How Important is Neutral Carbon to an Understanding of the Dense Interstellar Medium?

Abstract

In virtually all of the dense interstellar medium, H2 is the most abundant form of hydrogen, but it is not directly observable in the bulk of the gas. As a result, we are forced to use trace constituents of the gas as surrogates when we want to know the distribution of material in the dense ISM. Most commonly, we employ the lowest few rotational transitions of CO and its isotopes as the trace species. One of the most hotly debated issues in the study of the molecular ISM is the extent to which one can trust CO or isotopic CO lines to reflect reliably the underlying H2 distribution (see Shier, Rieke, & Rieke (1994), Sodroski et al. (1995) for recent comments on the I(12CO)/N(H2) ratio and Lada et al. (1994) for a recent analysis of the relationship between isotopic CO and total cloud column densities). CO becomes increasingly unreliable as a tracer of H2 as the average column density between cloud surfaces exposed to ultraviolet photons and the shielded centers of clouds becomes smaller. Young stars in the galactic plane perfuse atomic and molecular clouds with far-UV (λ > 91 nm) radiation. This radiation tends to dissociate CO more readily than it dissociates H2 (Van Dishoeck & Black 1988). The differences in susceptibility of H2 and CO to photo dissociation may lead to the existence of significant portions of the molecular medium where the usual trace species are underabundant or even absent. In addition, there is dense H I at the cloud boundaries, immediately outside the molecular material.