CRIRES⁺ detection of CO emissions lines and temperature inversions on the dayside of WASP-18b and WASP-76b

¹ Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
e-mail: yanfei@ustc.edu.cn
² Institut für Astrophysik und Geophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
³ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁴ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁶ Instituto de Astrofísica de Andalucía - CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁷ Max-Planck-Institute für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁸ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁹ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹⁰ UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK

Received: 4 November 2022
Accepted: 8 February 2023

Abstract

The dayside atmospheres of ultra-hot Jupiters (UHJs) are predicted to possess temperature inversion layers with extremely high temperatures at high altitudes. We observed the dayside thermal emission spectra of WASP-18b and WASP-76b with the new CRIRES⁺ high-resolution spectrograph at near-infrared wavelengths. Using the cross-correlation technique, we detected strong CO emission lines in both planets, which confirms the existence of temperature inversions on their dayside hemispheres. The two planets are the first UHJs orbiting F-type stars with CO emission lines detected; previous detections were mostly for UHJs orbiting A-type stars. Evidence of weak H₂O emission signals is also found for both planets. We further applied forward-model retrievals on the detected CO lines and retrieved the temperature-pressure profiles along with the CO volume mixing ratios. The retrieved logarithmic CO mixing ratio of WASP-18b (−2.2_−1.5^+1.4) is slightly higher than the value predicted by the self-consistent model assuming solar abundance. For WASP-76b, the retrieved CO mixing ratio (−3.6_−1.6^+1.8) is broadly consistent with the value of solar abundance. In addition, we included the equatorial rotation velocity (υ_eq) in the retrieval when analyzing the line profile broadening. The obtained υ_eq is 7.0 ± 2.9 km s⁻¹ for WASP-18b and 5.2_−3.0^+2.5 km s⁻¹ for WASP-76b, which are consistent with the tidally locked rotational velocities.