# Methyl cyanide (CH3CN) and propyne (CH3CCH) in the low mass protostar IRAS 16293-2422

## TL;DR

This study investigates the origins and distributions of methyl cyanide and propyne in the low-mass protostar IRAS 16293-2422, revealing their different emission regions and comparing observations with chemical models.

## Contribution

It provides the first detailed analysis of CH3CN and CH3CCH emission regions in IRAS 16293-2422 and compares observational data with gas-grain chemical model predictions.

## Key findings

- CH3CN emission originates from a warmer, inner envelope region.
- CH3CCH emission is from a cooler, outer envelope region.
- Model predicts CH3CCH peaks in outer envelope near 2000 au.

## Abstract

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 30m 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.