The overflowing atmosphere of WASP-121 b

High-resolution He I λ10833 transmission spectroscopy with VLT/CRIRES⁺

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
² Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090 GE Amsterdam, The Netherlands
³ Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, IRAP/UMR 5277, 14 avenue Edouard Belin, 31400 Toulouse, France
⁴ Universitäts-Sternwarte, Ludwig-Maximilians-Universität München, Scheinerstraße 1, 81679 München, Germany
⁵ Exzellenzcluster Origins, Boltzmannstraße 2, 85748 Garching, Germany
⁶ Institut für Astrophysik und Geophysik, Georg-August-Universität, Friedrich Hund Platz 1, 37077 Göttingen, Germany
⁷ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁸ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁹ Instituto de Astrofísica de Andalucía – CSIC, c/ Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁰ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
¹¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹² Laboratory for Atmospheric and Space Physics, University of Colorado at Boulder, 600 UCB, Boulder, CO 80303, USA
¹³ Department of Astronomy, University of Science and Technology of China, Hefei 230026, China

^★ Corresponding author; sczesla@tls-tautenburg.de

Received: 5 June 2024
Accepted: 14 November 2024

Abstract

Transmission spectroscopy is a prime method to study the atmospheres of extrasolar planets. We obtained a high-resolution spectral transit time series of the hot Jupiter WASP-121 b with CRIRES+ to study its atmosphere via transmission spectroscopy of the He I λ10833 triplet lines. Our analysis shows a prominent He I λ10833 absorption feature moving along with the planetary orbital motion, which shows an observed, transit-averaged equivalent width of approximately 30 mÅ, a slight redshift, and a depth of about 2%, which can only be explained by an atmosphere overflowing its Roche lobe. We carried out 3D hydrodynamic modeling to reproduce the observations, which favors asymmetric mass loss with a more pronounced leading tidal tail, possibly also explaining observational evidence for additional absorption stationary in the stellar rest frame. A trailing tail is not detectable. From our modeling, we derived estimates of ≥2 × 10¹³ g s⁻¹ for the stellar and 5.4 × 10¹² g s⁻¹ for the planetary mass loss rate, which is consistent with X-ray and extreme-ultraviolet (XUV) driven mass loss in WASP-121 b.