Atomic Carbon and CO Isotope Emission in the Vicinity of DR15

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We present observations of the 3P1-3P0 fine-structure transition of atomic carbon [CI], the J=3-2 transition of CO, and the J=1-0 transitions of 13CO and C18O toward DR15, an HII region associated with two mid-infrared dark clouds (IRDCs). The 13CO and C18O J=1-0 emissions closely follow the dark patches seen in optical wavelength, showing two self-gravitating molecular cores with masses of 2000 and 900 M., respectively, at the positions of the catalogued IRDCs. Our data show a rough spatial correlation between [CI] and 13CO J=1-0. Bright [CI] emission occurs in the relatively cold gas behind the molecular cores but does not occur in either highly excited gas traced by CO J=3-2 emission or in the HII region/molecular cloud interface. These results are inconsistent with those predicted by standard photodissociation region models, suggesting an origin for interstellar atomic carbon unrelated to photodissociation processes.