Gas phase Elemental abundances in Molecular cloudS (GEMS)

III. Unlocking the CS chemistry: the CS+O reaction

¹ Department of Physics, University of Firat, 23169 Elazig, Turkey
² Instituto de Física Fundamental (IFF-CSIC), C.S.I.C., Serrano 123, 28006 Madrid, Spain
e-mail: octavio.roncero@csic.es
³ Departamento de Química-Física Aplicada, Unidad asociada IFF-UAM, Universidad Autónoma de Madrid 28049, Spain
⁴ Institut des Sciences Moléculaires ISM), CNRS, Univ. Bordeaux, 351 cours de la Libération, 33400 Talence, France
⁵ Observatorio Astronómico Nacional (OAN-IGN), c/ Alfonso XII 3, 28014 Madrid, Spain
⁶ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁷ Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 92190 Meudon, France
⁸ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
⁹ Centre for Astrochemical Studies, Max-Planck-Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
¹⁰ Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
¹¹ Instituto Radioastronomía Milimétrica (IRAM), Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
¹² Observatorio de Yebes (IGN), Cerro de la Palera s/n, 19141 Yebes, Guadalajara, Spain
¹³ Department of Space, Chalmers University of Technology, Earth and Environment, 412 93 Gothenburg, Sweden
¹⁴ Faculty of Aerospace Engineering, Delft University of Technology, Delft, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁵ École Normale Supérieure de Lyon, CRAL, UMR CNRS 5574, Université Lyon I, 46 allée d’Italie, 69364, Lyon Cedex 07, France
¹⁶ Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
¹⁷ Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
¹⁸ Institute of Physics I, University of Cologne, Cologne, Germany
¹⁹ National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville VA 22901, USA
²⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 7 October 2020
Accepted: 10 December 2020

Abstract

Context. Carbon monosulphide (CS) is among the most abundant gas-phase S-bearing molecules in cold dark molecular clouds. It is easily observable with several transitions in the millimeter wavelength range, and has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. However, chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur.

Aims. The CS+O → CO + S reaction has been proposed as a relevant CS destruction mechanism at low temperatures, and could explain the discrepancy between models and observations. Its reaction rate has been experimentally measured at temperatures of 150−400 K, but the extrapolation to lower temperatures is doubtful. Our goal is to calculate the CS+O reaction rate at temperatures <150 K which are prevailing in the interstellar medium.

Methods. We performed ab initio calculations to obtain the three lowest potential energy surfaces (PES) of the CS+O system. These PESs are used to study the reaction dynamics, using several methods (classical, quantum, and semiclassical) to eventually calculate the CS + O thermal reaction rates. In order to check the accuracy of our calculations, we compare the results of our theoretical calculations for T ~ 150−400 K with those obtained in the laboratory.

Results. Our detailed theoretical study on the CS+O reaction, which is in agreement with the experimental data obtained at 150–400 K, demonstrates the reliability of our approach. After a careful analysis at lower temperatures, we find that the rate constant at 10 K is negligible, below 10⁻¹⁵ cm³ s⁻¹, which is consistent with the extrapolation of experimental data using the Arrhenius expression.

Conclusions. We use the updated chemical network to model the sulfur chemistry in Taurus Molecular Cloud 1 (TMC 1) based on molecular abundances determined from Gas phase Elemental abundances in Molecular CloudS (GEMS) project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate, ζ_H2, along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundance.