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H [CSC]i[/CSC] Self-Absorption and the Kinematic Distance Ambiguity: The Case of the Molecular Cloud GRSMC 45.6+0.3

2002; IOP Publishing; Volume: 566; Issue: 2 Linguagem: Inglês

10.1086/339715

1538-4357

James M. Jackson, T. M. Bania, R. Simon, M. A. Kolpak, D. P. Clemens, M. H. Heyer,

Stellar, planetary, and galactic studies

We report on a three-dimensional (l, b, VLSR) physical association of H I self-absorption with 13CO emission toward the Galactic molecular cloud GRSMC 45.6+0.3. Photochemical models indicate that a significant column density of cool (T ≲ 25 K) H I exists in the skins (Av ≲ 2) of typical Galactic molecular clouds. The H I self-absorption can thus be produced by the cool cloud skin absorbing the warm (TSPIN ~ 100 K) background H I emission that is ubiquitously distributed throughout the Galaxy. Liszt, Burton, and Bania have suggested that because cool H I in a molecular cloud will absorb warmer H I background line emission, H I self-absorption can be used to resolve the kinematic distance ambiguity for molecular clouds in the inner Galaxy. Clouds at the near distance should show H I self-absorption, whereas clouds at the far distance will not since there is no background to absorb. The near kinematic distance of 1.8 kpc for the GRSMC 45.6+0.3 molecular cloud is independently confirmed by its morphological correlation with foreground optical extinction.