Detection of thioacetaldehyde (CH3CHS) in TMC-1: sulfur-oxygen differentiation along the hydrogenation sequence

TL;DR

This study reports the first detection of thioacetaldehyde (CH3CHS) in space within TMC-1, compares its abundance to its oxygen analog, and explores its formation pathways, revealing sulfur chemistry nuances in cold interstellar environments.

Contribution

First identification of thioacetaldehyde in space, analysis of its abundance relative to oxygen analogs, and evaluation of its formation mechanisms in cold dense clouds.

Findings

CH3CHS is 36 times less abundant than CH3CHO in TMC-1.

Sulfur-bearing molecules are less favored in hydrogenation compared to oxygen analogs.

Chemical models underestimate the observed abundance of CH3CHS.

Abstract

In recent years the chemistry of sulfur in the interstellar medium has experienced a renewed interest due to the detection of a large variety of molecules containing sulfur. Here we report the first identification in space of a new S-bearing molecule, thioacetaldehyde (CH3CHS), which is the sulfur counterpart of acetaldehyde (CH3CHO). The astronomical observations are part of QUIJOTE, a Yebes 40m Q band line survey of the cold dense cloud TMC-1. We detected seven individual lines corresponding to A and E components of the four most favorable rotational transitions of CH3CHS covered in the Q band (31.0-50.3 GHz). Assuming a rotational temperature of 9 K, we derive a column density of 9.8e10 cm-2 for CH3CHS, which implies that it is 36 times less abundant than its oxygen counterpart CH3CHO. By comparing the column densities of the O- and S-bearing molecules detected in TMC-1, we find that as molecules increase their degree of hydrogenation, sulfur-bearing molecules become less abundant compared to their oxygen analog. That is, hydrogenation seems to be less favored for S-bearing molecules than for O-bearing ones in cold sources like TMC-1. We explored potential formation pathways to CH3CHS and implemented them into a chemical model, which however underestimates by several orders of magnitude the observed abundance of thioacetaldehyde. Quantum chemical calculations carried out for one of the potential formation pathways, the S + C2H5 reaction, indicate that formation of CH3CHS is only a minor channel in this reaction.