Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS)^★

II. A broadened sodium feature on the ultra-hot giant WASP-76b

¹ Observatoire astronomique de l’Université de Genève, chemin des Maillettes 51, 1290 Versoix, Switzerland
e-mail: julia.seidel@unige.ch
² Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
³ Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria
⁴ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁵ SNSF NCCR-PlanetS, Bern, Switzerland
⁶ Departamento Astrofísica, Centro de Astrobiología (INTA-CSIC), ESAC campus, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
⁷ Department of Physics, University of Warwick, Gibbet Hill Rd., Coventry CV4 7AL, UK
⁸ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁹ University of Bern, Center for Space and Habitability, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁰ Laboratoire d’Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
¹¹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile

Received: 4 December 2018
Accepted: 29 January 2019

Abstract

High-resolution optical spectroscopy is a powerful tool to characterise exoplanetary atmospheres from the ground. The sodium D lines, with their large cross sections, are especially suited to studying the upper layers of atmospheres in this context. We report on the results from Hot Exoplanet Atmosphere Resolved with Transit Spectroscopy survey (HEARTS), a spectroscopic survey of exoplanet atmospheres, performing a comparative study of hot gas giants to determine the effects of stellar irradiation. In this second installation of the series, we highlight the detection of neutral sodium on the ultra-hot giant WASP-76b. We observed three transits of the planet using the High-Accuracy Radial-velocity Planet Searcher (HARPS) high-resolution spectrograph at the European Southern Observatory (ESO) 3.6 m telescope and collected 175 spectra of WASP-76. We repeatedly detect the absorption signature of neutral sodium in the planet atmosphere (0.371 ± 0.034%; 10.75σ in a 0.75 Å passband). The sodium lines have a Gaussian profile with full width at half maximum (FWHM) of 27.6 ± 2.8 km s⁻¹. This is significantly broader than the line spread function of HARPS (2.7 km s⁻¹). We surmise that the observed broadening could trace the super-rotation in the upper atmosphere of this ultra-hot gas giant.