SIMBAD references

A wealth of observations of CO in absorption in diffuse clouds has accumulated in the past decade at uv and mm-wavelengths 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. 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 The variation of N(CO) with N(H₂) is explained by quiescent recombination of a steady fraction n(HCO⁺)/n(H₂)=2x10^–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)≲10¹⁶cm^–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. 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 A_V and N(H₂) just beyond the diffuse regime. For N(H₂)<7x10¹⁹cm^–2 the character of the chemistry changes inasmuch as CH is generally undetected while CO suffers no such break.