2026/06/25 by L. Giani, D. Zaremba, M. De Simone +4 · 1 voice
Physics and Astronomy · #astro-ph.GA
Chemical correlations between molecules provide powerful diagnostics to probe the physical conditions of protostellar outflows. In particular, the relationship between methanol (CH3OH) and methyl cyanide (CH3CN) offers a promising tool to investigate the chemistry and irradiation environment of shocked gas. In this Letter, we use the CH3OH/CH3CN 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 2-3×1013 cm-2 for CH3CN and 3-5×1015 cm-2 for CH3OH. The resulting CH3OH/CH3CN abundance ratio is nearly constant along the outflow, with values of ∼100-200, similar to those found in other protostellar environments. Using an up-to-date astrochemical model, we test whether gas-phase formation of CH3CN 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 ∼10-14 s-1. These results suggest that the CH3OH-CH3CN 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 CH3CN which could lead to a lower ζ\rm CR and to extend the analysis to a larger sample of sources.