First Detection of Interstellar S₂H

Abstract

We present the first detection of gas phase S 2 H in the Horsehead, a moderately UV-irradiated nebula. This confirms the presence of doubly sulfuretted species in the interstellar medium and opens a new challenge for sulfur chemistry. The observed S 2 H abundance is ∼5×10 −11 , only a factor 4-6 lower than that of the widespread H 2 S molecule. H 2 S and S 2 H are efficiently formed on the UV-irradiated icy grain mantles. We performed ice irradiation experiments to determine the H 2 S and S 2 H photodesorption yields. The obtained values are ∼1.2×10 −3 and <1×10 −5 molecules per incident photon for H 2 S and S 2 H, respectively. Our upper limit to the S 2 H photodesorption yield suggests that photo-desorption is not a competitive mechanism to release the S 2 H molecules to the gas phase. Other desorption mechanisms such as chemical desorption, cosmic-ray desorption and grain shattering can increase the gaseous S 2 H abundance to some extent. Alternatively, S 2 H can be formed via gas phase reactions involving gaseous H 2 S and the abundant ions S + and SH + . The detection of S 2 H in this nebula could be therefore the result of the coexistence of an active grain surface chemistry and gaseous photo-chemistry.

Millimeter- and submillimeter-wave spectroscopy of thioformamide and interstellar search toward Sgr B2(N)

“…Several small sulfur compounds have recently been observed in the interstellar medium (ISM): HS 2 (Fuente et al 2017), NS + (Cernicharo et al 2018), HCS, and HSC (Agúndez et al 2018). To date, 23 sulfur derivatives have been detected or tentatively detected in the ISM, which means that the sulfur atom holds the fifth position after hydrogen, carbon, oxygen, and nitrogen atoms; see, for instance "Molecules in Space" 1 in the Cologne Database for Molecular Spectroscopy.…”

Are disulfide bonds resilient to double ionization? Insights from coincidence spectroscopy and ab initio calculations

“…28 The S 2 unit, the smallest motif containing a sulfur-sulfur chemical bond, is known as the disulde bond (-S-S-). The rst interstellar molecule containing such a feature, S 2 H, was only recently observed by Fuente and co-workers 29 in the Horsehead nebula, possibly resulting from the proton detachment of an HSSH + intermediate. 30 Moreover, the disulde bond plays a very important role in the stability, solubility and folding of proteins, being present in around 10% of the proteins produced by mammalian cells.…”

Gas phase Elemental abundances in Molecular cloudS (GEMS)

“…This is also the case towards the prototypical photodissociation region (PDR) in the Horsehead Nebula, where the sulfur abundance is found to be very close to the undepleted value observed in the diffuse ISM (Goicoechea et al 2006), with an estimate of S/H = (3.5 ± 1.5) × 10 −6 . A wide variety of S-bearing molecules, including the doubly sulphuretted molecule S 2 H, were later detected in the HCO peak in this PDR (Fuente et al 2017;Rivière-Marichalar et al 2019). However, sulphur is thought to be depleted inside molecular clouds by a factor of 1000 compared to its estimated cosmic abundance (Graedel et al 1982;Agúndez & Wakelam 2013).…”

“…The gas-phase H 2 S formation rate is then proportional to the attenuated local UV interstellar flux 1.7χ UV F 0 e −0.9A v + Φ SP , where F 0 is the local interstellar flux, A v the visual extinction in magnitudes, Φ SP is the flux of secondary photons, and χ UV is the incident UV field in Draine units. This rate is also proportional to the photodesorption efficiency Y H 2 S , which is set to 1.2 × 10 −3 molecules per incident photon, as calculated by Fuente et al (2017), the fraction of desorption sites occupied by H 2 S ice ( f s,H 2 S ), and the probability of collision (n gr σ gr ). The destruction rate however must account for dissociation and freezing of H 2 S molecules.…”

“…We have developed an up-to-date sulfur chemical network, based on the KInetic Database for Astrochemistry (KIDA), including recent updates (Fuente et al 2017;Le Gal et al 2017Vidal et al 2017). Our chemical network is composed of 1126 species (588 in the gas phase and 538 in solid phase) linked together via 13 155 reactions, with 8526 reactions in gas phase and 4629 reactions in solid phase.…”