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

Abstract

Context.A wealth of observations of CO in absorption in diffuse clouds has accumulated in the past decade at and mm-wavelengthsAims.Our aims are threefold: a) To compare the and mm-wave results; b) to interpret CO and CO 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 H-bearing diffuse/H I cloudsResults.The variation of (CO) with (H) is explained by quiescent recombination of a steady fraction (HCO)/(H) = . Observed (CO))/(CO) 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 (CO), not (H), so the CO-H conversion factor varies widely; it attains typical values at (CO) cm. Models of CO rotational excitation account for the line brightnesses and CO-Hconversion 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 results generally agree well but the former show somewhat more enhancement of C in CO. In any case, fractionation may seriously bias C/C ratios measured in CO and other co-spatial molecules. Complete C CO conversion must occur over a very narrow range of and (H) just beyond the diffuse regime. For (H) cmthe character of the chemistry changes inasmuch as CH is generally undetected while CO suffers no such break. © 2007 ESO.