The Sulfur Plume in the Horsehead Nebula: New Detections of S 2 H, SH + , and CO +

Le résumé fourni par la source

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 S 2 H in the Horsehead Nebula, making the estimation of sulfur chains abundances difficult. Here we present total-power Atacama Large Millimeter/submillimeter Array images of H 2 S, S 2 H, SO 2 , 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 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 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.

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