H I Self-Absorption and the Kinematic Distance Ambiguity: The Case of the Molecular Cloud GRSMC 45.6+0.3

摘要

We report on a three-dimensional (l, b, V_LSR) 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 (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.