The sulphur saga in TMC-1: Discovery of HCSCN and HCSCCH

TL;DR

This study reports the first space detection of HCSCN and HCSCCH in TMC-1, revealing unexpected abundance ratios and suggesting new insights into sulphur chemistry in interstellar environments.

Contribution

First detection of HCSCN and HCSCCH in space, with analysis of their abundance ratios and implications for sulphur chemistry models.

Findings

HCSCN is four times more abundant than HCOCN.

HCSCCH is five times less abundant than HCOCCH.

Observed abundances exceed model predictions, indicating missing reactions or underestimated sulphur.

Abstract

We report the detection, for the first time in space, of cyano thioformaldehyde (HCSCN) and propynethial (HCSCCH) towards the starless core TMC-1. Cyano thioformaldehyde presents a series of prominent a- and b-type lines, which are the strongest previously unassigned features in our Q-band line survey of TMC-1. Remarkably, HCSCN is four times more abundant than cyano formaldehyde (HCOCN). On the other hand, HCSCCH is five times less abundant than propynal (HCOCCH). Surprisingly, we find an abundance ratio HCSCCH/HCSCN of 0.25, in contrast with most other ethynyl-cyanide pairs of molecules for which the CCH-bearing species is more abundant than the CN-bearing one. We discuss the formation of these molecules in terms of neutral-neutral reactions of S atoms with CH2CCH and CH2CN radicals as well as of CCH and CN radicals with H2CS. The calculated abundances for the sulphur-bearing species are, however, significantly below the observed values, which points to an underestimation of the abundance of atomic sulphur in the model or to missing formation reactions, such as ion-neutral reactions.