2024AJ....168..108E

We present new near-ultraviolet (NUV, λ = 2479-3306 Å) transmission spectroscopy of KELT-9b, the hottest known exoplanet, obtained with the Colorado Ultraviolet Transit Experiment CubeSat. Two transits were observed on 2022 September 28th and September 29th, referred to as Visits 1 and 2 respectively. Using a combined transit and systematics model for each visit, the best-fit broadband NUV light curves are R_p/R_⋆ = 0.136${}_{-0.0146}^{+0.0125}$ for Visit 1 and R_p/R_⋆ = 0.111${}_{-0.0190}^{+0.0162}$ for Visit 2, appearing an average of 1.54× larger in the NUV than at optical wavelengths. While the systematics between the two visits vary considerably, the two broadband NUV light curves are consistent with each other. A transmission spectrum with 25 Å bins suggests a general trend of excess absorption in the NUV, consistent with expectations for ultrahot Jupiters. Although we see an extended atmosphere in the NUV, the reduced data lack the sensitivity to probe individual spectral lines.