Forces on Dust Grains Exposed to Anisotropic Interstellar Radiation Fields

Grains exposed to anisotropic radiation fields are subjected to forces due tothe asymmetric photon-stimulated ejection of particles. These forces act inaddition to the ``radiation pressure'' due to absorption and scattering. Herewe model the forces due to photoelectron emission and the photodesorption ofadatoms. The ``photoelectric'' force depends on the ambient conditions relevantto grain charging. We find that it is comparable to the radiation pressure whenthe grain potential is relatively low and the radiation spectrum is relativelyhard. The calculation of the ``photodesorption'' force is highly uncertain,since the surface physics and chemsitry of grain materials are poorlyunderstood at present. For our simple yet plausible model, the photodesorptionforce dominates the radiation pressure for grains with size >~0.1 micronexposed to starlight from OB stars. We find that the anisotropy of theinterstellar radiation field is ~10% in the visible and ultraviolet. Weestimate size-dependent drift speeds for grains in the cold and warm neutralmedia and find that micron-sized grains could potentially be moved across adiffuse cloud during its lifetime.Comment: LaTeX(41 pages, 19 figures), submitted to Ap