Detecting H₂O with CRIRES+: WASP-20b^★

¹ Université Côte-d’Azur, Observatoire de la Côte d’Azur, CNRS, Lagrange, CS 34229 Nice, France
e-mail: maria-chiara.mmaimone@oca.eu
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁴ INAF-Osservatorio Astrofísico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
⁵ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁶ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85741 Garching, Germany
⁷ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
⁸ Leiden Observatory, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands
⁹ Département d’astronomie de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland

Received: 30 June 2022
Accepted: 22 September 2022

Abstract

Context. Infrared spectroscopy over a wide spectral range and at the highest resolving powers (R > 70 000) has proved to be one of the leading techniques to unveil the atmospheric composition of dozens of exoplanets. The recently upgraded spectrograph CRIRES instrument at the Very Large Telescope (CRIRES+) was operative for a first science verification in September 2021, and its new capabilities in atmospheric characterization were ready to be tested.

Aims. We analyzed transmission spectra of the hot Saturn WASP-20b in the K band (1981–2394 nm) that were acquired with CRIRES+ with the aim to detect the signature of H₂O and CO.

Methods. We used a principal component analysis to remove dominant time-dependent contaminating sources such as telluric bands and the stellar spectrum. We extracted the planet spectrum by cross-correlating observations with 1D and 3D synthetic spectra, without circulation.

Results. We present the tentative detection of molecular absorption from water vapor at a signal-to-noise ratio equal to 4.2 and 4.7 by using only H₂O 1D and 3D models, respectively. The peak of the cross-correlation function occurred at the same rest-frame velocity for both model types (V_rest = −1 ± 1 km s⁻¹ ) and at the same projected orbital velocity of the planet, but with different error bands (1D model: K_p = 131 ₋₂₉⁺¹⁸ k ms ⁻¹; 3D: K_p = 131 ₋₃₉⁺²³ k ms ⁻¹). Our results agree with the result expected in the literature (132.9 ± 2.7 km ⁻¹).

Conclusions. Although the observational conditions were not ideal and we had problems with the pipeline in calibrating and reducing our raw data set, we obtained the first tentative detection of water in the atmosphere of WASP-20b. We suggest a deeper analysis and additional observations to confirm our results and unveil the presence of CO.