Rotational Excitation of CO in the Diffuse Interstellar Medium: Effects of Line Emission from Dense Molecular Clouds
We discuss a somewhat neglected interstellar rotational excitation mechanism that may be significant for CO in diffuse gas (ntot<=300 cm-3). Where such gas is found in the vicinity of a dense interstellar cloud, we show that molecular excitation in the diffuse gas may be dominated by spectral line photons emitted by the denser material. We illustrate the mechanism analytically for a simple two-level case and present the numerical results from a multilevel calculation.In all six of six sight lines that we analyze, it is plausible that the observed rotational excitation results from radiative excitation, so there is no compelling evidence that collisions play an important role. Therefore, densities derived under the assumption of purely collisional excitation would give incorrect (too large) values. Line-of-sight analyses of CO excitation can be (and have been) misinterpreted because of neglect of this contribution to radiative excitation. Similar situations can arise whenever there is a small continuum opacity and a large contrast of gas density within, or around, an interstellar cloud. It is a situation in which the often-used Sobolev (or large-scale velocity gradient) approximation simply does not apply.