Gas-phase recombination, grain neutralization and cosmic-ray ionization in diffuse gas

Abstract

Atomic ions are mostly neutralized by small grains (or PAH molecules) in current theories of heating and cooling in cool diffuse clouds; in the main they do not recombine with free electrons. This alters the ionization balance by depressing n(H- ) and n(He+) while carbon generally remains nearly fully once-ionized; charge exchange with atomic oxygen and formation of H2 and OH also depress n(H+) in partly molecular gas. Seemingly restrictive empirical limits on ζH are relaxed and higher values for ζH are favored in a wide range of circumstances, when grain neutralization is recognized. Maintenance of the proton density at levels needed to reproduce observations of HD requires ζH ≳H 2 × 10-16 s-1, but such models naturally explain the presence of both HD and H3+ in relatively tenuous H I clouds. In dense gas, a higher ionization rate can account for high observed fractions of atomic hydrogen, and recognition of the effects of grain neutralization can resolve a major paradox in the formation of sulfur-bearing compounds.