The PEPSI Exoplanet Transit Survey (PETS). II. A Deep Search for Thermal Inversion Agents in KELT-20 b/MASCARA-2 b with Emission and Transmission Spectroscopy*

Recent observations have shown that the atmospheres of ultrahot Jupiters (UHJs) commonly possess temperature inversions, where the temperature increases with increasing altitude. Nonetheless, which opacity sources are responsible for the presence of these inversions remains largely observationally unconstrained. We used LBT/PEPSI to observe the atmosphere of the UHJ KELT-20 b in both transmission and emission in order to search for molecular agents which could be responsible for the temperature inversion. We validate our methodology by confirming a previous detection of Fe i in emission at 16.9 sigma. Our search for the inversion agents TiO, VO, FeH, and CaH results in non-detections. Using injection-recovery testing we set 4 sigma upper limits upon the volume mixing ratios for these constituents as low as similar to 1 x 10(-9) for TiO. For TiO, VO, and CaH, our limits are much lower than expectations from an equilibrium chemical model, while we cannot set constraining limits on FeH with our data. We thus rule out TiO and CaH as the source of the temperature inversion in KELT-20 b, and VO only if the line lists are sufficiently accurate.

Automated summary

Recent observations have shown that the atmospheres of ultrahot Jupiters commonly possess temperature inversions. However, it remains unclear which opacity sources are responsible for these inversions. In this study, the atmosphere of the ultrahot Jupiter KELT-20 b was observed in both transmission and emission to search for molecular agents that could contribute to the temperature inversion. Through testing and analysis, it was found that TiO and CaH can be ruled out as the sources, while more accurate line lists are needed to determine the possibility of VO contributing to the inversion.