ISM organosulfur reaction pathways remain largely unknown

Determine the formation and destruction pathways of organosulfur molecules in the interstellar medium by establishing the elementary gas-phase reactions, rate coefficients, and product branching fractions under low-temperature (10–100 K) and low-density (approximately 10^4–10^6 molecules cm^-3) conditions that are typical of interstellar clouds, so that observed abundances can be consistently reproduced by astrochemical models.

Background

The paper highlights that despite significant advances in astronomical detection, many interstellar chemical processes—especially those involving sulfur-bearing species—are not quantitatively understood at the low temperatures and densities characteristic of the interstellar medium (ISM).

The authors note that discrepancies between observations and models often stem from missing or uncertain kinetic data (e.g., rate coefficients and branching fractions) for reactions of radicals with sulfur-containing molecules. This motivates the development of their HILTRAC apparatus to enable measurements under ISM-like conditions.

References

However, the formation and destruction pathways of such molecules, particularly for reactions of organosulfur species, at temperatures and gas densities ranging from 10 – 100 K and 10 – 10 molecule cm , respectively, remain largely unknown.

— Design and Implementation of a New Apparatus for Astrochemistry: Kinetic Measurements of the CH + OCS Reaction and Frequency Comb Spectroscopy in a Cold Uniform Supersonic Flow  (2405.18814 - Lucas et al., 2024) in Section I. Introduction

In the gas phase, the forward hydrogenation of \ce{CH3CHS} is certainly governed by the addition of H to the sulfur atom, although the final reaction products remain unknown in our investigation.

— Thioacetaldehyde (CH3CHS) on interstellar ices: a key molecule to unravel two chemical dichotomies in the ISM  (2609.20506 - Mallo et al., 17 Sep 2026) in Section 3.1, “Kinetic Analysis of the First Hydrogenation”