Radiative torque alignment: Essential Physical Processes

TL;DR

This paper investigates the physical processes influencing grain alignment by radiative torques, emphasizing the roles of thermal fluctuations, gaseous bombardment, and H$_2$ formation torques, validating the analytical model against numerical calculations.

Contribution

It extends the analytical model of RATs to include thermal fluctuations and gaseous effects, demonstrating their impact on grain alignment and validating the model with numerical simulations.

Findings

Gaseous bombardment can enhance grain alignment by moving grains to high-J attractor points.

Thermal fluctuations create low-J attractor points in grains with triaxial ellipsoid shapes.

H$_2$ formation torques influence angular momentum at high-J attractor points.

Abstract

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 Paper I. We focus our discussion on the alignment by anisotropic radiation flux with respect to magnetic field, which defines the axis of grain Larmor precession. Such an alignment does not invoke paramagnetic dissipation (i.e. 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 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. We study effects of stochastic gaseous bombardment and show that gaseous bombardment can drive grains from low-$J$ to high-$J$ attractor points in cases when the 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 in 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 point, 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 validate the use of the AMO for realistic situations of RAT alignment.