The Sulfur Plume in the Horsehead Nebula: New Detections of S2H, SH+, and CO+

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 (Sn) 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 Atacama Large Millimeter/submillimeter Array images of H2S, S2H, SO2, CO+, and SH+ toward 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 nonthermal 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.