Radiative Torque Alignment: Essential Physical Processes

We study physical processes that affect the alignment of grains subject to radiative torques (RATs). To describe the action of the RATs we use the analytical model (AMO) of RATs introduced in Paper I, namely, in Lazarian & Hoang (2007). We focus our discussion on the RAT alignment by anisotropic radiation flux in respect to magnetic field. The latter defines the axis of grain Larmor precession. Such an alignment does not invoke paramagnetic, i.e. Davis-Greenstein, dissipation, but, nevertheless, grains tend to align with long axes perpendicular to magnetic field. We use phase space trajectory maps to describe the alignment. When we account for thermal fluctuations within grain material, we show that for grains, which 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. Value at the latter point is the order of thermal angular momentum corresponding to the grain temperature. We show that the alignment of grains with long axes parallel to magnetic field (“wrong alignment”) reported in Paper I, for situations when the direction of radiative flux was nearly perpendicular to magnetic field, disappears in the present of thermal fluctuations. Thus all grains get aligned with their long axes perpendicular to magnetic field. To get physical insight into the origin and stability of the low-J attractor points we provide a detailed study of the dynamics of the crossovers in the presence of both RATs and thermal fluctuations. We show effects that stochastic gaseous bombardment drives grains from low-J to high-J attractor points in the cases when the high-J attractor points are present. As the alignment of grain axes in respect to angular momentum is higher for higher values of J , counter-intuitively, gaseous bombardment can increase the degree of grain alignment in respect to magnetic field. We consider the effects of torques, induced by H2 formation and show that those change the value of angular momentum at high-J attractor point, but marginally affect low-J attractor points. We compare the AMO results with those obtained with direct numerical calculations of RATs acting upon irregular grains.