation process. The leading method to observe B orientation is to use polarized thermal emission from aligned dust grains. In dense environments such as protostellar cores, however, dust grains may be inefficiently aligned owing to strong gas randomizations, making the use of dust polarization to trace Buncertain. The study of Hoang and Lazarian in 2016 demonstrated that grain alignment by radiative torques is enhanced if dust grains contain embedded iron inclusions. Here we extend the polaris code to study the effect of iron inclusions on grain alignment and thermal dust polarization towards a protostellar core, assuming uniform B. We found that paramagnetic grains produce a low polarization degree of $p \sim 1{{\ \rm per\ cent}}$ in the envelope and a negligible $p \ll 1{{\ \rm per\ cent}}$ in the central region owing to the loss of grain alignment. In contrast, grains with a high level of iron inclusions have perfect alignment and produce a high $p \sim 40{{\ \rm per\ cent}}$ in the envelope and a low $p \le 10{{\ \rm per\ cent}}$ in the central region. Grains with a moderate level of iron inclusions induce the polarization flipping from P ‖ B at millimetre to P ⊥ B at submillimetre wavelengths owing to the change in the internal alignment caused by slow internal relaxation. The weak alignment of very large grains with $a \ge 10\, {\mu \rm {m}}$ reduces dichroic extinction efficiency at submillimetre wavelengths. We found a positive correlation between p and the level of iron inclusions, which introduces a new option to constrain the abundance of solid iron locked in dust through dust polarimetry.