Exploring the Atmospheric Dynamics of the Extreme Ultrahot Jupiter KELT-9b Using TESS Photometry

Abstract

We carry out a phase-curve analysis of the KELT-9 system using photometric observations from NASA's Transiting Exoplanet Survey Satellite (TESS). The measured secondary eclipse depth and peak-to-peak atmospheric brightness modulation are-+ 650 15 14 and 566±16 ppm, respectively. The planet's brightness variation reaches maximum 31±5minutes before the midpoint of the secondary eclipse, indicating a 5°.2±0°.9 eastward shift in the dayside hot spot from the substellar point. We also detect stellar pulsations on KELT-9 with a period of 7.58695±0.00091hr. The dayside emission of KELT-9b in the TESSbandpass is consistent with a blackbody brightness temperature of 4600±100 K. The corresponding nightside brightness temperature is 3040±100 K, comparable to the dayside temperatures of the hottest known exoplanets. In addition, we detect a significant phase-curve signal at the first harmonic of the orbital frequency and a marginal signal at the second harmonic. While the amplitude of the first harmonic component is consistent with the predicted ellipsoidal distortion modulation assuming equilibrium tides, the phase of this photometric variation is shifted relative to the expectation. Placing KELT-9b in the context of other exoplanets with phase-curve observations, we find that the elevated nightside temperature and relatively low day-night temperature contrast agree with the predictions of atmospheric models that include H 2 dissociation and recombination. The nightside temperature of KELT-9b implies an atmospheric composition containing about 50% molecular and 50% atomic hydrogen at 0.1bar, a nightside emission spectrum that deviates significantly from a blackbody, and a 0.5-2.0 μm transmission spectrum that is featureless at low resolution.

Atmospheric Dynamics of Hot Giant Planets and Brown Dwarfs

“…The molecular abundances of almost all species, including H 2 , are expected to vary both vertically and horizontally, CO being the only molecule to not dissociate at these temperatures due to its higher binding energy . In the hottest planets the dissociation of H 2 on the dayside and its recombination on the nightside is expected to significantly enhance the day to night heat transport through latent heat transport (Bell and Cowan 2018;Komacek and Tan 2018;Tan and Komacek 2019;Mansfield et al 2020;Wong et al 2019).…”

The TESS Phase Curve of KELT-1b Suggests a High Dayside Albedo

“…Given the precise prior information we have about KELT-1ʼs physical properties and the radial velocity orbit of the system, we placed a Gaussian prior on the Doppler boosting amplitude based on our aforementioned estimate when fitting the TESS light curve. Meanwhile, we wanted to empirically test whether the measured ellipsoidal distortion amplitude is consistent with the predictions of theory (see Wong et al 2020bWong et al , 2020d for an in-depth discussion of various caveats and assumptions inherent within the classical theory of stellar tidal response). Therefore, in our analysis, we fit for the ellipsoidal distortion signal by allowing either the amplitude A ellip (in the case of polynomial detrending; Section 2.2.1) or the mass ratio M BD /M * (in the case of Gaussian-process (GP) regression; Section 2.2.2) to vary freely.…”

“…This systematics detrending method has been utilized in several previously published TESS phase-curve analyses (Shporer et al 2019;Wong et al 2020aWong et al , 2020d. Using the ExoTEP pipeline (see, for example, Benneke et al 2019), we divided the KELT-1 light curve into six segments separated by the momentum dumps.…”

“…Though many of these planets have had their phase curves observed in the near-infrared (NIR), until recently, there were only 15 hot Jupiters around bright stars with optical phase-curve data-all from Kepler (Angerhausen et al 2015;Esteves et al 2015). The TESS mission has dramatically changed this picture, and there are now 13 additional systems with published results (Shporer et al 2019;Bourrier et al 2020;von Essen et al 2020;Wong et al 2020aWong et al , 2020cWong et al , 2020d, with many more on the way as TESS completes its 2 yr primary mission.…”

TOI-1518b: A Misaligned Ultra-hot Jupiter with Iron in Its Atmosphere

“…Our analysis methodology closely mirrors the techniques utilized in the extensive previous work on TESS phase curves (e.g., Shporer et al 2019;Wong et al 2020aWong et al , 2020bWong et al , 2020c; consult those references for a detailed description of the data processing and light-curve fitting. The full PDCSAP light curve of TOI-1518 is shown in Figure 1.…”