Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS)

IV. A spectral inventory of atoms and molecules in the high-resolution transmission spectrum of WASP-121 b

¹ Observatoire de Genève, Université de Genève, 51 Chemin des Maillettes, 1290 Sauverny, Switzerland
² Center for Space and Habitability, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
e-mail: jens.hoeijmakers@space.unibe.ch
³ Lund Observatory, Department of Astronomy and Theoretical Physics, Lunds Universitet, Solvegatan 9, 222 24 Lund, Sweden
⁴ Anton Pannekoek Institute of Astronomy, Universiteit van Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁵ INAF – Osservatorio Astrofisico di Arcetri Largo Enrico Fermi 5, 50125 Firenze, Italy
⁶ Institute of Astronomy, Katholieke Universiteit Leuven, Celestijnenlaan 200D 3001 Leuven, Belgium
⁷ Centre for Exoplanet Science, University of St Andrews, North Haugh, St. Andrews, KY169SS, UK
⁸ SUPA, School of Physics & Astronomy, University of St. Andrews, North Haugh, St Andrews, KY169SS, UK
⁹ Physikalisches Institut, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁰ Department of Physics, ETH Zürich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
¹¹ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹² Department of Mathematics and applied Physics, Universidad Católica de la Santísima Concepción, Alonso de Rivera 2850, Concepción, Chile
¹³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹⁴ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Regin Metropolitana, Chile
¹⁵ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
¹⁶ Departament of Astrophysics, Universidad de La Laguna (ULL), 38206, La Laguna, Tenerife, Spain
¹⁷ Department of Physics and Astronomy, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁸ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France

Received: 7 May 2020
Accepted: 19 June 2020

Abstract

Context. WASP-121 b is a hot Jupiter that was recently found to possess rich emission (day side) and transmission (limb) spectra, suggestive of the presence of a multitude of chemical species in the atmosphere.

Aims. We survey the transmission spectrum of WASP-121 b for line-absorption by metals and molecules at high spectral resolution and elaborate on existing interpretations of the optical transmission spectrum observed with the Hubble Space Telescope (HST).

Methods. We applied the cross-correlation technique and direct differential spectroscopy to search for sodium and other neutral and ionised atoms, TiO, VO, and SH in high-resolution transit spectra obtained with the HARPS spectrograph. We injected models assuming chemical and hydrostatic equilibrium with a varying temperature and composition to enable model comparison, and employed two bootstrap methods to test the robustness of our detections.

Results. We detect neutral Mg, Na, Ca, Cr, Fe, Ni, and V, which we predict exists in equilibrium with a significant quantity of VO, supporting earlier observations by HST/WFC3. Non-detections of Ti and TiO support the hypothesis that Ti is depleted via a cold-trap mechanism, as has been proposed in the literature. Atomic line depths are under-predicted by hydrostatic models by a factor of 1.5 to 8, confirming recent findings that the atmosphere is extended. We predict the existence of significant concentrations of gas-phase TiO₂, VO₂, and TiS, which could be important absorbers at optical and near-IR wavelengths in hot Jupiter atmospheres. However, accurate line-list data are not currently available for them. We find no evidence for absorption by SH and find that inflated atomic lines can plausibly explain the slope of the transmission spectrum observed in the near-ultraviolet with HST. The Na I D lines are significantly broadened (FWHM ~50 to 70 km s⁻¹) and show a difference in their respective depths of ~15 scale heights, which is not expected from isothermal hydrostatic theory. If this asymmetry is of astrophysical origin, it may indicate that Na I forms an optically thin envelope, reminiscent of the Na I cloud surrounding Jupiter, or that it is hydrodynamically outflowing.