Cosmic-ray-driven enhancement of the C0/CO abundance ratio in W 51 C

We examine spatial variations of the C⁰$/$CO abundance ratio (X_C/CO) in the vicinity of the γ-ray supernova remnant W 51 C, based on [C I] (³P₁-³P₀), ¹²CO(1-0), and ¹³CO(1-0) observations with the ASTE and Nobeyama 45 m telescopes. We find that X_C/CO varies in a range of 0.02-0.16 (0.05 in median) inside the molecular clouds of A_V > 100 mag, where photodissociation of CO by the interstellar UV is negligible. Furthermore, X_C/CO is locally enhanced by a factor of up to four near the W 51 C center, depending on the projected distance from the W 51 C center. In high-A_V molecular clouds, X_C/CO is determined by the ratio of the cosmic-ray (CR) ionization rate to the H₂ density, and we find no clear spatial variation of the H₂ density against the projected distance. Hence, the high CR ionization rate may locally enhance X_C/CO near the W 51 C center. We also find that the observed spatial extent of the enhanced X_C/CO (~17 pc) is consistent with the diffusion distance of CRs with an energy of 100 MeV. This fact suggests that the low-energy CRs accelerated in W 51 C enhance X_C/CO. The CR ionization rate in the X_C/CO-enhanced cloud is estimated to be 3 × 10^-16 s^-1 on the basis of time-dependent photodissociation region simulations of X_C/CO, the value of which is 30 times higher than that in the standard Galactic environment. These results demonstrate that [C I] is a powerful probe to investigate the interaction between CRs and the interstellar medium for a wide area in the vicinity of supernova remnants.