The Production of Hiin Photodissociation Regions and a Comparison with CO (1–0) Emission

Abstract

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 H 2 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 G 0 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 G 0 separately in distant molecular clouds. This diagnostic is particularly useful in the range G 0 P100, 10 cm À3 P n P10 5 cm À3 , 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 G216À2.5, 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 À3 and G 0 % 0:8, consistent with other data.

A New Method for Obtaining the Star Formation Law in Galaxies

“…We use a simple "slab" model described initially by Sternberg (1988) and in more detail by Allen (2004), with improved coefficients provided by Allen et al (2004).…”

“…We use the formulation in Appendix A of Allen et al (2004). Briefly, the model is a simple semi-infinite slab geometry in statistical equilibrium with FUV radiation incident on one side, and a H i ↔ H 2 dissociation-reformation equilibrium in the slab on the right side.…”

“…Briefly, the model is a simple semi-infinite slab geometry in statistical equilibrium with FUV radiation incident on one side, and a H i ↔ H 2 dissociation-reformation equilibrium in the slab on the right side. The solution appropriate for our present purposes gives the steady state H i column density along a line of sight perpendicular to the face of the slab as a function of G 0 , the incident UV intensity scaling factor (see Appendix B of Allen et al 2004, for a definition of G 0 ), and the total volume density n of H nuclei in the slab. The result is…”

“…Using the parameter values in this equation adopted by Allen et al (2004), and writing explicitly the dependence on the dust-to-gas ratio (Allen 2004), we have…”

Excitation of H2in photodissociation regions as seen bySpitzer

“…As part of the SPECPDR 1 program dedicated to the study of PDRs with Spitzer, we used this capability to study the H 2 line emission in regions with χ ∼ 5−10 3 times the Solar Neighborhood Far-UV interstellar radiation field 2 as given by Draine (1978). Our 1 See http://www.cesr.fr/~joblin/SPECPDRpublic/SPECPDR.html 2 For a discussion on the definitions of the mean interstellar radiation field used in the literature on PDRs see Appendix B of Allen et al (2004) or Appendix C of Le Petit et al (2006). sample consists of well-known PDRs, so that they are good test cases for models.…”