2025A&A...693L..20A

In recent years, the chemistry of sulfur in the interstellar medium has experienced renewed interest due to the detection of a large variety of molecules containing sulfur. 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 40 m 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 9.8 × 10¹⁰ 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 than their oxygen analog. That is, hydrogenation seems to be less favored for S-bearing molecules than for O-bearing ones in cold sources such as TMC-1. We explored potential formation pathways to CH₃CHS and implemented them into a chemical model, which underestimates the observed abundance of thioacetaldehyde by several orders of magnitude, however. 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.