Formation, fractionation and excitation of carbon monoxide in diffuse clouds

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) = 2×10. 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(CO) <∼ 10 cm. 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 C in CO. 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−2 the character of the chemistry changes inasmuch as CH is generally undetected while CO suffers no such break.