Radiative Torques on Interstellar Grains. III. Dynamics with Thermal Relaxation

In the previous papers in this series, we found that radiative torques canplay a major role in the alignment of grains with the interstellar magneticfield. Since the radiative torques can drive the grains to suprathermalrotational speeds, in previous work we made the simplifying assumption that thegrain principal axis of greatest moment of inertia is always parallel to thegrain angular momentum. This enabled us to describe many of the features of thegrain dynamics. However, this assumption fails when the grains enter periods ofthermal rotation, which occur naturally in the radiative torque alignmentscenario. In the present paper, we relax this assumption and explore theconsequences for the grain dynamics. We develop a treatment to follow the graindynamics including thermal fluctuations and ``thermal flipping'', and showresults for one illustrative example. By comparing with a treatment withoutthermal fluctuations, we see that inclusion of thermal fluctuations can lead toqualitative changes in the grain dynamics. In a future installment in thisseries, we will use the more complete dynamical treatment developed here toperform a systematic study of grain alignment by radiative torques.Comment: 54 pages, 16 figures, submitted to Ap