Formation, fractionation, and excitation of carbon monoxide in diffuse clouds
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, 22903-2475, USA e-mail: email@example.com
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 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 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. 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) 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 CO conversion must occur over a very narrow range of AV and N(H2) just beyond the diffuse regime. For N(H2) < 7 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