Molecular chemistry induced by J-shock toward supernova remnant W51C

Abstract

Shock waves from supernova remnants (SNRs) have strong influence on the physical and chemical properties of molecular clouds (MCs). Shocks propagating into magnetized MCs can be classified into "jump" J-shock and "continuous" C-shock. The molecular chemistry in the re-formed molecular gas behind J-shock is still not well understood, which will provide a comprehensive view of the chemical feedback of SNRs and the chemical effects of J-shock. We conducted a W-band (71.4-89.7 GHz) observation toward a re-formed molecular clump behind a J-shock induced by SNR W51C with the Yebes 40 m radio telescope to study the molecular chemistry in the re-formed molecular gas. Based on the local thermodynamic equilibrium (LTE) assumption, we estimate the column densities of HCO+, HCN, C2H and o-c-C3H2, and derive the maps of their abundance ratios with CO. The gas density is constrained by non-LTE analysis of the HCO+ J=1-0 line. We obtain the following abundance ratios: $N({\rm HCO^+})/N({\rm CO})\sim (1.0\text{--}4.0)\times 10^{-4}$, $N({\rm HCN})/N({\rm CO})\sim (1.8\text{--}5.3)\times 10^{-4}$, $N({\rm C_2H})/N({\rm CO})\sim (1.6\text{--}5.0)\times 10^{-3}$, and $N({o\text{-}c\text{-}{\rm C_3H_2}})/N({\rm CO})\sim (1.2\text{--}7.9)\times 10^{-4}$. The non-LTE analysis suggests that the gas density is $n_{\rm H_2}\gtrsim 10^4\rm \ cm^{-3}$. We find that the N(C2H)/N(CO) and N(o-c-C3H2)/N(CO) are higher than typical values in quiescent MCs and shocked MCs by 1-2 orders of magnitude, which can be qualitatively attributed to the abundant C+ and C at the earliest phase of molecular gas re-formation. The Paris-Durham shock code can reproduce, although not perfectly, the observed abundance ratios, especially the enhanced N(C2H)/N(CO) and N(o-c-C3H2)/N(CO), with J-shocks propagating in to both non-irradiated and irradiated molecular gas with a preshock density of $n_{\rm H}=2\times 10^3\rm \ cm^{-3}$.