Abstract
Abstract We present dayside thermal emission observations of the hottest exoplanet KELT-9b using the new MAROON-X spectrograph. We detect atomic lines in emission with a signal-to-noise ratio of 10 using cross-correlation with binary masks. The detection of emission lines confirms the presence of a thermal inversion in KELT-9b’s atmosphere. We also use M-dwarf stellar masks to search for TiO, which has recently been invoked to explain the unusual Hubble Space Telescope WFC3 spectrum of the planet. We find that the KELT-9b atmosphere is inconsistent with the M-dwarf masks. Furthermore, we use an atmospheric retrieval approach to place an upper limit on the TiO volume mixing ratio of 10−8.5 (at 99% confidence). This upper limit is inconsistent with the models used to match the WFC3 data, which require at least an order of magnitude more TiO, thus suggesting the need for an alternate explanation of the space-based data. Our retrieval results also strongly prefer an inverted temperature profile and atomic/ion abundances largely consistent with the expectations for a solar composition gas in thermochemical equilibrium. The exception is the retrieved abundance of Fe+, which is about 1–2 orders of magnitude greater than predictions. These results highlight the growing power of high-resolution spectrographs on large ground-based telescopes to characterize exoplanet atmospheres when used in combination with new retrieval techniques.
Highlights
Since its discovery just a few years ago, the ultrahot-Jupiter KELT-9b has become a touchstone for studying the extreme physics and chemistry in the atmospheres of highly irradiated planets (Gaudi et al 2017)
The overall picture that is painted by these observations is consistent with the predictions of models that assume roughly solar composition, thermochemical equilibrium, and standard atmospheric physics (Bell & Cowan 2018; Kitzmann et al 2018; Lothringer et al 2018; Parmentier et al 2018; Tan & Komacek 2019; Fossati et al 2021)
The TiO molecule has historically been challenging to detect in exoplanet atmospheres with high-resolution techniques due to problems with the line list
Summary
Since its discovery just a few years ago, the ultrahot-Jupiter KELT-9b has become a touchstone for studying the extreme physics and chemistry in the atmospheres of highly irradiated planets (Gaudi et al 2017). Changeat & Edwards (2021) used Hubble Space Telescope (HST) WFC3 data to show that the planet’s low-resolution thermal emission spectrum in the near-infrared is highly featured They suggested that these features could be explained by opacity from the molecules TiO, VO, and FeH. We present ground-based high-resolution observations of the dayside emission from KELT-9b that were obtained using the new MAROON-X instrument on the Gemini-North telescope (Seifahrt et al 2016, 2018, 2020) We analyzed these data to test the claimed detection of molecules by Changeat & Edwards (2021) and to further explore the chemistry of this unique planet.
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