Probing outflow physics through CH$_3$CN and CH$_3$OH chemistry

Abstract

Chemical correlations between molecules provide powerful diagnostics to probe the physical conditions of protostellar outflows. In particular, the relationship between methanol (CH$_3$OH) and methyl cyanide (CH$_3$CN) offers a promising tool to investigate the chemistry and irradiation environment of shocked gas. In this Letter, we use the CH$_3$OH/CH$_3$CN abundance ratio to constrain the physical properties of the outflow driven by the Class 0 protostar S68N using ALMA Band 3 and Band 6 observations. Assuming local thermodynamic equilibrium (LTE), we derive excitation temperatures of 50-60 K and column densities of 2-3$\times$10$^{13}$ cm$^{-2}$ for CH$_3$CN and 3-5$\times$10$^{15}$ cm$^{-2}$ for CH$_3$OH. The resulting CH$_3$OH/CH$_3$CN abundance ratio is nearly constant along the outflow, with values of $\sim$100-200, similar to those found in other protostellar environments. Using an up-to-date astrochemical model, we test whether gas-phase formation of CH$_3$CN can account for the observed ratios. We find that they are reproduced only by assuming enhanced cosmic-ray ionization rates $ζ_{\rm CR}$ up to $\sim$10$^{-14}$ s$^{-1}$. These results suggest that the CH$_3$OH-CH$_3$CN correlation can be used as a probe of the irradiation conditions in protostellar outflows. Further studies are required to explore the possible contribution of grain-surface formation of CH$_3$CN which could lead to a lower $ζ_{\rm CR}$ and to extend the analysis to a larger sample of sources.