Journal
ASTRONOMY & ASTROPHYSICS
Volume 398, Issue 2, Pages 621-630Publisher
E D P SCIENCES
DOI: 10.1051/0004-6361:20021660
Keywords
ISM : general; ISM : cosmic rays; ISM : atoms; ISM : molecules
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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 zeta(H) are relaxed and higher values for zeta(H) 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 zeta(H) greater than or similar to 2 x 10(-16) s(-1), but such models naturally explain the presence of both HD and H-3(+) 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.
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