Gas phase Elemental abundances in Molecular cloudS (GEMS)

IX. Deuterated compounds of H₂S in starless cores

¹ Observatorio Astronómico Nacional (OAN), Alfonso XII 3, 28014 Madrid, Spain
e-mail: m.rodriguezbaras@oan.es
² Centro de Astrobiología (CSIC-INTA), Ctra. de Ajalvir, km 4, Torrejón de Ardoz, 28850 Madrid, Spain
³ Centre for Astrochemical Studies, Max-Planck-Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Institut des Sciences Moléculaires (ISM), CNRS, Univ. Bordeaux, 351 cours de la Libération, 33400 Talence, France
⁵ LERMA, Observatoire de PARIS, PSL Research University, CNRS, Sorbonne Université, 92190 Meudon, France
⁶ Département d’Astrophysique (DAp), Commissariat à l’Énergie Atomique et aux Énergies Alternatives (CEA), Paris Saclay, Orme des Merisiers, Bât. 709, 91191 Gif sur Yvette, France
⁷ Departamento de Estadística e Investigación Operativa, Facultad de Ciencias Matemáticas, Universidad Complutense de Madrid, Plaza de las Ciencias 3, 28040 Madrid, Spain
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA

Received: 11 May 2023
Accepted: 30 August 2023

Abstract

Context. H₂S is predicted to form by hydrogenation of atomic sulphur on grains and is thought to be the main sulphur reservoir in interstellar ice, being therefore a key molecule to understanding sulphur chemistry in the star formation process and to solving the missing sulphur problem in molecular clouds and star-forming regions. The study of the H₂S deuterium fraction can be used to constrain its molecule formation pathways.

Aims. The aim of this work is to investigate for the first time the deuteration of H₂S in a large sample of starless cores.

Methods. We used observations of the GEMS IRAM 30 m Large Program and complementary IRAM 30 m telescope observations. We considered a sample of 19 starless cores located in the Taurus, Perseus, and Orion molecular clouds, detecting HDS in ten of these starless cores, and D₂S in five. The single and double H₂S deuterium fractions were analysed with regard to their relation with the cloud physical parameters, comparisons with values obtained for other interstellar sources, and comparisons with deuterium fractions in early-stage star-forming sources of abundant molecules: c-C₃H₂, H₂CS, H₂O, H₂CO, and CH₃OH.

Results. We obtain a range of X(HDS)/X(H₂S) ~ 0.025–0.2 in the starless cores with HDS detections. The five starless cores with D₂S detections show values of X(D₂S)/X(HDS) ~ 0.05–0.3. H₂S single deuteration shows an inverse relation with the cloud kinetic temperature, but no trend is found with molecular hydrogen density or visual extinction. H₂S deuteration values in starless cores are similar to those observed in Class 0, although this may be a consequence of an observational bias due to the limited spatial resolution. Comparison with c-C₃H₂, H₂CS, H₂O, H₂CO, and CH₃OH in other interstellar sources reveals a general trend of decreasing deuteration with increasing temperature, with lower values for Class I and massive star-forming sources. In starless cores and Class 0 objects, H₂CS and H₂CO present higher deuteration fractions than c-C₃H₂, H₂S, H₂O, and CH₃OH. H₂O shows single and double deuteration values one order of magnitude lower than those of H₂S and CH₃OH.

Conclusions. Differences between c-C₃H₂, H₂CS and H₂CO deuterium fractions and those of H₂S, H₂O, and CH₃OH are related to deuteration processes produced in gas or solid phases, respectively. We interpret the differences between H₂S and CH₃OH deuterations and that of H₂O as a consequence of differences in the formation routes in the solid phase, which can particularly be explained in terms of the different occurrence of the D-H and H-D substitution reactions in the ice, together with the chemical desorption processes. Further interferometric observations and laboratory experiments are needed to understand the deuteration processes.