Toward a General Theory of Grain Alignment and Disruption by Radiative Torques and Magnetic Relaxation

Abstract We generalize the magnetically enhanced radiative torque (MRAT) theory for astrophysical environments described by a dimensionless parameter U /( n 1 T 2 ) with U the radiation strength, n 1 = n H /(10 cm −3 ) the normalized hydrogen density, and T 2 = 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 /( n 1 T 2 ). 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 /( n 1 T 2 ) ≤ 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 /( n 1 T 2 ) &gt; 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 /( n 1 T 2 ) &gt; 10 4 , 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, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">high</mml:mi> <mml:mo>−</mml:mo> <mml:mi mathvariant="normal">J</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">fast</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> , for different RAT models, magnetic relaxation, and U /( n 1 T 2 ), and found that the maximum <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi>f</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">high</mml:mi> <mml:mo>−</mml:mo> <mml:mi mathvariant="normal">J</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">fast</mml:mi> </mml:mrow> </mml:msubsup> </mml:math> can reach 45% by MRATs and 22% by RATs. We discuss the implications of our results for describing alignment and disruption of astrophysical dust.