Possible origin of the non-detection of metastable He I in the upper atmosphere of the hot Jupiter WASP-80b

¹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
e-mail: Luca.Fossati@oeaw.ac.at
² INAF – Osservatorio Astronomico di Palermo, P.zza Parlamento 1, 90134 Palermo, Italy
³ Institute of Laser Physics, SB RAS, Lavrentyev Prospekt 13, Novosibirsk 630090, Russia
⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807, Merate (LC), Italy
⁵ Instituto de Astrofísica de Canarias (IAC), C. Vía Láctea 1, 38205 La Laguna, Tenerife, Spain
⁶ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁷ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025, Pino Torinese, Italy
⁸ Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation of the Russian Academy of Sciences (IZMIRAN), Kaluzhskoe Hwy 4, Troitsk, Moscow 108840, Russia

Received: 12 December 2022
Accepted: 13 March 2023

Abstract

Aims. We aim to constrain the origin of the non-detection of the metastable He I triplet at ≈10 830 Å obtained for the hot Jupiter WASP-80b.

Methods. We measure the X-ray flux of WASP-80 from archival observations and use it as input to scaling relations accounting for the coronal [Fe/O] abundance ratio in order to infer the extreme-ultraviolet (EUV) flux in the 200–504 Å range, which controls the formation of metastable He I. We run three-dimensional (magneto) hydrodynamic simulations of the expanding planetary upper atmosphere interacting with the stellar wind to study the impact on the He I absorption of the stellar high-energy emission, the He/H abundance ratio, the stellar wind, and the possible presence of a planetary magnetic field up to 1 G.

Results. For low-stellar-EUV emission, which is favoured by the measured log R′_HK value, the He I non-detection can be explained by a solar He/H abundance ratio in combination with a strong stellar wind, by a subsolar He/H abundance ratio, or by a combination of the two. For a high stellar EUV emission, the non-detection implies a subsolar He/H abundance ratio. A planetary magnetic field is unlikely to be the cause of the non-detection.

Conclusions. The low-EUV stellar flux driven by the low [Fe/O] coronal abundance is the likely primary cause of the He I non-detection. High-quality EUV spectra of nearby stars are urgently needed to improve the accuracy of high-energy emission estimates, which would then enable the employment of observations to constrain the planetary He/H abundance ratio and the stellar wind strength. This would greatly enhance the information that can be extracted from He I atmospheric characterisation observations.