A Yebes W band Line Survey towards an Unshocked Molecular Cloud of Supernova Remnant 3C391: Evidence of Cosmic-Ray-Induced Chemistry

TL;DR

This study uses a W band line survey of an unshocked molecular cloud near SNR 3C391 to reveal evidence of cosmic-ray-induced chemistry, showing elevated ionization rates and altered molecular abundances.

Contribution

It provides the first detailed molecular line survey of 3C391:NML and estimates the cosmic-ray ionization rate, highlighting the impact of cosmic rays on dense cloud chemistry.

Findings

Detected 18 molecular species in 3C391:NML.

Estimated cosmic-ray ionization rate is ≥ 2.7×10⁻¹⁴ s⁻¹.

Observed molecular abundance ratios indicate enhanced cosmic-ray activity.

Abstract

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 as 3C391:NML) associated with supernova remnant (SNR) 3C391. 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 the CR ionization rate of 3C391:NML is $\zeta\gtrsim 2.7\times 10^{-14}\rm \ s^{-1}$ with an analytic method. However, we caution on 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 $\sim 10^{-15}$ or $\sim 10^{-14}\rm \ s^{-1}$) is needed to reproduce the observation with chemical model. This is higher than the values found in typical MCs by 2--3 orders of magnitude.