First Detection of Interstellar S2H

Abstract

We present the first detection of gas phase S₂H 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 S₂H abundance is ~5×10^-11, only a factor 4-6 lower than that of the widespread H₂S molecule. H₂S and S₂H are efficiently formed on the UV-irradiated icy grain mantles. We performed ice irradiation experiments to determine the H₂S and S₂H photodesorption yields. The obtained values are ~1.2×10^-3 and <1×10^-5 molecules per incident photon for H₂S and S₂H, respectively. Our upper limit to the S₂H photodesorption yield suggests that photo-desorption is not a competitive mechanism to release the S₂H molecules to the gas phase. Other desorption mechanisms such as chemical desorption, cosmic-ray desorption and grain shattering can increase the gaseous S₂H abundance to some extent. Alternatively, S₂H can be formed via gas phase reactions involving gaseous H₂S and the abundant ions S⁺ and SH⁺. The detection of S₂H in this nebula could be therefore the result of the coexistence of an active grain surface chemistry and gaseous photo-chemistry.

Keywords: Astrochemistry; Horsehead; ISM: abundances; ISM: molecules; methods: laboratory: solid state; photon-dominated region (PDR).

Figure 1

In the central panel, we show the integrated intensity map of the H₂S 1_1,0→1_0,1 line (168.763 GHz). UV-illumination from σOri comes from the west (right). The beam is drawn in the bottom-left corner. Black circles around the surveyed positions indicate the beam of the S₂H detections. Spectra of the four S₂H lines detected towards the two positions targetted in the Whisper spectral are plotted in the left (DCO⁺ peak) and right (HCO peak) panels. The frequency in GHz is indicated in the top-left corner. In red, the Gaussian fits shown in Table 1.

Figure 2

Photodesorption of H₂S (black) detected by the QMS during irradiation of a pure H₂S ice sample. No increase of the measured ion current for the mass fragment m/z = 64 (blue, corresponding to any sulfur-sulfur photo-product) was detected. Irradiation intervals are indicated with vertical dashed lines. Signals are shifted for clarity.