Photoelectric heating of interstellar gas

Abstract

Photoelectric emission from interstellar grains and its consequences for the heating and cooling of interstellar gas are reexamined. The physics of photoemission is briefly reviewed, the photoemissive properties of interstellar grains are examined, and the absorption efficiency and total geometric cross section of the grains are estimated. Some aspects of gas-grain interactions are considered, the interstellar UV energy density is discussed, and the grain potential and heating rate are evaluated for the interstellar UV background as well as for a wide range of electron densities and temperatures. Steady-state solutions for the gas are obtained, and their relevance to the interstellar medium is briefly assessed. The time-dependent cooling of a fossil H II region is analyzed, and it is suggested that fossil H II in the wakes of runaway O stars can produce significant amounts of intermediate-temperature (about 500 to 3000 K) H I. It is concluded that: (1) interstellar grains are likely to have effective photoelectric thresholds of approximately 8 eV and (2) photoemission from grains is a major contributor to the heating of the cool phase of the interstellar medium.