Space and laboratory discovery of HC3S+

TL;DR

This paper reports the first detection of the protonated molecule HC3S+ in space, using radio observations supported by laboratory and theoretical data, and discusses its formation pathways and abundance ratios in TMC-1.

Contribution

It presents the first observational detection and characterization of HC3S+ in space, supported by laboratory measurements and ab initio calculations, and analyzes its formation mechanisms.

Findings

HC3S+ detected in TMC-1 with a column density of (2.0 +/- 0.5)e11 cm-2

The abundance ratio C3S/HC3S+ is about 65, comparable to CS/HCS+ ratio

HC3S+ is mainly formed through proton transfer, S+ reactions, and neutral sulfur reactions.

Abstract

We report the detection in TMC-1 of the protonated form of C3S. The discovery of the cation HC3S+ was carried through the observation of four harmonically related lines in the Q band using the Yebes 40m radiotelescope, and is supported by accurate ab initio calculations and laboratory measurements of its rotational spectrum. We derive a column density N(HC3S+) = (2.0 +/- 0.5)e11 cm-2, which translates to an abundance ratio C3S/HC3S+ of 65 +/- 20. This ratio is comparable to the CS/HCS+ ratio (35 +/- 8) and is a factor of about ten larger than the C3O/HC3O+ ratio previously found in the same source. However, the abundance ratio HC3O+/HC3S+ is 1.0 +/- 0.5, while C3O/C3S is just 0.11. We also searched for protonated C2S in TMC-1, based on ab initio calculations of its spectroscopic parameters, and derive a 3sigma upper limit of N(HC2S+) < 9e11 cm-2 and a C2S/HC2S+ > 60. The observational results are compared with a state-of-the-art gas-phase chemical model and conclude that HC3S+ is mostly formed through several pathways: proton transfer to C3S, reaction of S+ with c-C3H2, and reaction between neutral atomic sulfur and the ion C3H3+.