Electric Dipole Radiation from Spinning Dust Grains

We discuss the rotational excitation of small interstellar grains and the resulting electric dipole radiation from spinning dust. Attention is given to excitation and damping of grain rotation by: collisions with neutrals; collisions with ions; “plasma drag”; emission of infrared radiation; emission of electric dipole radiation; photoelectric emission; and formation of H2 on the grain surface. Electrostatic “focussing” can substantially enhance the rate of rotational excitation of grains colliding with ions. Under some conditions, “plasma drag” – due to interaction of the electric dipole moment of the grain with the electric field produced by passing ions – dominates both rotational damping and rotational excitation. We introduce dimensionless functions F and G which allow direct comparison of the contributions of different mechanisms to rotational drag and excitation. Emissivities are estimated for dust in different phases of the interstellar medium, including diffuse HI clouds, warm HI, low-density photoionized gas, and cold molecular gas. Spinning dust grains could explain much, and perhaps all, of the 14 50 GHz background component recently observed by Kogut et al. (1996), de Oliveira-Costa et al. (1997) and Leitch et al. (1997). Future sensitive measurements of angular structure in the microwave sky brightness from the ground and from space should detect this emission from high-latitude HI clouds. It should be possible to detect rotational emission from small grains by ground-based pointed observations of molecular clouds. Subject headings: ISM: Atomic Processes, Dust, Radiation; Cosmic Microwave Background