Thioacetaldehyde (CH3CHS) On Interstellar Ices: A Key Molecule To Unravel Two Chemical Dichotomies In The ISM

[astro-ph.GA] Thioacetaldehyde (CH₃CHS), recently detected in TMC-1, has an abundance approximately 36 times lower than its oxygen analog, acetaldehyde (CH₃CHO). This makes the CH₃CHS/CH₃CHO pair the one with the largest column density difference among the detected oxygen/sulfur analogue pairs in this cloud.

We investigate the hydrogenation pathways of CH₃CHS to address two chemical dichotomies in the ISM: (i) the differenciation between CH₃CHS and CH₃CHO, and (ii) the apparent absence of both CH₃CHS in the G+0.693-0.027 molecular cloud and ethyl mercaptan (CH₃CH₂SH), in TMC-1.

Our results reveal a complex scheme that involves multiple competing reactions, highlighting an efficient sequence of consecutive hydrogenations that can lead to CH₃CH₂SH. This finding suggests that the high S/O ratio observed for thioacetaldehyde in TMC-1 (~36), and even more pronounced in G+0.693-0.027 (≥112), may result from its conversion via hydrogenation on the ice surface, contrary to the case of CH₃CHO, which is more resistant to that chemical process.

The straightforward hydrogenation of CH₃CHS on ices, which can also take place even in the gas-phase at 150 K, provides a reliable explanation for its non-detection in G+0.693-0.027, where grain-surface chemistry is expected to play an important role, favoring the conversion of CH₃CHS into CH₃CH₂SH, which is indeed detected in G+0.693-0.027.

In contrast, TMC-1 represents a more pristine gas-phase environment, where grain-surface chemistry has a lower impact. Under these conditions, CH₃CHS can persist, while CH₃CH₂SH remains undetected.

Overall, our results show the entirely different reactivity that contributes to the chemical complexity of two of the largest interstellar sulfur factories.

Reaction pathways of the CH₃CHS + H reaction on the ice surface, including the schematic representation of reactant and products. — [astro-ph.GA]

María Mallo, Miguel Sanz-Novo, Marcelino Agúndez, José Cernicharo, Gisela Esplugues, Víctor M. Rivilla, Izaskun Jiménez-Serra, Carlos Cabezas, Germán Molpeceres

Comments: Accepted for publication in MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2609.20506 [astro-ph.GA] (or arXiv:2609.20506v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2609.20506
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Submission history
From: Maria Mallo
[v1] Thu, 17 Sep 2026 14:50:33 UTC (1,402 KB)
https://arxiv.org/abs/2609.20506

Astrobiology, Astrochemistry,