EDP Sciences
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Volume 398, Number 2, February I 2003
Page(s) 621 - 630
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20021660

A&A 398, 621-630 (2003)
DOI: 10.1051/0004-6361:20021660

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

H. 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 $\zeta_{\rm H}$ are relaxed and higher values for $\zeta_{\rm H}$ 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 $\zeta_{\rm H}$ $\ga 2\times 10^{-16}$ 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

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