KAgoshima Galactic Object survey with Nobeyama 45 m telescope by Mapping in Ammonia lines (KAGONMA): Discovery of parsec-scale CO depletion in the Canis Major star-forming region

In observational studies of infrared dark clouds, the number of detections of CO freeze-out on to dust grains (CO depletion) at the pc scale is extremely limited, and the conditions for its occurrence are, therefore, still unknown. We report a new object where pc-scale CO depletion is expected. As part of the Kagoshima Galactic Object survey with Nobeyama 45 m telescope by Mapping in Ammonia lines (KAGONMA), we have made mapping observations of NH3 inversion transition lines towards the star-forming region associated with Canis Major OB1, including IRAS 07077−1026, IRAS 07081−1028, and PGCC G224.28−0.82. By comparing the spatial distributions of NH3 (1,1) and C18O (J = 1–0), an intensity anti-correlation was found in IRAS 07077−1026 and IRAS 07081−1028 on the ∼1 pc scale. Furthermore, we obtained a lower abundance of C18O at least in IRAS 07077−1026 than in the other parts of the star-forming region. After examining high-density gas dissipation, photodissociation, and CO depletion, we concluded that the intensity anti-correlation in IRAS 07077−1026 is due to CO depletion. On the other hand, in the vicinity of the center of PGCC G224.28−0.82, the emission line intensities of both NH3 (1,1) and C18O (J = 1–0) were strongly detected, although the gas temperature and density were similar to IRAS 07077−1026. This indicates that there are situations where C18O (J = 1–0) cannot trace dense gas on the pc scale and implies that the conditional differences in which C18O (J = 1–0) can and cannot trace dense gas are unclear.