Nobeyama 45 m Cygnus-X CO Survey. I. Photodissociation of Molecules Revealed by the Unbiased Large-scale CN and C18O Maps

We present an unbiased large-scale (9 deg²) CN (N = 1-0) and C¹⁸O (J = 1-0) survey of Cygnus-X conducted with the Nobeyama 45 m Cygnus-X CO survey. CN and C¹⁸O are detected in various objects toward the Cygnus-X North and South (e.g., DR17, DR18, DR21, DR22, DR23, and W75N). We find that CN/C¹⁸O integrated intensity ratios are systematically different from region to region, and are especially enhanced in DR17 and DR18, which are irradiated by the nearby OB stars. This result suggests that CN/C¹⁸O ratios are enhanced via photodissociation reactions. We investigate the relation between the CN/C¹⁸O ratio and strength of the UV radiation field. As a result, we find that CN/C¹⁸O ratios correlate with the far-UV intensities, G ₀. We also find that CN/C¹⁸O ratios decrease inside molecular clouds, where the interstellar UV radiation is reduced due to the interstellar dust extinction. We conclude that the CN/C¹⁸O ratio is controlled by the UV radiation, and is a good probe of photon-dominated regions.