Discovery of the elusive thioketenylium, HCCS+, in TMC-1

TL;DR

This paper reports the first detection of the protonated radical HCCS+ in a cold dark cloud, TMC-1, using radio telescopes, and discusses its formation and abundance in interstellar chemistry.

Contribution

The discovery of HCCS+ in TMC-1 is the first identification of a protonated radical in such an environment, expanding knowledge of interstellar molecular ions.

Findings

HCCS+ detected with 26 hyperfine components

Column density of HCCS+ is (1.1±0.1)×10^12 cm^-2

HCCS+/CCS ratio is 50±10

Abstract

We report the detection in TMC-1 of the cation HCCS+ (3Sigma-), which is the protonated form of the widespread radical CCS. This is the first time that a protonated radical has been detected in a cold dark cloud. Twenty-six hyperfine components from twelve rotational transitions have been observed with the Yebes 40m and IRAM 30m radio telescopes. We confidently assign the characteristic rotational spectrum pattern to HCCS+ based on the good agreement between the astronomical and theoretical spectroscopic parameters. The column density of HCCS+ is (1.1+/-0.1)e12 cm-2, and the CCS/HCCS+ abundance ratio is 50+/-10, which is very similar to that of CS/HCS+ (35+/-8) and CCCS/HCCCS+ (65+/-20). From a state-of-the-art gas-phase chemical model, we conclude that HCCS+ is mostly formed by reactions of proton transfer from abundant cations such as HCO+, H3O+, and H3+ to the radical CCS.