Formation, fractionation and excitation of carbon monoxide in diffuse clouds

TL;DR

This paper investigates the formation, fractionation, and excitation of CO in diffuse clouds, comparing observational data with models to understand the physical processes and how they influence CO abundance and excitation temperatures.

Contribution

It introduces a simple phenomenological model linking CO formation to HCO+ recombination and evaluates how well models explain observed CO properties in diffuse clouds.

Findings

N(CO) variation explained by steady HCO+ recombination

N(12CO)/N(13CO) ratios mainly result from competing processes

CO line brightness correlates with N(CO), affecting the CO-H2 conversion factor

Abstract

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.