A Ground-based Near-ultraviolet Secondary Eclipse Observation of KELT-9b

KELT-9b is a recently discovered exoplanet with a 1.49 day orbit around a B9.5/A0-type star. The unparalleled levels of ultraviolet irradiation that it receives from its host star put KELT-9b in its own unique class of ultra-hot Jupiters, with an equilibrium temperature >4000 K. The high quantities of dissociated hydrogen and atomic metals present in the dayside atmosphere of KELT-9b bear more resemblance to a K-type star than a gas giant. We present a single observation of KELT-9b during its secondary eclipse, taken with the Wide Field Camera on the Isaac Newton Telescope (INT). This observation was taken in the U-band, a window particularly sensitive to Rayleigh scattering. We do not detect a secondary eclipse signal, but our 3σ upper limit of 181 ppm on the depth allows us to constrain the dayside temperature of KELT-9b at pressures of ∼30 mbar to 4995 K (3σ). Although we can place an observational constraint of Ag < 0.14, our models suggest that the actual value is considerably lower than this due to H− opacity. This places KELT-9b squarely in the albedo regime populated by its cooler cousins, almost all of which reflect very small components of the light incident on their daysides. This work demonstrates the ability of ground-based 2 m class telescopes like the INT to perform secondary eclipse studies in the near-ultraviolet, which have previously only been conducted from space-based facilities.