The Production of H I in Photodissociation Regions and a Comparison with CO (1-0) Emission

The gas at the surfaces of molecular clouds in galaxies is heated and dissociated by photons from young stars both near and far. H I resulting from the dissociation of molecular hydrogen H2 emits hyperfine line emission at 21 cm, and warmed CO emits dipole rotational lines such as the 2.6 mm line of CO (1-0). We use previously developed models for photodissociation regions (PDRs) to compute the intensities of these H I and CO (1-0) lines as a function of the total volume density n in the cloud and the far-ultraviolet (FUV) flux G0 incident on it and present the results in units familiar to observers. The intensities of these two lines behave differently with changing physical conditions in the PDR, and taken together, the two lines can provide a ground-based radio astronomy diagnostic for determining n and G0 separately in distant molecular clouds. This diagnostic is particularly useful in the range G0 ≲ 100, 10 cm-3 ≲ n ≲ 105 cm-3, which applies to a large fraction of the volume of the interstellar medium in galaxies. If the molecular cloud is located near discrete sources of far-UV (FUV) emission, the PDR-generated H I and CO (1-0) emission on the cloud surface can be more easily identified, appearing as layered "blankets" or "blisters" on the side of the cloud nearest the FUV source. As an illustration, we consider the Galactic object G216-2.5, i.e., "Maddalena's Cloud," which has been previously identified as a large PDR in the Galaxy. We determine that this cloud has n ≈ 200 cm-3 and G0 ≈ 0.8, consistent with other data.