Colorado Ultraviolet Transit Experiment Near-Ultraviolet Transmission Spectroscopy of the Ultra-hot Jupiter KELT-9b

TL;DR

This study presents near-ultraviolet transmission spectroscopy of the ultra-hot Jupiter KELT-9b using the CUTE CubeSat, revealing excess NUV absorption and an extended atmosphere, consistent with expectations for such extreme exoplanets.

Contribution

First NUV transmission spectrum of KELT-9b obtained with a CubeSat, demonstrating the presence of excess absorption and an extended atmosphere in the NUV.

Findings

NUV radius ratio larger than optical

Consistent broadband NUV light curves across visits

Evidence of excess NUV absorption in the atmosphere

Abstract

We present new near-ultraviolet (NUV, $\lambda$ = 2479 $-$ 3306 $\r{A}$) transmission spectroscopy of KELT-9b, the hottest known exoplanet, obtained with the Colorado Ultraviolet Transit Experiment ($CUTE$) CubeSat. Two transits were observed on September 28th and September 29th 2022, 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$_{\text{p}}$/R$_{\star}$ $=$ 0.136$_{0.0146}^{0.0125}$ for Visit 1 and R$_{\text{p}}$/R$_{\star}$ $=$ 0.111$_{0.0190}^{0.0162}$ for Visit 2, appearing an average of 1.54$\times$ 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 $\r{A}$ bins suggests a general trend of excess absorption in the NUV, consistent with expectations for ultra-hot Jupiters. Although we see an extended atmosphere in the NUV, the reduced data lack the sensitivity to probe individual spectral lines.