Five new hot Jupiters 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 lightcurves 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 XMM-Newton in the case of WASP-62. 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 Teq = 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 micron. We demonstrate that the Swift-UVOT filters could be sensitive to absorption from aerosols in exoplanet atmospheres. [arXiv:2110.01579]