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 $Jₗₒ = 2 to Jₗₒ= 13 of HOCS+ covered by our survey, we derive a column density ofN$ = (9 $±$ 2)$×$10$¹²$ cm$⁻²$, translating into a fractional abundance relative to H$_2$ of $~$7$×$10$⁻¹¹. Conversely, the S-protonated HSCO+ isomer remains undetected, and we derive an upper limit to its abundance with respect to H_2$ of $≤$3$×$10$⁻¹¹$, a factor of $≥2.3 less abundant than HOCS+. We obtain a 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 ionisation 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 a {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.