A&A 398, 621-630 (2003)
Gas-phase recombination, grain neutralization and cosmic-ray ionization in diffuse gasH. Liszt
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
(Received 17 September 2002 / Accepted 12 November 2002)
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 H 2 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.
Key words: ISM: general -- ISM: cosmic rays -- ISM: atoms -- ISM: molecules
© ESO 2003