Unusually high HCO+/CO ratios in and outside supernova remnant W49B
  Galactic supernova remnants (SNRs) and their environments provide the nearest
laboratories to study SN feedback. We performed molecular observations toward
SNR W49B -- the most luminous Galactic SNR in the X-ray band, aiming to explore
signs of multiple feedback channels of SNRs on nearby molecular clouds (MCs).
We found very broad HCO+ lines with widths of dv = 48--75 km/s in the SNR
southwest, providing strong evidence that W49B is perturbing MCs at a systemic
velocity of $V_{LSR}=61$--65 km/s, and placing W49B at a distance of $7.9\pm
0.6$ kpc. We observed unusually high intensity ratios of HCO+ J=1-0/CO J=1-0
not only at shocked regions ($1.1\pm 0.4$ and $0.70\pm 0.16$), but also in
quiescent clouds over 1 pc away from the SNR's eastern boundary (> 0.2). By
comparing with the magnetohydrodynamics (MHD) shock models, we interpret that
the high ratio in the broad-line regions can result from a cosmic-ray (CR)
induced chemistry in shocked MCs, where the CR ionization rate is enhanced to
around 10--100 times of the Galactic level. The high HCO+/CO ratio outside the
SNR is probably caused by the radiation precursor, while the luminous X-ray
emission of W49B can explain a few properties in this region. The above results
provide observational evidence that SNR can strongly influence the molecular
chemistry in and outside the shock boundary via their shocks, CRs, and
radiation. We propose that the HCO+/CO ratio is a potentially useful tool to
probe SNR's multi-channel influence on MCs.
