Issue |
A&A
Volume 461, Number 1, January I 2007
|
|
---|---|---|
Page(s) | 205 - 214 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20065070 | |
Published online | 15 September 2006 |
Time-dependent H2 formation and protonation in diffuse clouds
National Radio Astronomy Observatory, 520 Edgemont Road, 22903-2475 Charlottesville, VA, USA e-mail: hliszt@nrao.edu
Received:
22
February
2006
Accepted:
14
September
2006
Aims.To demonstrate the time-approach to equilibrium of H2-formation and protonation in models of diffuse or H I interstellar gas clouds previously published by the author.
Methods.The microscopic equations of H2-formation and protonation are integrated numerically over time in such a manner that the overall structures evolve self-consistently under benign conditions.
Results.The equilibrium H2 formation timescale in
an H I cloud with N(H) is
yr, nearly independent of the assumed density or
H2 formation rate on grains, etc. Attempts to speed up
the evolution of the H2-fraction would require densities well
beyond the range usually considered typical of diffuse gas.
The calculations suggest that, under benign, quiescent conditions,
H2 in the diffuse ISM formation of H2 is favored in larger
regions having moderate density, consistent with the rather high mean
kinetic temperatures measured in H2, 70-80 K.
Formation of H
is essentially complete when H2-formation
equilibrates but the final abundance of H
appears more nearly
at the very last instant.
Chemistry in a weakly-molecular gas has particular properties so
that the abundance patterns change appreciably as gas becomes more fully
molecular, either in model sequences or with time in a single model.
One manifestation of this is that the predicted abundance of H
is much more weakly dependent on
the cosmic-ray ionization rate when n(H2)/n(H
. In
general, high abundances of H
do not enhance
the abundances of other species (e.g. HCO+) but
late-time OH formation proceeds most vigourously in more diffuse
regions having modest density, extinction and H2 fraction and somewhat
higher fractional ionization, suggesting that atypically high OH/H2
abundance ratios might be found optically in diffuse clouds having
modest extinction.
Key words: ISM: molecules / ISM: atoms / ISM: clouds
© ESO, 2006
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