Re-Interpreting UV-Optical Transmission Spectra of Hot and Ultra-Hot Jupiters

The low-resolution transmission spectra of ultra-hot Jupiters observed shortward of 0.5 µm indicate strong absorption at short-wavelengths. Previous explanations have included scattering, photochemistry, and disequilibrium chemistry. Using the PHOENIX atmosphere model, we show that slopes and features shortward of 0.5 µm can be caused by opacity not commonly considered in atmosphere models of exoplanets but guaranteed to be present if conditions are near chemical equilibrium including Fe I, Fe II, Ti I, Ni I, Ca I, Ca II, and SiO. Even relatively trace species (e.g., Cr and V) can contribute through strong lines in the UV and blue-optical. Many of these species have also recently been detected at high-resolution in multiple ultra-hot Jupiters. We show that the presence of these species in the transit spectrum of hot and ultra-hot Jupiter can be a probe of condensation and rainout. Furthermore, our models can match low-resolution transit observations of the ultra-hot Jupiters WASP-12b, WASP-76b, and WASP-121b. These results not only verify our self-consistent models, but also further solidify ultra-hot Jupiters as the most spectrally rich exoplanets to-date, opening the door to even more detailed characterization.