17:15 〜 18:30
[PAE16-P04] Measurement of Spin-orbit Obliquity of WASP-33b by Doppler Tomography and Transit Photometry
キーワード：系外惑星、惑星軌道、ホットジュピター、高温星
There are three major orbital evolution models of hot Jupiter. One model drops a hot Jupiter near its host star by the gravitational interaction between the planet and the gas disk (planet-disk interactive model) and often aligns the orbit with the stellar spin. Another model scatters more than three hot Jupiters by their mutual interactions (planet-planet scattering model), and the stellar rotation axis is inclined easily to the planetary orbital axis. The other model oscillates the orbit of hot Jupiter by the Kozai effect (Kozai-Lidov mechanism) and tends to make the obliquity distribution more widely than one via the planet-planet scattering. While the first model keeps an orbit circular, the second and the third ones make an orbit elliptic. Especially, the eccentric orbit gets circular by tidal evolution keeping its misaligned orbit.
However, a solar-like star, whose effective temperature is less than 6250K, is realigned with a planetary orbit because its thick convective has been affected by the tidal dissipation. On the other hand, a hot star, whose effective temperature is more than 7000K, has no convective zones and hardly undergoes realignment. Thus, the spin-orbit obliquity of hot Jupiter around the hot star is the clue to understanding its origin.
Doppler tomography is a powerful method to measure the spin-orbit obliquity of hot Jupiters around hot stars by transit spectroscopy. Nevertheless, a single observation can only measure the projected one and never detect the real one φ. The nodal precession occurs and moves the transit trajectory when a hot Jupiter revolves around a hot star, which tends to be oblate by its fast spin, in the misaligned orbit. The larger stellar quadrupole moment J2, which is the index of the stellar oblateness, makes the speed of the nodal precession faster. This movement changes projected spin-orbit obliquity and impact parameter, which is available by more than one observation. In terms of measuring an impact parameter, transit photometry is also a valuable method for the nodal precession. Finally, we can measure the real spin-orbit obliquity φ by the two variations.
WASP-33b, a hot Jupiter around a hot star, is the only planet whose real spin-orbit obliquity φ has been measured by the nodal precession via Doppler tomographic observation. However, because the previous study measured it by only two-epoch Doppler tomographic datasets, it is not enough to confirm WASP-33b's nodal precession. Thus, we observed WASP-33b using HIgh Dispersion Echelle Spectrograph (HIDES) in 2019 by Doppler tomographic observation. We utilized this data from HIDES. We also analyzed Doppler tomographic data by High Dispersion Spectrograph (HDS) in 2011, and Robert G. Tull Coudé Spectrograph (TS23) in 2008, 2014, and 2016 to search the variations of its projected spin-orbit obliquity and its impact parameter. We also observed the hot Jupiter for adding data points of its impact parameters by Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT) in 2017 and the second generation (MuSCAT2) in 2018. We calculated its φ and J2 by MCMC with the time variation models of the two orbital parameters.
We confirmed the more precise WASP-33b's nodal precession with more extended and more observations than the previous study. We obtained WASP-33b's real spin-orbit obliquity, φ=110.2+1.3-1.4deg. This planetary orbit has evolved with the mechanisms which makes the orbit misaligned easily such as the planet-planet scattering and the Kozai migration. This research is the first step to make a histogram of the real spin-orbit obliquity φ by Doppler tomography and transit photometry. We also found J2=1.11+0.15-0.11×10-4, which is slightly smaller than the calculated one in theory. This may indicate the possibility that its actual stellar internal structure is different from the theoretical one. In the future, we will clarify how hot Jupiters have migrated by increasing the measured samples.
However, a solar-like star, whose effective temperature is less than 6250K, is realigned with a planetary orbit because its thick convective has been affected by the tidal dissipation. On the other hand, a hot star, whose effective temperature is more than 7000K, has no convective zones and hardly undergoes realignment. Thus, the spin-orbit obliquity of hot Jupiter around the hot star is the clue to understanding its origin.
Doppler tomography is a powerful method to measure the spin-orbit obliquity of hot Jupiters around hot stars by transit spectroscopy. Nevertheless, a single observation can only measure the projected one and never detect the real one φ. The nodal precession occurs and moves the transit trajectory when a hot Jupiter revolves around a hot star, which tends to be oblate by its fast spin, in the misaligned orbit. The larger stellar quadrupole moment J2, which is the index of the stellar oblateness, makes the speed of the nodal precession faster. This movement changes projected spin-orbit obliquity and impact parameter, which is available by more than one observation. In terms of measuring an impact parameter, transit photometry is also a valuable method for the nodal precession. Finally, we can measure the real spin-orbit obliquity φ by the two variations.
WASP-33b, a hot Jupiter around a hot star, is the only planet whose real spin-orbit obliquity φ has been measured by the nodal precession via Doppler tomographic observation. However, because the previous study measured it by only two-epoch Doppler tomographic datasets, it is not enough to confirm WASP-33b's nodal precession. Thus, we observed WASP-33b using HIgh Dispersion Echelle Spectrograph (HIDES) in 2019 by Doppler tomographic observation. We utilized this data from HIDES. We also analyzed Doppler tomographic data by High Dispersion Spectrograph (HDS) in 2011, and Robert G. Tull Coudé Spectrograph (TS23) in 2008, 2014, and 2016 to search the variations of its projected spin-orbit obliquity and its impact parameter. We also observed the hot Jupiter for adding data points of its impact parameters by Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT) in 2017 and the second generation (MuSCAT2) in 2018. We calculated its φ and J2 by MCMC with the time variation models of the two orbital parameters.
We confirmed the more precise WASP-33b's nodal precession with more extended and more observations than the previous study. We obtained WASP-33b's real spin-orbit obliquity, φ=110.2+1.3-1.4deg. This planetary orbit has evolved with the mechanisms which makes the orbit misaligned easily such as the planet-planet scattering and the Kozai migration. This research is the first step to make a histogram of the real spin-orbit obliquity φ by Doppler tomography and transit photometry. We also found J2=1.11+0.15-0.11×10-4, which is slightly smaller than the calculated one in theory. This may indicate the possibility that its actual stellar internal structure is different from the theoretical one. In the future, we will clarify how hot Jupiters have migrated by increasing the measured samples.