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Paper: HI Self-Absorption Toward Molecular Clouds: Theoretical Models
Volume: 317, Milky Way Surveys: The Structure and Evolution of Our Galaxy
Page: 44
Authors: Flynn, E.S.; Jackson, J.M.; Simon, R.; Shah, R.Y.; Bania, T.M.; Wolfire, M.
Abstract: We study HI 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 find that all model molecular clouds contain enough opacity in cold HI to exhibit self-absorption against strong 21 cm backgrounds (TBG∼ 75—100 K). Cold atomic hydrogen formed by cosmic ray chemistry deep in the interiors of clouds dominates the H I opacity. If all molecular clouds contain as much cold atomic hydrogen as the models suggest, then the presence or absence of HI self-absorption toward a molecular cloud can constrain its location in the Galaxy. In particular, in the inner Galaxy, the HI self-absorption may resolve the kinematic distance ambiguity and, therefore, establish accurate distances to galactic molecular clouds.
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