A new pathway to SO 2

Astronomy and Astrophysics, vol. 693, pp. A132

Abstract

Context . Photochemistry is a key process that drives planetary atmospheres away from local thermodynamic equilibrium. Recent observations of the H 2 dominated atmospheres of hot gas giants have detected SO 2 as one of the major products of this process. Aims . We investigated which chemical pathways lead to the formation of SO 2 in an atmosphere, and we investigated which part of the flux from the host star is necessary to initiate SO 2 production. Methods . We used the publicly available S–N–C–H–O photochemical network in the VULCAN chemical kinetics code to compute the disequilibrium chemistry of an exoplanetary atmosphere. Results . We find that there are two distinct chemical pathways that lead to the formation of SO 2 . The formation of SO 2 at higher pressures is initiated by stellar flux >200 nm, whereas the formation of SO 2 at lower pressures is initiated by stellar flux <200 nm. In deeper layers of the atmosphere, OH is provided by the hydrogen abstraction of H 2 O, and sulfur is provided by the photodissociation of SH and S 2 , which leads to a positive feedback cycle that liberates sulfur from the stable H 2 S molecule. In the upper layers of the atmosphere, OH is provided by the photodissociation of H 2 O, and sulfur can be liberated from H 2 S either by the photodissociation of SH and S 2 , or by the hydrogen abstraction of SH. Conclusions . We conclude that the stellar flux in the 200–350 nm wavelength range as well as the ratio of near-UV to UV radiation are important parameters determining the observability of SO 2 . In addition, we find that there is a diversity of chemical pathways to the formation of SO 2 . This is crucial for the interpretation of SO 2 detections and derived elemental abundance ratios, and for overall metallicities.

Authors 6

1. Wiebe de Gruijter

   University of Amsterdam

   Affiliation as printed

   Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090GE Amsterdam, The Netherlands

2. Shang‐Min Tsai

   University of California, Riverside

   Affiliation as printed

   Department of Earth and Planetary Sciences, University of California, Riverside, CA, USA

3. Michiel Min Aachen SRON Netherlands Institute for Space Research

   Space Research Organisation Netherlands

   Affiliation as printed

   SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands

4. Rens Waters Aachen SRON Netherlands Institute for Space Research

   Radboud University Nijmegen · Space Research Organisation Netherlands

   Affiliation as printed

   Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands

   SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands

5. T. Konings

   KU Leuven

   Affiliation as printed

   Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

6. L. Decin

   KU Leuven

   Affiliation as printed

   Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium