Molecular chemistry induced by a J-shock toward supernova remnant W51C

¹ School of Astronomy & Space Science, Nanjing University, 163 Xianlin Avenue, Nanjing 210023, China
² Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
³ Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, Ministry of Education, Nanjing 210023, China

^★ Corresponding author; ygchen@nju.edu.cn

Received: 17 September 2024
Accepted: 9 December 2024

Abstract

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.