Cosmic-ray-driven enhancement of the C${}^0$/CO abundance ratio in W51C

Abstract

We examine spatial variations of the C${}^0$/CO abundance ratio ($X_{C/CO}$) in the vicinity of the ${\hat{I}}^3$-ray supernova remnant W51C, based on [CI] (${}^3{P}_1$-${}^3{P}_0$), ${}^{12}$CO(1-0), and ${}^{13}$CO(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 up to by a factor of four near the W51C center, depending on the projected distance from the W51C center. In high-$A_V$ molecular clouds, $X_{C/CO}$ is determined by the ratio of the cosmic-ray (CR) ionization rate to the H${}_2$ density, and we find no clear spatial variation of the H${}_2$ density against the projected distance. Hence, the high CR ionization rate may locally enhance $X_{C/CO}$ near the W51C center. We also find that the observed spatial extent of the enhanced $X_{C/CO}$ ($\sim$17 pc) is consistent with the diffusion distance of CRs with the energy of 100 MeV. The fact suggests that the low-energy CRs accelerated in W51C enhance $X_{C/CO}$. The CR ionization rate at the $X_{C/CO}$-enhanced cloud is estimated to be 3$\times$10${}^{-16}$ s${}^{-1}$ on the basis of time-dependent PDR simulations of $X_{C/CO}$, the value of which is 30 times higher than that in the standard Galactic environment. These results demonstrate that [CI] is a powerful probe to investigate the interaction between CRs and the interstellar medium for a wide area in the vicinity of supernova remnants.

17 pages, 8 figures, accepted for publication in PASJ