Broadband transmission spectroscopy of HD 209458b with ESPRESSO: evidence for Na, TiO, or both^★

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: nuno@astro.up.pt
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
³ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁵ Observatoire Astronomique, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
⁶ INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
⁷ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Región Metropolitana, Chile
⁸ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
¹⁰ Centro de Astrobiología (CSIC-INTA), Crta. Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
¹¹ INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
¹² Consejo Superior de Investigaciones Cientícas, Spain
¹³ Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
¹⁴ Institute for Fundamental Physics of the Universe, Via Beirut 2, 34151 Grignano, Trieste, Italy
¹⁵ Instituto de Astrofísica e Ciências do Espaço, Faculdade de Ciências da Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
¹⁶ Fundación G. Galilei – INAF (Telescopio Nazionale Galileo), Rambla J. A. Fernández Pérez 7, 38712 Breña Baja, La Palma, Spain
¹⁷ Faculdade de Ciências da Universidade de Lisboa (Departamento de Física), Edifício C8, 1749-016 Lisboa, Portugal
¹⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching b. München, Germany
¹⁹ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal

Received: 16 September 2020
Accepted: 28 October 2020

Abstract

Context. The detection and characterization of exoplanet atmospheres is currently one of the main drivers pushing the development of new observing facilities. In this context, high-resolution spectrographs are proving their potential and showing that high-resolution spectroscopy will be paramount in this field.

Aims. We aim to make use of ESPRESSO high-resolution spectra, which cover two transits of HD 209458b, to probe the broadband transmission optical spectrum of the planet.

Methods. We applied the chromatic Rossiter–McLaughin method to derive the transmission spectrum of HD 209458b. We compared the results with previous HST observations and with synthetic spectra.

Results. We recover a transmission spectrum of HD 209458b similar to the one obtained with HST data. The models suggest that the observed signal can be explained by only Na, only TiO, or both Na and TiO, even though none is fully capable of explaining our observed transmission spectrum. Extra absorbers may be needed to explain the full dataset, though modeling approximations and observational errors can also be responsible for the observed mismatch.

Conclusions. Using the chromatic Rossiter–McLaughlin technique, ESPRESSO is able to provide broadband transmission spectra of exoplanets from the ground, in conjunction with space-based facilities, opening good perspectives for similar studies of other planets.