Magnetic fields (B) are an important factor that controls the star
formation process. The leading method to observe B is using
polarized thermal emission from dust grains aligned with B. However,
in dense environments such as protostellar cores, dust grains may have
inefficient alignment due to strong gas randomizations, so that using dust
polarization to trace B is uncertain. Hoang & Lazarian (2016)
demonstrated that the grain alignment by RAdiative Torques is enhanced if dust
grains contain embedded iron inclusions. Here we extend POLARIS code to study
the effect of iron inclusions on grain alignment and thermal dust polarization
toward a protostellar core, assuming uniform magnetic fields. We found that
paramagnetic grains produce a low polarization degree of p∼1% in the
envelope and negligible p≪1% in the central region due to the loss of
grain alignment. In contrast, grains with a high level of iron inclusions can
have perfect alignment and produce high p∼40% in the envelope and low
p≤10% in the central region. Grains with a moderate level of iron
inclusions induce the polarization flipping from P∥B at millimeter to P⊥B at
submillimeter due to the change in the internal alignment caused by slow
internal relaxation. The weak alignment of very large grains of a≥10μm reduces the polarization by dichroic extinction at submillimeter
wavelengths. We found a positive correlation between p and the level of iron
inclusions, which opens a new window to constrain the abundance of irons locked
in dust through dust polarimetry.Comment: 35 pages, 25 figure