Detection of thioacetaldehyde (CH<sub>3</sub>CHS) in TMC-1: Sulfur-oxygen differentiation along the hydrogenation sequence
M. Agúndez, Germán Molpeceres, C. Cabezas, N. Marcelino, B. Tercero, R. Fuentetaja, P. de Vicente, J. Cernicharo
Abstract
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 (CH 3 CHS), which is the sulfur counterpart of acetaldehyde (CH 3 CHO). 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 CH 3 CHS 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 × 10 10 cm −2 for CH 3 CHS, which implies that it is 36 times less abundant than its oxygen counterpart CH 3 CHO. 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 CH 3 CHS 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 + C 2 H 5 reaction, indicate that formation of CH 3 CHS is only a minor channel in this reaction.