标题： 关于辐射扭矩和磁弛豫引起晶粒对齐和破裂的通用理论的研究 显示英文标题

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

摘要： 我们将磁增强辐射扭矩（MRAT）对齐理论推广到由无量纲参数$U/(n_{1}T_{2})$描述的一般天体物理环境中，其中$U$表示局部辐射强度，$n_{1}=n_{\rm H}/(10{\rm cm}^{-3})$表示氢密度，$T_{2}=T_{\rm gas}/100{\rm K}$表示气体温度。我们首先推导出产生不同RAT模型和局部条件下高J吸引子所需的临界磁弛豫$\delta_{\rm mag,cri}$，并发现对于更强的辐射场，$\delta_{\rm mag,cri}$必须更大。然后，我们通过数值研究考虑气体碰撞和磁场涨落的情况下，颗粒通过MRAT机制的对齐和旋转破坏。 我们发现，在碰撞主导（CD）区域（$U/(n_{1}T_{2})\leq 1$），碰撞和磁场激发可以缓慢地将大颗粒从低-J旋转传输到高-J吸引子，由于磁弛豫引起的各向异性转动（MRATs），在$\sim 10-100$阻尼时间内导致完美的{\it 慢速对齐}。然而，在辐射主导（RD）区域（$U/(n_{1}T_{2})>1$），只有部分颗粒能够通过MRATs在高-J吸引子处快速对齐，由于强烈的辐射扭矩，大多数颗粒被困在低-J旋转中，这是一种我们称之为{\it 辐射扭矩（RAT）捕获}的新效应。对于极端的辐射场（$U/(n_{1}T_{2})>10^{4}$），磁弛豫对颗粒对齐的效率受到抑制，颗粒只能有完全由各向异性转动（RATs）决定的快速对齐和破坏。 我们量化了不同RAT模型、磁场弛豫和$U/(n_{1}T_{2})$条件下，在高J吸引子处快速对齐的颗粒比例$f_{\rm high-J}^{\rm fast}$，发现最大$f_{\rm high-J}^{\rm fast}$可以达到由MRATs引起的$45\%$和由RATs引起的$22\%$。 显示英文摘要

摘要： 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.