Letter to the Editor

First detection of HS₂ in a cold dark cloud

¹ Observatorio Astronómico Nacional (OAN), Alfonso XII, 3, 28014 Madrid, Spain
² Observatorio de Yebes, IGN, Cerro de la Palera s/n, E-19141 Yebes, Guadalajara, Spain
³ Instituto de Física Fundamental, CSIC, Calle Serrano 123, E-28006 Madrid, Spain

^⋆ Corresponding authors: g.esplugues@oan.es; jose.cernicharo@csic.es

Received: 5 May 2025
Accepted: 3 June 2025

Abstract

We report the first detection of HS₂ towards the cold dark cloud TMC-1. This is the first observation of a chemical species containing more than one sulphur atom in this type of sources. The astronomical observations are part of QUIJOTE, a line survey of TMC-1 in the Q band (31–50 GHz). The detection is confirmed by the observation of the fine and hyperfine components of two rotational transitions (2_0, 2–1_0, 1 and 3_0, 3–2_0, 2). Assuming a rotational temperature of 7 K, we derived an HS₂ column density of 5.7 × 10¹¹ cm⁻², using a local thermodynamic equilibrium model that reproduces the observed spectra. The abundance of HS₂ relative to H₂ is 5.7 × 10⁻¹¹, which means that it is about seven times more abundant than its oxygenated counterpart HSO. We also explored the main formation and destruction mechanisms of HS₂ using a chemical model, which reproduces the observed abundance of HS₂ and indicates that dissociative recombination reactions from the ions H₂S₂⁺ and H₃S₂⁺ play a major role in forming HS₂.