2024ApJ...965..149S

We present the first detection in space of O-protonated carbonyl sulfide (HOCS⁺), in the midst of an ultradeep molecular line survey toward the G+0.693-0.027 molecular cloud. From the observation of all K_a = 0 transitions ranging from J_lo = 2 to J_lo = 13 of HOCS⁺ covered by our survey, we derive a column density of N = (9 ± 2) × 10¹² cm^–2, translating into a fractional abundance relative to H₂ of ∼7 × 10^–11. Conversely, the S-protonated HSCO⁺ isomer remains undetected, and we derive an upper limit to its abundance with respect to H₂ of ≤3 × 10^–11, a factor of ≥2.3 less abundant than HOCS⁺. We obtain an HOCS⁺/OCS ratio of ∼2.5 × 10^–3, in good agreement with the prediction of astrochemical models. These models show that one of the main chemical routes to the interstellar formation of HOCS⁺ is likely the protonation of OCS, which appears to be more efficient at the oxygen end. Also, we find that high values of cosmic-ray ionization rates (10^–15-10^–14 s^–1) are needed to reproduce the observed abundance of HOCS⁺. In addition, we compare the O/S ratio across different interstellar environments. G+0.693-0.027 appears as the source with the lowest O/S ratio. We find an HOCO⁺/HOCS⁺ ratio of ∼31, in accordance with other O/S molecular pairs detected toward this region and also close to the O/S solar value (∼37). This fact indicates that S is not significantly depleted within this cloud due to the action of large-scale shocks, unlike in other sources where S-bearing species remain trapped on icy dust grains.