Interstellar Detection Of O-protonated Carbonyl Sulfide, HOCS+

By Keith Cowing
Press Release
February 26, 2024
Interstellar Detection Of O-protonated Carbonyl Sulfide, HOCS+
Transitions of HOCS+ identified toward the G+0.693–0.027 molecular cloud (listed in Table 1). The result of the best LTE fit of HOCS+ is shown with a red line, the green line plots the predicted emission of HNC34S, and the blue line plots the emission from all the molecules identified to date in our survey (including the latter two). The observed spectra are plotted as gray histograms. The structure of HOCS+, taken from Fortenberry et al. (2012), is also shown (carbon atoms in gray; oxygen atoms are in red, sulfur atoms in yellow and hydrogen atoms in white). Note that HNC34S is blended with HOCS+ because of their similarity of B + C but its contribution can be well constrained based on the HNC32S/HNC34S isotopic ratio (see text). — astro-ph.GA

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 Ka = 0 transitions ranging from Jlo = 2 to Jlo = 13 of HOCS+ covered by our survey, we derive a column density of N = (9 ± 2)×1012 cm−2, translating into a fractional abundance relative to H2 of ∼7×10−11. Conversely, the S-protonated HSCO+ isomer remains undetected, and we derive an upper limit to its abundance with respect to H2 of ≤3×10−11, 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.

Miguel Sanz-Novo, Víctor M. Rivilla, Izaskun Jiménez-Serra, Jesús Martín-Pintado, Laura Colzi, Shaoshan Zeng, Andrés Megías, Álvaro López-Gallifa, Antonio Martínez-Henares, Sarah Massalkhi, Belén Tercero, Pablo de Vicente, David San Andrés, Sergio Martín, Miguel A. Requena-Torres

Comments: Forthcoming paper in The Astrophysical Journal (in press)
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2402.15405 [astro-ph.GA] (or arXiv:2402.15405v1 [astro-ph.GA] for this version)
Submission history
From: Miguel Sanz-Novo
[v1] Fri, 23 Feb 2024 16:07:09 UTC (8,895 KB)
Astrobiology, Astrochemistry,

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻