Presentation #624.39 in the session Planetary Atmospheres - Hot Jupiters.
We present new near-ultraviolet (NUV) transmission spectroscopic observations of KELT-9b, the hottest known exoplanet to date, obtained with the Colorado Ultraviolet Transit Experiment (CUTE) CubeSat. A low resolution (R ~ 112) transmission spectrum between 2479 - 3306 A shows several features with effective altitudes beyond the planet’s Roche lobe of 1.91 Rp, though none with a 3 σ detection. A 25 A bin containing Mg2 exhibits a radius ratio of Rp/R* = 0.160 ± 0.0417%, corresponding to an optically thick radius of R = 2.56 Rp. Three 25 A bins between 2500 A and 2600 A exhibit absorption consistent with R = 2.021 Rp. A very strongly absorbing feature at 2850 A with R = 2.42 Rp is not predicted by current NUV models. The transmission spectrum is too low to isolate specific atomic absorbers, but we speculate that metals such as Fe I, Fe II, and Ti II, which have been detected in the planet’s thermosphere with optical transmission spectroscopy, are responsible for the enhanced transit depths.