Shock and Cosmic Ray Chemistry Associated with the Supernova Remnant W28

TL;DR

This study investigates how supernova remnant W28 influences nearby molecular cloud chemistry through shock waves and cosmic rays, revealing enhanced molecular abundances and providing insights into shock and cosmic ray induced chemical processes.

Contribution

It presents new molecular line observations and chemical analysis of W28, demonstrating the impact of shocks and cosmic rays on molecular abundances in the cloud.

Findings

Higher HCO+ to CO ratio in shocked regions indicating shock and CR influence.

Chemical models reproduce observed abundances with high-density assumptions.

Estimated cosmic ray ionization rate consistent with previous high CR activity measurements.

Abstract

Supernova remnants (SNRs) exert strong influence on the physics and chemistry of the nearby molecular clouds (MCs) through shock waves and the cosmic rays (CRs) they accelerate. To investigate the SNR-cloud interaction in the prototype interacting SNR W28 (G6.4$-$0.1), we present new observations of $\rm HCO^+$, HCN and HNC $J=1\text{--}0$ lines, supplemented by archival data of CO isotopes, $\rm N_2H^+$ and $\rm H^{13}CO^+$. We compare the spatial distribution and spectral line profiles of different molecular species. Using local thermodynatic equilibrium (LTE) assumption, we obtain an abundance ratio $N({\rm HCO^+})/N({\rm CO})\sim10^{-4}$ in the northeastern shocked cloud, which is higher by an order of magnitude than the values in unshocked clouds. This can be accounted for by the chemistry jointly induced by shock and CRs, with the physical parameters previously obtained from observations: preshock density $n_{\rm H}\sim 2\times 10^{5}\rm \ cm^{-3}$, CR ionization rate $\zeta=2.5\times 10^{-15} \rm \ s^{-1}$ and shock velocity $V_{\rm s}=15\text{--}20\rm \ km\ s^{-1}$. Towards a point outside the northeastern boundary of W28 with known high CR ionization rate, we estimate the abundance ratio $ N({\rm HCO^+})/N({\rm N_2H^+}) \approx 0.6\text{--}3.3$, which can be reproduced by a chemical simulation if a high density $n_{\rm H}\sim 2\times 10^5 \ \rm cm^{-3}$ is adopted.