EDP Sciences
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Volume 476, Number 1, December II 2007
Page(s) 291 - 300
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20078502

A&A 476, 291-300 (2007)
DOI: 10.1051/0004-6361:20078502

Formation, fractionation, and excitation of carbon monoxide in diffuse clouds

H. S. Liszt

National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, 22903-2475, USA
    e-mail: hliszt@nrao.edu

(Received 18 August 2007 / Accepted 3 October 2007)

Context.A wealth of observations of CO in absorption in diffuse clouds has accumulated in the past decade at uv and mm-wavelengths
Aims.Our aims are threefold: a) To compare the uv and mm-wave results; b) to interpret 13CO and 12CO abundances in terms of the physical processes which separately and jointly determine them; c) to interpret observed J = 1-0 rotational excitation and line brightness in terms of ambient gas properties.
Methods.A simple phenomenological model of CO formation as the immediate descendant of quiescently-recombining HCO+ is used to study the accumulation, fractionation and rotational excitation of CO in more explicit and detailed models of H2-bearing diffuse/H I clouds
Results.The variation of N(CO) with N(H2) is explained by quiescent recombination of a steady fraction n(HCO+)/n(H2) = 2 $\times$ 10-9. Observed N(12CO))/N(13CO) ratios generally do not require a special chemistry but result from competing processes and do not provide much insight into the local gas properties, especially the temperature. J = 1-0 CO line brightnesses directly represent N(CO), not N(H2), so the CO-H2 conversion factor varies widely; it attains typical values at N(12CO) $\la$ 1016 cm-2. Models of CO rotational excitation account for the line brightnesses and CO-H2conversion factors but readily reproduce the observed excitation temperatures and optical depths of the rotational transitions only if excitation by H-atoms is weak - as seems to be the case for the very most recent calculations of these excitation rates.
Conclusions.Mm-wave and uv results generally agree well but the former show somewhat more enhancement of 13C in 13CO. In any case, fractionation may seriously bias 12C/13C ratios measured in CO and other co-spatial molecules. Complete C $\rightarrow$ CO conversion must occur over a very narrow range of $A_{\rm V}$ and N(H2) just beyond the diffuse regime. For N(H2) < 7 $\times$ 1019 cm-2the character of the chemistry changes inasmuch as CH is generally undetected while CO suffers no such break.

Key words: astrochemistry -- molecular processes -- ISM: clouds -- ISM: molecules

© ESO 2007