2018MNRAS.481.5651A

Methyl cyanide (CH₃CN) and propyne (CH₃CCH) are two molecules commonly used as gas thermometers for interstellar gas. They are detected in several astrophysical environments and in particular towards protostars. Using data of the low-mass protostar IRAS 16293-2422 obtained with the IRAM 30-m single-dish telescope, we constrained the origin of these two molecules in the envelope of the source. The line shape comparison and the results of a radiative transfer analysis both indicate that the emission of CH₃CN arises from a warmer and inner region of the envelope than the CH₃CCH emission. We compare the observational results with the predictions of a gas-grain chemical model. Our model predicts a peak abundance of CH₃CCH in the gas-phase in the outer part of the envelope, at around 2000 au from the central star, which is relatively close to the emission size derived from the observations. The predicted CH₃CN abundance only rises at the radius where the grain mantle ices evaporate, with an abundance similar to the one derived from the observations.