Volume 637, May 2020
|Number of page(s)||21|
|Section||Interstellar and circumstellar matter|
|Published online||12 May 2020|
Gas phase Elemental abundances in Molecular cloudS (GEMS)
II. On the quest for the sulphur reservoir in molecular clouds: the H2S case
Observatorio Astronómico Nacional (OAN),
Alfonso XII, 3,
2 Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
3 Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
4 Faculty of Aerospace Engineering, Delft University of Technology, Delft, The Netherlands
5 University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
6 Centre for Astrochemical Studies, Max-Planck-Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
7 Institut des Sciences Moléculaires (ISM), CNRS, Univ. Bordeaux, 351 cours de la Libération, 33400, Talence, France
8 Observatoire de Paris, PSL Research University, CNRS, École Normale Supérieure, Sorbonne Universités, UPMC Univ. Paris 06, 75005 Paris, France
9 Instituto Radioastronomía Milimétrica (IRAM), Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
10 Sorbonne Université, Observatoire de Paris, Université PSL, CNRS, LERMA, 92190 Meudon, France
11 École Normale Supérieure de Lyon, CRAL, UMR CNRS 5574, Université Lyon I, 46 allée d’Italie, 69364 Lyon Cedex 07, France
12 National Radio Astronomy Observatory, 520 Edgemont Rd., Charlottesville VA 22901, USA
13 Instituto de Física Fundamental (CSIC), Calle Serrano 123, 28006 Madrid, Spain
14 Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
15 Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
16 Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
17 Institute of Physics I, University of Cologne, Cologne, Germany
18 Chalmers University of Technology, Department of Space, Earth and Environment, 412 93 Gothenburg, Sweden
19 Leiden Observatory, Leiden University, PO Box 9513, 2300, Leiden, The Netherlands
Accepted: 30 March 2020
Context. Sulphur is one of the most abundant elements in the Universe. Surprisingly, sulphuretted molecules are not as abundant as expected in the interstellar medium and the identity of the main sulphur reservoir is still an open question.
Aims. Our goal is to investigate the H2S chemistry in dark clouds, as this stable molecule is a potential sulphur reservoir.
Methods. Using millimeter observations of CS, SO, H2S, and their isotopologues, we determine the physical conditions and H2S abundances along the cores TMC 1-C, TMC 1-CP, and Barnard 1b. The gas-grain model NAUTILUS is used to model the sulphur chemistry and explore the impact of photo-desorption and chemical desorption on the H2S abundance.
Results. Our modeling shows that chemical desorption is the main source of gas-phase H2S in dark cores. The measured H2S abundance can only be fitted if we assume that the chemical desorption rate decreases by more than a factor of 10 when nH > 2 × 104. This change in the desorption rate is consistent with the formation of thick H2O and CO ice mantles on grain surfaces. The observed SO and H2S abundances are in good agreement with our predictions adopting an undepleted value of the sulphur abundance. However, the CS abundance is overestimated by a factor of 5−10. Along the three cores, atomic S is predicted to be the main sulphur reservoir.
Conclusions. The gaseous H2S abundance is well reproduced, assuming undepleted sulphur abundance and chemical desorption as the main source of H2S. The behavior of the observed H2S abundance suggests a changing desorption efficiency, which would probe the snowline in these cold cores. Our model, however, highly overestimates the observed gas-phase CS abundance. Given the uncertainty in the sulphur chemistry, we can only conclude that our data are consistent with a cosmic elemental S abundance with an uncertainty of a factor of 10.
Key words: astrochemistry / ISM: abundances / ISM: kinematics and dynamics / ISM: molecules / stars: formation / stars: low-mass
© ESO 2020
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