Atomic carbon and CO isotope emission in the vicinity of DR 15

We present observations of the ³P₁-³P₀ fine-structure transition of atomic carbon [C I], the J = 3-2 transition of CO, and the J = 1-0 transitions of ¹³CO and C¹⁸O toward DR 15, an H 11 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 cataloged IRDCs. Our data show a rough spatial correlation between [C I] and ¹³CO J = 1-0. Bright [C I] 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 H II 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.