The sulfur plume in the Horsehead nebula: New detections of S2H, 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 n) have been proposed as possible reservoirs, but the only detected interstellar molecule with a disulfide bond is S2H in the Horsehead Nebula, making the estimation of sulfur chains abundances difficult. Here we present total-power ALMA images of H2S, S2H, SO2, CO+, and SH+ towards the Horsehead nebula. These observations, with unprecedented sensitivity (rms 1.5 mK), provide the first detections of SH+ and CO+ in this region, together with the identification of a new S2H line. The comparison of the spectroscopic images of H2S, S2H, SO2, CO+ and SH+ shows that the S2H emission originates from a warm gas layer adjacent to the photodissociation front. The emission peak of S2H is offset from those of reactive ions such as SH+, CO+, and SO+, suggesting that gas-phase reactions involving SH+ and H2S are not the dominant formation pathway of S2H. Instead, we propose that S2H 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.
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