2011ApJ...740L...4C

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 ( ≳ 1 GeV) protons produce γ-rays by π⁰-meson decay and (2) low-energy (≲ 1 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, 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.