Momentum Exchange Between Small Particles and Light

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Abstract
The transfer of linear and angular momentum between small particles and a radiation field is discussed, with a view to evaluating the importance of the exchange of angular momentum to the alignment of interstellar grains. To this end expressions for the two diffusion coefficients in the Fokker–Planck equation governing the steady-state momentum distribution are derived. When the separate contributions of absorption, emission and scattering are identified it becomes clear that in the interstellar medium emission dominates the angular momentum transfer. This supports the earlier conclusion made by Purcell & Spitzer that the disorienting effects of this process are the most important. Some numerical examples are given to compare the efficiency of the process with that of other alignment mechanisms; in particular the possible consequences for magnetic alignment are examined.

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