Formation of S-bearing complex organic molecules in interstellar clouds via ice reactions with C2H2, HS, and atomic H

TL;DR

This study investigates how sulfur-bearing organic molecules form in interstellar ices through laboratory experiments and computational modeling, revealing key reaction pathways and products relevant to interstellar chemistry.

Contribution

It provides the first experimental evidence of sulfur reaction networks in icy mantles involving C2H2 and SH radicals, identifying new sulfurated organic molecules formed at 10 K.

Findings

Confirmed formation of six sulfurated products including CH3CH2SH and CH2CS.

Main product CH3CH2SH increases with higher H ratios.

Revealed reaction pathways through computational analysis.

Abstract

The chemical network governing interstellar sulfur has been the topic of unrelenting discussion for the past decades due to the conspicuous discrepancy between its expected and observed abundances in different interstellar environments. More recently, the astronomical detections of CH3CH2SH and CH2CS highlighted the importance of interstellar formation routes for sulfur-bearing organic molecules with two carbon atoms. In this work, we perform a laboratory investigation of the solid-state chemistry resulting from the interaction between C2H2 molecules and SH radicals -- both thought to be present in interstellar icy mantles -- at 10 K. Reflection absorption infrared spectroscopy and quadrupole mass spectrometry combined with temperature-programmed desorption experiments are employed as analytical techniques. We confirm that SH radicals can kick-start a sulfur reaction network under interstellar cloud conditions and identify at least six sulfurated products: CH3CH2SH, CH2CHSH, HSCH2CH2SH, H2S2, and tentatively CH3CHS and CH2CS. Complementarily, we utilize computational calculations to pinpoint the reaction routes that play a role in the chemical network behind our experimental results. The main sulfur-bearing organic molecule formed under our experimental conditions is CH3CH2SH and its formation yield increases with the ratios of H to other reactants. It serves as a sink to the sulfur budget within the network, being formed at the expense of the other unsaturated products. The astrophysical implications of the chemical network proposed here are discussed.