Formation, fractionation and excitation of carbon monoxide in diffuse clouds

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 x 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 10^{16}cm^{-2}. Models of CO rotational excitation account for the line brightnesses and CO-H2 conversion 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.