Nodal Precession of a Hot Jupiter Transiting the Edge of a Late A-Type Star TOI-1518

TOI-1518b, a hot Jupiter around a late A-type star, is one of the few planetary systems that transit the edge of the stellar surface (the impact parameter b~0.9) among hot Jupiters around hot stars (Cabot et al. 2021). The host star’s high rotation speed (Vsini_s~85 km s^-1) and the planet’s nearly polar orbit (λ~120 deg) may cause a nodal precession. If the orbit of TOI-1518b shifts toward a larger b due to the nodal precession, its transit trajectory moves further toward the edge, and the transit of TOI-1518b will cease in a few decades. In this study, we report that TOI-1518b undergoes nodal precession. This system is the fourth planetary system in which nodal precession has been detected. We investigate the time change in b from the photometric data of TOI-1518 acquired in 2019 and 2022 with the space telescope TESS and from the spectral transit data of TOI-1518b obtained in 2020 with the high dispersion spectrographs CARMENES and EXPRES. We find that the value of b is decreasing with db/dt = -0.0116±0.0036 year^-1, indicating that the transit trajectory is moving toward the center of the stellar surface at present. We also estimate the minimum value of TOI-1518’s quadrupole mass moment J_2,min = 4.41×10^-5 and the logarithm of TOI-1518’s Love number logk₂ = -2.17±0.33 from the nodal precession.