Carbon monoxide in collapsing interstellar clouds

Abstract

Calculations are made for the energy loss rates, brightness temperatures, and line profiles of carbon monoxide in collapsing interstellar clouds. The most recent data for the H2-CO collision rates have been used in the calculations; a useful extrapolation of these data to high rotational levels is given. The density distribution and velocity field inside the cloud are varied, with one model applying to a cloud of uniform density that is collapsing in free fall and a second to a cloud in isothermal collapse. Analytical relations for total CO cooling at low and high densities are derived. The effects of varying the cross sections as well as the density distribution, velocity field, and geometry of the clouds are discussed. The line profiles are a sensitive tool for testing the models against the observations. The observed CO lines can be explained by collapsing models in which both density and velocity gradients occur. The intensity ratio of the second-to-first deexcitation transition and the first-to-ground deexcitation transition CO lines in Orion is consistent with such models. It is proposed that CO infrared lines from dense interstellar clouds should be observable with instruments of high spectral resolution at wavelengths greater than 200 microns.