Forces on Dust Grains Exposed to Anisotropic Interstellar Radiation Fields

Grains exposed to anisotropic radiation fields are subjected to forces due to the asymmetric photon-stimulated ejection of particles. These forces act in addition to the ``radiation pressure'' due to absorption and scattering. Here we model the forces due to photoelectron emission and the photodesorption of adatoms. The ``photoelectric'' force depends on the ambient conditions relevant to grain charging. We find that it is comparable to the radiation pressure when the grain potential is relatively low and the radiation spectrum is relatively hard. The calculation of the ``photodesorption'' force is highly uncertain, since the surface physics and chemsitry of grain materials are poorly understood at present. For our simple yet plausible model, the photodesorption force dominates the radiation pressure for grains with size >~0.1 micron exposed to starlight from OB stars. We find that the anisotropy of the interstellar radiation field is ~10% in the visible and ultraviolet. We estimate size-dependent drift speeds for grains in the cold and warm neutral media and find that micron-sized grains could potentially be moved across a diffuse cloud during its lifetime.Comment: LaTeX(41 pages, 19 figures), submitted to Ap

Two-Dimensional Surface Magnetism in the Bulk Paramagnetic Intermetallic Alloy CoAl(100)

Utilizing a combination of the in situ magneto-optical Kerr effect and scanning tunneling microscopy and spectroscopy measurements, we show that the (100) surface of the B2 bulk paramagnetic CoAl is an excellent representation of a two-dimensional ferromagnet. The order-parameter critical exponent beta=0.22 +/- 0.02 is determined, which is the universal signature of a finite-size two-dimensional XY behavior. The Curie temperature is found to be T-c=90 K. The magnetism can be explained by the appearance of Co antisite atoms at the surface