Atmospheric Rossiter–McLaughlin effect and transmission spectroscopy of WASP-121b with ESPRESSO^★

¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
e-mail: francesco.borsa@inaf.it
² Département d’astronomie, Université de Genève, Ch. des Maillettes 51, 1290 Versoix, Switzerland
³ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁴ Fundación G. Galilei – INAF (TNG), Rambla J. A. Fernández Pérez 7, 38712 Breña Baja (La Palma), Spain
⁵ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁶ Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, 38205 La Laguna (Tenerife), Spain
⁷ Universidad de La Laguna (ULL), Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁸ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁹ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento, 1, 90134, Palermo, Italy
¹⁰ Instituto de Astrofísica e Ciênçias do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹¹ Departamento de Física e Astronomia, Faculdade de Ciençias, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹² Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching b. Munchen, Germany
¹⁴ Universitat Bern, Physikalisches Institut, Siedlerstrasse 5, 3012 Bern, Switzerland
¹⁵ Faculdade de Ciênçias da Universidade de Lisboa (Departamento de Física), Edifício C8, 1749-016 Lisboa, Portugal
¹⁶ Instituto de Astrofísica e Ciênçias do Espaço, Universidade de Lisboa, Edifício C8, 1749-016 Lisboa, Portugal
¹⁷ Consejo Superior de Investigaciones Científicas, 28006 Madrid, Spain
¹⁸ ESO, European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago
¹⁹ Institute for Fundamental Physics of the Universe, IFPU, Via Beirut 2, 34151 Grignano, Trieste, Italy

Received: 4 September 2020
Accepted: 26 October 2020

Abstract

Context. Ultra-hot Jupiters are excellent laboratories for the study of exoplanetary atmospheres. WASP-121b is one of the most studied; many recent analyses of its atmosphere report interesting features at different wavelength ranges.

Aims. In this paper we analyze one transit of WASP-121b acquired with the high-resolution spectrograph ESPRESSO at VLT in one-telescope mode, and one partial transit taken during the commissioning of the instrument in four-telescope mode.

Methods. We take advantage of the very high S/N data and of the extreme stability of the spectrograph to investigate the anomalous in-transit radial velocity curve and study the transmission spectrum of the planet. We pay particular attention to the removal of instrumental effects, and stellar and telluric contamination. The transmission spectrum is investigated through single-line absorption and cross-correlation with theoretical model templates.

Results. By analyzing the in-transit radial velocities we were able to infer the presence of the atmospheric Rossiter–McLaughlin effect. We measured the height of the planetary atmospheric layer that correlates with the stellar mask (mainly Fe) to be 1.052 ± 0.015 R_p and we also confirmed the blueshift of the planetary atmosphere. By examining the planetary absorption signal on the stellar cross-correlation functions we confirmed the presence of a temporal variation of its blueshift during transit, which could be investigated spectrum-by-spectrum thanks to the quality of our ESPRESSO data. We detected significant absorption in the transmission spectrum for Na, H, K, Li, Ca II, and Mg, and we certified their planetary nature by using the 2D tomographic technique. Particularly remarkable is the detection of Li, with a line contrast of ~0.2% detected at the 6σ level. With the cross-correlation technique we confirmed the presence of Fe I, Fe II, Cr I, and V I. Hα and Ca II are present up to very high altitudes in the atmosphere (~1.44 R_p and ~2 R_p, respectively), and also extend beyond the transit-equivalent Roche lobe radius of the planet. These layers of the atmosphere have a large line broadening that is not compatible with being caused by the tidally locked rotation of the planet alone, and could arise from vertical winds or high-altitude jets in the evaporating atmosphere.