H₂S and SO₂ detectability in hot Jupiters

Sulphur species as indicators of metallicity and C/O ratio

¹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
² SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
e-mail: m.min@sron.nl
³ Sterrewacht leiden, University of Leiden, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
⁴ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

Received: 31 July 2022
Accepted: 14 November 2022

Abstract

Context. The high cosmic abundance, the intermediate volatility, and the chemical properties of sulphur allow sulphur-bearing species to be used as tracers of the chemical processes in the atmospheres of hot Jupiter exoplanets. Nevertheless, despite their properties and relevance as tracers of the giant planets’ formation histories, little attention has been paid to these species in the context of hot Jupiter atmospheres.

Aims. In this paper, we provide an overview of the abundances of sulphur-bearing species in hot Jupiter atmospheres under different conditions and explore their observability.

Methods. We used the photochemical kinetics code VULCAN to model hot Jupiter atmospheric disequilibrium chemistry. Transmission spectra for these atmospheres were created using the modelling framework ARCiS. We varied model parameters such as the diffusion coefficient K_zz, and we studied the importance of photochemistry on the resulting mixing ratios. Furthermore, we varied the chemical composition of the atmosphere by increasing the metallicity from solar to ten times solar. We also explored different C/O ratios.

Results. We find that H₂S and SO₂ are the best candidates for detection between 1 and 10 μm, using a spectral resolution that is representative of the instruments on board the James Webb Space Telescope (JWST). H₂S is easiest to detect at an equilibrium temperature of ~1500 K, and with C/O ratios between 0.7 and 0.9, with the ideal value increasing slightly for increasing metallicity. SO₂ is most likely to be detected at an equilibrium temperature of ~1000 K at low C/O ratios and high metallicities. Nevertheless, among these two molecules, we expect SO₂ detection to be more common, as it is detectable in scenarios more favoured by formation models.

Conclusions. We conclude that H₂S and SO₂ will most likely be detected in the coming years with the JWST, and that the detection of these species will provide information on atmospheric processes and planet formation scenarios.