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

TL;DR

This paper extends the theory of grain alignment by radiative torques and magnetic relaxation, analyzing how different astrophysical conditions influence grain alignment efficiency and disruption, with implications for understanding interstellar dust behavior.

Contribution

It generalizes the MRAT alignment theory to various environments, derives critical magnetic relaxation parameters, and studies the effects of radiation and magnetic fluctuations on grain alignment and disruption.

Findings

Magnetic relaxation critical value depends on radiation strength.

Slow alignment occurs in collision-dominated regimes over 10-100 damping times.

Maximum fast high-J alignment fraction reaches 45% with MRATs.

Abstract

We generalize the magnetically enhanced radiative torque (MRAT) alignment theory for general astrophysical environments described by a dimensionless parameter $U/(n_{1}T_{2})$ with $U$ local radiation strength, $n_{1}=n_{\rm H}/(10{\rm cm}^{-3})$ the hydrogen density, and $T_{2}=T_{\rm gas}/100{\rm K}$ the gas temperature. We first derive the critical magnetic relaxation $\delta_{\rm mag,cri}$ required to produce high-J attractors for different RAT models and local conditions and find that $\delta_{\rm mag,cri}$ must be larger for stronger radiation fields. We then numerically study the grain alignment and rotational disruption by the MRAT mechanism taking into account gas collisions and magnetic fluctuations. We find that, for the collision-dominated (CD) regime ($U/(n_{1}T_{2})\leq 1$), collisional and magnetic excitations can slowly transport large grains from low-J rotation to high-J attractors, leading to the perfect {\it slow alignment} within $\sim 10-100$ damping times due to MRATs. However, for the radiation-dominated (RD) regime ($U/(n_{1}T_{2})>1$), only a fraction of grains can be fast aligned at high-J attractors by MRATs, and the majority of grains are trapped at low-J rotation due to strong radiative torques, a new effect we term {\it radiative torque (RAT) trapping}. For extreme radiation fields of $U/(n_{1}T_{2})>10^{4}$, 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, $f_{\rm high-J}^{\rm fast}$, for different RAT models, magnetic relaxation, and $U/(n_{1}T_{2})$, and found that the maximum $f_{\rm high-J}^{\rm fast}$ can reach $45\%$ by MRATs and $22\%$ by RATs.