2024ApJ...974..262T

Cosmic rays (CRs) have strong influences on the chemistry of dense molecular clouds (MCs). To study the detailed chemistry induced by CRs, we conducted a Yebes W-band line survey towards an unshocked MC (which we named 3C391:NML) associated with supernova remnant 3C 391. We detected emission lines of 18 molecular species in total and estimated their column densities with local thermodynamic equilibrium (LTE) and non-LTE analysis. Using the abundance ratio N(HCO⁺)/N(CO) and an upper limit of N(DCO⁺)/N(HCO⁺), we estimated that the CR ionization rate of 3C391:NML is ζ ≳ 2.7 × 10^–14 s^–1 with an analytic method. However, we caution against adopting this value because chemical equilibrium, which is a prerequisite of using the equations, is not necessarily reached in 3C391:NML. We observed lower N(HCO⁺)/N(HOC⁺), higher N(HCS⁺)/N(CS), and higher X(l-C₃H⁺) by an order of magnitude in 3C391:NML than the typical values in quiescent dense MCs. We found that an enhanced CR ionization rate (of order ∼10^–15 or ∼10^–14 s^–1) is needed to reproduce the observation with a chemical model. This is higher than the values found in typical MCs by 2-3 orders of magnitude.