Photoelectric heating of the interstellar gas.

A model is developed in which the interstellar gas in both clouds and the intercloud medium is heated predominantly by photoelectrons from dust grains. In this model, the intercloud medium is ionized by a steady flux of low-energy cosmic rays, and the cloud medium is ionized by photons longward of 912 A. Ionization equilibrium and chemical abundances are discussed, and the heating and cooling of the gas are calculated. An equation of state for the interstellar gas is obtained for different values of the primary-cosmic-ray ionization rate (zeta) and the photoelectric emission efficiency of the dust (y). One steady-state two-phase model is then compared with observational data. It is concluded that: (1) heating by photoelectrons from dust grains can account for a hot neutral intercloud gas; (2) the interstellar gas is stable against thermal instability during heating by photoelectrons from positively charged dust grains; and (3) reasonable agreement with observed parameters of the interstellar gas can be obtained in a model with zeta equal to 10 to the -16th power per sec and y of 0.1.