Supernova-enhanced Cosmic-Ray Ionization and Induced Chemistry in a Molecular Cloud of W51C

Cosmic rays (CRs) pervade the Galaxy and are thought to be accelerated in supernova shocks. The interaction of CRs with dense interstellar matter has two important effects: (1) high-energy (gsim1 GeV) protons produce γ-rays by π⁰-meson decay and (2) low-energy (lsim1 GeV) CRs (protons and electrons) ionize the gas. We present here new observations toward a molecular cloud close to the W51C supernova remnant and associated with a recently discovered TeV γ-ray source. Our observations show that the cloud ionization degree is highly enhanced, implying a CR ionization rate ~10^-15 s^-1, i.e., 100 times larger than the standard value in molecular clouds. This is consistent with the idea that the cloud is irradiated by an enhanced flux of freshly accelerated low-energy CRs. In addition, the observed high CR ionization rate leads to an instability in the chemistry of the cloud, which keeps the electron fraction high, ~10^-5, in a large fraction (A _v >= 6 mag) of the cloud and low, ~10^-7, in the interior. The two states have been predicted in the literature as high- and low-ionization phases (HIP and LIP). This is the observational evidence of their simultaneous presence in a cloud.