Detecting H2O with CRIRES+: WASP-20b

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 (Vrest = −1 ± 1 km s−1 ) and at the same projected orbital velocity of the planet, but with different error bands (1D model: Kp = 131 −29+18 k ms −1; 3D: Kp = 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.