Interstellar Detection of O-protonated Carbonyl Sulfide, HOCS⁺

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) x 1012 cm-2, translating into a fractional abundance relative to H2 of similar to 7 x 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 x 10-11, a factor of >= 2.3 less abundant than HOCS+. We obtain an HOCS+/OCS ratio of similar to 2.5 x 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 similar to 31, in accordance with other O/S molecular pairs detected toward this region and also close to the O/S solar value (similar to 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.