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

N. C. Santos, E. A. S. Cristo, O. Demangeon, M. Oshagh, R. Allart, S. C. C. Barros, F. Borsa, V. Bourrier, N. Casasayas-Barris, D. Ehrenreich, J. P. Faria, P. Figueira, J. H. C. Martins, G. Micela, E. Pallé, A. Sozzetti, H. M. Tabernero, M. R. Zapatero-Osorio, F. Pepe, S. Cristiani, R. Rebolo López, V. Zh. Adibekyan, C. Allende Prieto, Y. Alibert, F. Bouchy, A. Cabral, H. Dekker, P. Di Marcantonio, V. D'Odorico, X. Dumusque, J. I. González Hernández, B. Lavie, G. Lo Curto, C. Lovis, A. Manescau, C. J. A. P. Martins, D. Mégevand, A. Mehner, P. Molaro, N. J. Nunes, E. Poretti, M. Riva, S. G. Sousa, A. Suárez Mascareño, S. Udry

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.

Keywords
planetary systems; planets and satellites: atmospheres; techniques: spectroscopic; Astrophysics - Earth and Planetary Astrophysics

Notes
Based on Guaranteed Time Observations collected at the European Southern Observatory under ESO programme 1102.C-0744 by the ESPRESSO Consortium.

Astronomy and Astrophysics
Volume 644, Article Number A51, Number of pages 13
2020 January

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia