2025AJ....170..178G

The ultraviolet and optical wavelength ranges have proven to be a key addition to infrared observations of exoplanet atmospheres because they offer unique insights into the properties of clouds and hazes and are sensitive to signatures of disequilibrium chemistry. Here, we present the 0.2-0.8 μm transmission spectrum of the T_eq = 2000 K Jupiter KELT-7b, acquired with Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3)/UVIS G280 as part of the HUSTLE Treasury program. We combined this new spectrum with the previously published HST WFC3/IR G141 (1.1-1.7 μm) spectrum and Spitzer photometric points at 3.6 and 4.5 μm, to reveal a generally featureless transmission spectrum between 0.2 and 1.7 μm, with a slight downward slope toward bluer wavelengths, and an asymmetric water feature in the 1.1-1.7 μm band. Retrieval models conclude that the 0.2-1.7 μm spectrum is primarily explained by a high H^– abundance (∼10^–5), significantly above the equilibrium chemistry prediction (∼10^–12), suggesting disequilibrium in KELT-7b's upper atmosphere. Our retrievals also suggest the presence of bright inhomogeneities in the stellar surface, and tentative evidence of CO₂ at the Spitzer wavelengths. We demonstrate that with the UV-optical coverage provided by WFC3 UVIS/G280, we are able to confirm the presence and constrain the abundance of H^–, and obtain evidence for bright stellar inhomogeneities that would have been overlooked using infrared data alone. Observations redward of 1 μm with JWST should be able to further constrain the abundance of H^–, as well as confirm the presence of CO₂ inferred by the two Spitzer data points.