2025ApJ...994..115H

We generalize the magnetically enhanced radiative torque (MRAT) theory for astrophysical environments described by a dimensionless parameter U/(n₁T₂) with U the radiation strength, n₁ = n_H/(10 cm^–3) the normalized hydrogen density, and T₂ = T_gas/100 K the normalized gas temperature. We derive the critical magnetic relaxation δ_mag,cri required to produce high-J attractors for different radiative torque (RAT) models and local conditions, and find that δ_mag,cri must increase with increasing U/(n₁T₂). We numerically study grain alignment and rotational disruption by the MRAT mechanism, considering both gas collisions and magnetic fluctuations. We find that, for the collision-dominated regime (U/(n₁T₂) ≤ 1), collisional and magnetic excitations slowly transport large grains from low-J to high-J attractors, leading to perfect slow alignment within ∼10-100 damping times due to MRAT. However, for the radiation-dominated regime (U/(n₁T₂) > 1), only a fraction of grains can be fast aligned at high-J attractors by MRAT, and the majority of grains are trapped at low-J rotation due to strong RATs, a new effect termed RAT trapping. For extreme conditions of U/(n₁T₂) > 10⁴, the efficiency of magnetic relaxation on grain alignment is suppressed, and grains only have fast alignment and disruption purely determined by RATs. We quantified the fraction of grains with fast alignment at high-J attractors, ${f}_{{\rm{high}}-{\rm{J}}}^{{\rm{fast}}}$, for different RAT models, magnetic relaxation, and U/(n₁T₂), and found that the maximum ${f}_{{\rm{high}}-{\rm{J}}}^{{\rm{fast}}}$ can reach 45% by MRATs and 22% by RATs. We discuss the implications of our results for describing alignment and disruption of astrophysical dust.