Unusually high HCO+/CO ratios in and outside supernova remnant W49B

TL;DR

This study investigates how supernova remnant W49B influences nearby molecular clouds, revealing high HCO+/CO ratios caused by shocks, cosmic rays, and radiation, thus demonstrating SNRs' significant impact on molecular chemistry.

Contribution

It provides observational evidence of SNR W49B's multi-channel influence on molecular clouds, highlighting the role of shocks, cosmic rays, and radiation in altering molecular chemistry.

Findings

Broad HCO+ lines indicate perturbation of molecular clouds by W49B.

Unusually high HCO+/CO ratios found both in shocked and quiescent regions.

Cosmic-ray induced chemistry and radiation precursors explain high ratios.

Abstract

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 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 SNRs 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 an SNR's multichannel influence on MCs.