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

We report on a three-dimensional ( l , b , V LSR ) physical association of H I self-absorption with 13 CO 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 ( A v ≲ 2) of typical Galactic molecular clouds. The H I self-absorption can thus be produced by the cool cloud skin absorbing the warm ( T SPIN ~ 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.