Detection of atmospheric silicon in the dayside of two ultra-hot Jupiters at high spectral resolution

Ultra-hot Jupiters (UHJs) are giant exoplanets with very high dayside temperatures such that most of the molecules in their atmospheres are expected to dissociate. Investigating the atmospheres of these planets allows to measure their elemental composition and hence, enables us to constraint their formation history. We observed the two UHJs WASP-33b and KELT-20b/MASCARA-2b in the near-infrared wavelength range with CARMENES. Applying the cross-correlation technique allowed detecting the spectral signature of silicon (Si) for the first time in exoplanet atmospheres. This is an important finding, since Si is expected to be a fundamental element for the formation of silicate clouds on the planetary nightsides. The detections of Si in its gaseous phase in the two planets indicate that the dayside atmospheres of UHJs are hotter than the condensation temperatures of Si-bearing molecules. Also, the spectral lines are detected in emission, which confirms the presence of an inverted temperature profile on the planetary daysides. Investigating the presence of additional chemical species and retrieving the atmospheric temperature profiles will provide further advances in characterizing the structure of exoplanet atmospheres.