Radiation transport and non-LTE analysis of interstellar molecular lines. I - Carbon monoxide

We present numerical solutions to the coupled equations of radiative transfer and statistical equilibrium in the 10 lowest rotational transitions of carbon monoxide. Theoretical models are constructed for giant molecular clouds associated with H ii regions and for simple dark dust clouds, using static, spherical models and a microturbulent velocity field. Results are presented for a range of CO abundances and optical depths. Available observational evidence is summarized and detailed comparisons are made in terms of the intensity, shape, and width of the computed line profiles. Our calculations reproduce successfully a large body of observations of 12CO and `8CO in thej = 1-0 and j = 2-1 transitions, as long as the relative abundance of 12CO to molecular hydrogen [12CO]I[H2] (1-4) x 10- . Consequently, the 2CO optical depths in the j = 1-0 line are smaller than previously assumed, by a factor of 10, giving r (1-4). A non-LTE analysis of the excitation mechanism is made and the importance of line interlocking at small optical depths is discussed. Enhancement of the 13CO line under non-LTE conditions and large saturation effects in the 12CO line (even at r > 1) help to reduce the intensity ratio of `2CO/13CO in the j = 1-0 transition to the observed value of 4, even with an isotopic ratio of 40. Other implications of these results are discussed. Subject headings: interstellar: molecules - radiative transfer