Radiative torque alignment: essential physical processes
Radiative torque alignment: essential physical processes
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DOI：
10.1111/j.1365-2966.2008.13249.x
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发表时间：
2008-07-21
影响因子：
4.8
通讯作者：
Lazarian, A.
中科院分区：
文献类型：
--
作者：
Hoang, Thiem;Lazarian, A.
我们研究的物理过程中，影响排列的晶粒受到辐射扭矩（RAT）。为了描述RAT的作用，我们使用了在我们以前的论文中介绍的RAT的分析模型（AMO）。我们的讨论集中在各向异性辐射通量相对于磁场的取向上，磁场定义了晶粒拉莫尔进动的轴。这种排列不引起顺磁耗散（即戴维斯-格林斯坦机制），但是，尽管如此，晶粒倾向于与垂直于磁场的长轴对齐。当我们考虑到晶粒内的热波动，我们表明，对于其特征在于由一个三轴椭球体的惯性，在我们早期的研究中获得的零J吸引子点发展成一个低J吸引子点的晶粒。低J吸引子点处的角动量值与对应于晶粒温度的热角动量数量级。我们表明，当辐射通量的方向几乎垂直于磁场的情况下，与长轴平行的磁场（即“错误的排列”）在我们以前的论文中报道的晶粒的排列，消失在热波动的存在。因此，所有的晶粒都以其长轴垂直于磁场的方式排列。我们研究了随机气体轰击的影响，并表明，气体轰击可以驱动晶粒从低J高J吸引点的情况下，当高J吸引点。由于晶粒轴相对于角动量的对准对于J的值越高越高，因此与直觉相反，气体轰击可以增加晶粒相对于磁场的对准程度。我们还研究了由H（2）形成引起的力矩的影响，并表明它们可以改变高J吸引子点的角动量值，但对低J吸引子点的角动量值的影响很小。我们比较AMO的结果与那些使用直接数值计算的RAT作用于不规则的晶粒，我们验证使用AMO的现实情况下的RAT对齐。
We study the physical processes that affect the alignment of grains subject to radiative torques (RATs). To describe the action of RATs, we use the analytical model (AMO) of RATs introduced in our previous paper. We focus our discussion on the alignment by anisotropic radiation flux with respect to the magnetic field, which defines the axis of grain Larmor precession. Such an alignment does not invoke paramagnetic dissipation (i.e. the Davis-Greenstein mechanism), but, nevertheless, grains tend to be aligned with long axes perpendicular to the magnetic field. When we account for thermal fluctuations within grains, we show that for grains that are characterized by a triaxial ellipsoid of inertia, the zero-J attractor point obtained in our earlier study develops into a low-J attractor point. The value of angular momentum at the low-J attractor point is of the order of the thermal angular momentum corresponding to the grain temperature. We show that, for situations when the direction of radiative flux is nearly perpendicular to a magnetic field, the alignment of grains with long axes parallel to the magnetic field (i.e. 'wrong alignment') reported in our previous paper, disappears in the presence of thermal fluctuations. Thus, all grains are aligned with their long axes perpendicular to the magnetic field. We study the effects of stochastic gaseous bombardment and show that gaseous bombardment can drive grains from low-J to high-J attractor points in cases when high-J attractor points are present. As the alignment of grain axes with respect to angular momentum is higher for higher values of J, counter-intuitively, gaseous bombardment can increase the degree of grain alignment with respect to the magnetic field. We also study the effects of torques induced by H(2) formation and show that they can change the value of angular momentum at high-J attractor points, but marginally affect the value of angular momentum at low-J attractor points. We compare the AMO results with those obtained using the direct numerical calculations of RATs acting upon irregular grains and we validate the use of the AMO for realistic situations of RAT alignment.