On Internal and External Alignment of Dust Grains in Protostellar Environments

eld (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. © 2022. The Author(s). Published by the American Astronomical Society.