Interstellar detection of O-protonated carbonyl sulfide, HOCS

We present the first detection in space of O-protonated carbonyl sulfide (\ch{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}\( = 13 of \ch{HOCS+} covered by our survey, we derive a column density of \)N\( = (9 \)\pm\( 2)\)\times\(10\)^{12}\( cm\)^{-2}\(, translating into a fractional abundance relative to H\)_2\( of \)\sim\(7\)\times\(10\)^{-11}\(. Conversely, the S-protonated \ch{HSCO+} isomer remains undetected, and we derive an upper limit to its abundance with respect to H\)_2\( of \)\leq\(3\)\times\(10\)^{-11}\(, a factor of \)\geq\(2.3 less abundant than \ch{HOCS+}. We obtain a \ch{HOCS+}/OCS ratio of \)\sim\(2.5\)\times\(10\)^{-3}\(, in good agreement with the prediction of astrochemical models. These models show that one of the main chemical routes to the interstellar formation of \ch{HOCS+} is likely the protonation of OCS, which appears to be more efficient at the oxygen end. Also, we find that high values of cosmic-ray ionisation rates (10\)^{-15}\(-10\)^{-14}\( s\)^{-1}\() are needed to reproduce the observed abundance of \ch{HOCS+}. In addition, we compare the O/S ratio across different interstellar environments. G+0.693-0.027 appears as the source with the lowest O/S ratio. We find a \ch{HOCO+}/\ch{HOCS+} ratio of \)\sim\(31, in accordance with other O/S molecular pairs detected toward this region and also close to the O/S solar value (\)\sim$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.