Shock and Cosmic-Ray Chemistry Associated with the Supernova Remnant W28

Supernova remnants (SNRs) exert a strong influence on the physics and chemistry of the nearby molecular clouds 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 the HCO+, HCN , and HNC J = 1-0 lines, supplemented by archival data of CO isotopes, N2H+ and H13CO+. We compare the spatial distribution and spectral line profiles of different molecular species. Using the local thermodynamic equilibrium assumption, we obtain an abundance ratio of N(HCO+)/N(CO) similar to 10-4 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 H similar to 2 x 105 cm-3, CR ionization rate zeta = 2.5 x 10-15 s-1, and shock velocity V s = 15-20 km s-1. Toward a point outside the northeastern boundary of W28 with a known high CR ionization rate, we estimate the abundance ratio N(HCO+)/N(N2H+) approximate to 0.6-3.3, which can be reproduced by a chemical simulation if a high density n H similar to 2 x 105 cm-3 is adopted.