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

National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA

Corresponding author: hliszt@nrao.edu

Received: 17 September 2002
Accepted: 12 November 2002

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  are relaxed and higher values for  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    s^-1, but such models naturally explain the presence of both HD and H3p 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.