Hot Exoplanet Atmospheres Resolved with Transit Spectroscopy (HEARTS)

I. Detection of hot neutral sodium at high altitudes on WASP-49b^⋆

¹ Geneva Observatory, University of Geneva, ch. des Maillettes 51, 1290 Versoix, Switzerland
e-mail: aurelien.wyttenbach@unige.ch
² Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
³ Astrophysics Research Centre, School of Mathematics & Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK
⁴ University of Bern, Center for Space and Habitability, Sidlerstrasse 5, 3012 Bern, Switzerland
⁵ European Southern Observatory, Alonso de Cordova 3107, Vitacura Casilla 19001, Santiago 19, Chile
⁶ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
⁷ CNRS, IPAG, 38000 Grenoble, France
⁸ Aix Marseille Univ, CNRS, LAM, Laboratoire d’Astrophysique de Marseille, 13388 Marseille, France

Received: 15 November 2016
Accepted: 23 January 2017

Abstract

High-resolution optical spectroscopy during the transit of HD 189733b, a prototypical hot Jupiter, allowed the resolution of the Na i D sodium lines in the planet, giving access to the extreme conditions of the planet upper atmosphere. We have undertaken HEARTS, a spectroscopic survey of exoplanet upper atmospheres, to perform a comparative study of hot gas giants and determine how stellar irradiation affect them. Here, we report on the first HEARTS observations of the hot Saturn-mass planet WASP-49b. We observed the planet with the HARPS high-resolution spectrograph at ESO 3.6 m telescope. We collected 126 spectra of WASP-49, covering three transits of WASP-49b. We analyzed and modeled the planet transit spectrum, while paying particular attention to the treatment of potentially spurious signals of stellar origin. We spectrally resolve the Na i D lines in the planet atmosphere and show that these signatures are unlikely to arise from stellar contamination. The large contrasts of 2.0 ± 0.5% (D₂) and 1.8 ± 0.7% (D₁) require the presence of hot neutral sodium (2950⁺⁴⁰⁰_-500 K) at high altitudes (~1.5 planet radius or ~45 000 km). From estimating the cloudiness index of WASP-49b, we determine its atmosphere to be cloud free at the altitudes probed by the sodium lines. WASP-49b is close to the border of the evaporation desert and exhibits an enhanced thermospheric signature with respect to a farther-away planet such as HD 189733b.