Five New Hot Jupiter Transits Investigated with Swift-UVOT

Short-wavelength exoplanet transit measurements have been used to probe mass loss in exoplanet atmospheres. We present the Swift-UVOT transit light curves for five hot Jupiters orbiting UV-bright F-type stars: XO-3, KELT-3, WASP-3, WASP-62, and HAT-P-6. We report one positive transit detection of XO-3b and one marginal detection of KELT-3b. We place upper limits on the remaining three transit depths. The planetary radii derived from the NUV transit depths of both potential detections are 50%-100% larger than their optical radius measurements. We examine the ratio R _NUV/R _opt for trends as a function of estimated mass-loss rate, which we derive from X-ray luminosity obtained from the Swift-XRT or, in the case of WASP-62, XMM-Newton. We find no correlation between the energy-limited photoevaporative mass-loss rate and the R _NUV/R _opt ratio. We also search for trends based on the equilibrium temperature of the hot Jupiters. We find a possible indication of a transition in the R _NUV/R _opt ratio around T _eq = 1700 K, analogous to the trends found for NIR water features in transmission spectra. This might be explained by the formation of extended cloud decks with silicate particles ≤1 μm. We demonstrate that the Swift-UVOT filters could be sensitive to absorption from aerosols in exoplanet atmospheres.