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.

Keywords

• Interstellar molecules
• Astrochemistry
• Spectral line identification
• Galactic center
• Interstellar clouds