[112.20U] H I Self-Absorption Toward Molecular Clouds: Theoretical Models

E. Flynn, J.M. Jackson, R. Simon, R.Y. Shah, T.M. Bania (Institute for Astrophysical Research, Boston University), M. Wolfire (University of Maryland)

Although 21 cm H I self-absorption is commonly observed toward molecular clouds, the origin of the cold atomic hydrogen remains unclear. Two mechanisms have been proposed: (1) photodissociation of molecular hydrogen on the skins of the molecular clouds exposed to ultraviolet radiation, or (2) cosmic ray chemistry deep inside the molecular clouds. We study H I self-absorption toward molecular clouds by modeling the chemical and thermal structure of molecular clouds exposed to ultraviolet radiation fields and calculating the radiative transfer of 21 cm radiation through these model clouds. We use the models of Kaufmann et al. (1999), as modified by Wolfire et al. (2003), which compute the physical and chemical conditions in a molecular cloud as a function of extinction for various ultraviolet radiation fields, densities, and metallicities. We find that all model molecular clouds contain enough opacity in cold H I to exhibit self-absorption against strong 21 cm backgrounds (TB ~50 - 100 K). The H I absorption opacity is dominated by cold atomic hydrogen formed by cosmic ray chemistry deep in the interiors of clouds. If all molecular clouds contain as much cold atomic hydrogen as the models suggest, then the presence or absence of H I self-absorption toward a molecular cloud can constrain its location in the Galaxy. In particular, in the inner Galaxy, the H I self-absorption can be used to resolve the kinematic distance ambiguity and therefore establish accurate distances to galactic molecular clouds

This work was supported by NSF grant AST-9800334 and AST--0098562.