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In observational studies of infrared dark clouds, the number of detections ofCO freeze-out onto dust grains (CO depletion) at pc-scale is extremely limited,and the conditions for its occurrence are, therefore, still unknown. We reporta new object where pc-scale CO depletion is expected. As a part of KagoshimaGalactic Object survey with Nobeyama 45-m telescope by Mapping in Ammonia lines(KAGONMA), we have made mapping observations of NH3 inversion transition linestowards the star-forming region associated with the CMa OB1 including IRAS07077-1026, IRAS 07081-1028, and PGCC G224.28-0.82. By comparing the spatialdistributions of the NH3 (1,1) and C18O (J=1-0), an intensity anti-correlationwas 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 07077-1026than in the other parts of the star-forming region. After examining highdensity gas dissipation, photodissociation, and CO depletion, we concluded thatthe intensity anti-correlation in IRAS 07077-1026 is due to CO depletion. Onthe other hand, in the vicinity of the centre of PGCC G224.28-0.82, theemission line intensities of both the NH3 (1,1) and C18O (J=1-0) were stronglydetected, although the gas temperature and density were similar to IRAS07077-1026. This indicates that there are situations where C18O (J=1-0) cannottrace dense gas on the pc scale and implies that the conditional differencesthat C18O (J=1-0) can and cannot trace dense gas are unclear.

publications of the astronomical society of japan

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