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 ~(13)CO emission toward the Galactic molecular cloud GRSMC 45.6+0.3. Photochemical models indicate that a significant column density of cool (T approx< 25 K) H Ⅰ exists in the skins (A_v approx< 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.