On Internal and External Alignment of Dust Grains in Protostellar Environments

Multiwavelength observations toward protostars reveal complex properties of dust polarization, which are challenging to interpret. Here we study the physical processes inducing the alignment of the grain axis of the maximum inertia moment with the angular momentum ( J ; i.e., internal alignment) and of J with the magnetic field (i.e., external alignment) of very large grains (VLGs; of radius a > 10 μm) using the alignment framework based on radiative torques (RATs) and mechanical torques (METs). We derive analytical formulae for critical sizes of grain alignment, assuming grains aligned at low-J and high-J attractors by RATs (METs). For protostellar cores, we find that super-Barnett relaxation induces efficient internal alignment for VLGs with large iron inclusions, but inelastic relaxation is efficient for VLGs regardless of composition aligned at high-J attractors by RATs (METs). For external alignment, VLGs with iron inclusions aligned at high-J attractors have magnetic alignment by RATs (B-RAT) or METs (B-MET), enabling dust polarization as a reliable tracer of magnetic fields in dense regions. Still, grains at low-J attractors or without iron inclusions have alignment with J along the radiation direction (k-RAT) or gas flow (v-MET). For protostellar disks, we find that super-Barnett relaxation is efficient for grains with large iron inclusions in the outer disk thanks to spin-up by METs, but inelastic relaxation is inefficient. VLGs aligned at low-J attractors can have k-RAT (v-MET) alignment, but grains aligned at high-J attractors likely exhibit B-RAT (B-MET) alignment. We also find that grain alignment by METs is more important than that by RATs in protostellar disks.