Interstellar Detection of O-protonated Carbonyl Sulfide, HOCS +

Abstract

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) × 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 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.