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Context. Shock waves from supernova remnants (SNRs) strongly affect the physical and chemical properties of molecular clouds (MCs). Shocks propagating into magnetized MCs can be classified into jump or J-shocks and continuous or C-shocks. The molecular chemistry in the re-formed molecular gas behind J-shocks is still only poorly understood. It is expected to provide a comprehensive view of the chemical feedback of SNRs and the chemical effects of J-shocks.
Aims. 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.
Methods. Assuming local thermodynamic equilibrium (LTE), we estimated the column densities of HCO⁺, HCN, C₂H and o-c-C₃H₂, and derived their abundance ratio maps with CO. The gas density was constrained by a non-LTE analysis of the HCO⁺J = 1-0 line. The abundance ratios were compared with the values in typical quiescent MCs and shocked MCs, and they were also compared with the results of chemical simulations with the Paris-Durham shock code to verify and investigate the chemical effects of J-shocks.
Results. We obtained the following abundance ratios: N(HCO⁺)/N(CO) ∼ (1.0-4.0) × 10^–4, N(HCN)/N(CO) ∼ (1.8-5.3) × 10^–4, N(C₂H)/N(CO) ∼ (1.6-5.0) × 10^–3, and N(o-c-C₃H₂)/N(CO) ∼ (1.2-7.9) × 10^–4. The non-LTE analysis suggests that the gas density is n_H2 ≳ 10⁴ 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 in 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(c-C₃H₂)/N(CO), with J-shocks propagating into both nonirradiated and irradiated molecular gas with a preshock density of n_H = 2 × 10³ cm^–3.