The sulfur plume in the Horsehead nebula: New detections of S${}_2$H, SH${}^{+}$, and CO${}^{+}$

Abstract

Sulfur is essential for life, but its abundance and distribution in the interstellar medium remain uncertain, with over 90% of sulfur undetected in cold molecular clouds. Sulfur allotropes (S${}_{\mathrm{n}}$) have been proposed as possible reservoirs, but the only detected interstellar molecule with a disulfide bond is S${}_2$H in the Horsehead Nebula, making the estimation of sulfur chains abundances difficult. Here we present total-power ALMA images of H${}_2$S, S${}_2$H, SO${}_2$, CO${}^{+}$, and SH${}^{+}$ towards the Horsehead nebula. These observations, with unprecedented sensitivity (rms $\sim$ 1.5 mK), provide the first detections of SH${}^{+}$ and CO${}^{+}$ in this region, together with the identification of a new S${}_2$H line. The comparison of the spectroscopic images of H${}_2$S, S${}_2$H, SO${}_2$, CO${}^{+}$ and SH${}^{+}$ shows that the S${}_2$H emission originates from a warm gas layer adjacent to the photodissociation front. The emission peak of S${}_2$H is offset from those of reactive ions such as SH${}^{+}$, CO${}^{+}$, and SO${}^{+}$, suggesting that gas-phase reactions involving SH${}^{+}$ and H${}_2$S are not the dominant formation pathway of S${}_2$H. Instead, we propose that S${}_2$H is desorbed from irradiated grain surfaces by non-thermal processes. The SH${}^{+}$ detection indicates that sulfur is not significantly depleted at the UV-irradiated edge of the molecular cloud, arguing against a major refractory sulfur reservoir in the interior of molecular clouds.

arXiv comment12 pages, 4 figures, accepted for publication in Astrophysical Journal Letters

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