Nodal precession of a hot Jupiter transiting the edge of a late A-type star TOI-1518

Publications of the Astronomical Society of Japan, 2024, 76(3), 374–385 https://doi.org/10.1093/pasj/psae019 Advance access publication date: 2024 May 2 Nodal precession of a hot Jupiter transiting the edge of a late A-type star TOI-1518 Noriharu WATANABE ,1 ,∗ Norio NARITA ,2 ,3 ,4 and Yasunori HORI 3 ,5 1 ∗ E-mail: Abstract 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, AJ, 162, 218). The high rotation speed of the host star (∼85 km s−1 ) and the nearly polar orbit of the planet (∼120◦ ) may cause a nodal precession. In this study, we report the nodal precession undergone by TOI-1518 b. This system is the fourth planetary system in which nodal precession is detected. We investigate the time change in b from the photometric data of TOI-1518 acquired in 2019 and 2022 with TESS and from the spectral transit data of TOI-1518b obtained in 2020 with two high-dispersion spectrographs; CARMENES and EXPRES. We find that the value of b is decreasing with db/dt = −0.0116 ± 0.0036 yr−1 , indicating that the transit trajectory is moving toward the center of the stellar surface. We also estimate the minimum value of the quadrupole mass moment of TOI-1518, J2,min = 4.41 × 10−5 , and the logarithm of the Love number of TOI-1518, log k2 = −2.17 ± 0.33, from the nodal precession. Keywords: planetary systems — planets and satellites: individual (TOI-1518b) — techniques: photometric — techniques: spectroscopic 1 Introduction To date, 20 hot Jupiters have been discovered around hot stars whose effective temperatures are above 7000 K. These hot stars have a wide range of obliquities, that is, angles between the stellar rotational and orbital axes. The observed spin–orbit misalignment trends of hot Jupiters around hot stars imply that they did not undergo tidal realignment because of their shallow convective envelopes (Albrecht et al. 2012). Hot stars barely sustain stellar winds that lose their spin angular momentum due to magnetic braking. Hot stars tend to rotate rapidly, as is known for the Kraft break (Kraft 1967). The oblateness of fast-rotating stars causes nodal precession of hot Jupiters in misaligned orbits. Nodal precession was detected for three hot Jupiters on nearly polar orbits around rapidly rotating hot stars: Kepler-13Ab (Szabó et al. 2012, 2014; Herman et al. 2018), WASP-33b (Johnson et al. 2015; Watanabe et al. 2020, 2022; Stephan et al. 2022), and KELT-9b (Stephan et al. 2022). The nodal precession of a planet enables us to restrict the quadrupole mass moment J2 and the Love number k2 . J2 indicates the oblateness of the host star and its internal mass redistribution due to its rapid rotation. The Love number k2 expresses the rigidity of the internal structure, which could be an important clue for understanding the susceptibility to the tidal effects. Cabot et al. (2021) have discovered a hot Jupiter (planetary radius: Rp = 1.875 ± 0.053RJ , orbital period: Porb = 1.902603 ± 0.000011 d, scaled semi-major axis: a/Rs = .057 4.291+0 −0.061 ) around the rapidly-rotating late A-type star TOI1518 with an effective temperature Teff = 7300 ± 100 K and projected rotational speed Vsin is = 85.1 ± 6.3 km s−1 , where is is the angle between the stellar spin axis and the line of sight. They measured the projected spin–orbit obliquity .98 +0.0061 λ = −119◦ 66+0 −0.93 and impact parameter b = 0.9036−0.0053 , indicating that the planet transits the edge of the stellar surface in a near-polar orbit.1 If the orbit of TOI-1518 b shifts toward a larger b via nodal precession, then the transit of TOI-1518 b will finish in several decades. Section 2 presents our measurements of the impact parameter of TOI-1518b from the photometric data of The Transiting Exoplanet Survey Satellite (TESS; Ricker et al. 2015) and transit spectral data from high-resolution spectrographs. In section 3, we describe the change of the impact parameter and discuss the nodal precession results. Finally, we present our conclusions in section 4. 2 Observations and analyses 2.1 TESS photometry TESS observed TOI-1518 from UT 2019 October 7 to November 27 (Sectors 17 and 18) and from UT 2022 September 30 to November 26 (Sectors 57 and 58). Cabot et al. (2021) utilized the TESS Science Processing Operations Center (SPOC) High-Level Science Product (HLSP) light curves (Caldwell et al. 2020) in 2019. In addition to these datasets, 1 They defined the range of λ as 0◦ < λ < 360◦ , so they measured λ = .93 ◦ ◦ 240◦ 34+0 −0.98 . In this study, we define this range as −180 < λ < 180 . Thus, +0.98 ◦ we write λ = −119 66−0.93 . Received: 2024 January 17; Accepted: 2024 February 24 © The Author(s) 2024. Published by Oxford University Press on behalf of the Astronomical Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan 2 Komaba Institute for Science, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan 3 Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 4 Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain 5 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan Publications of the Astronomical Society of Japan (2024), Vol. 76, No. 3 375 we acquired the datasets of the SPOC light curves (Jenkins et al. 2016) in 2022 for TOI-1518 from the Mikulski Archive for Space Telescopes (MAST).2 The exposure times are 30 min in Sectors 17 and 18, and 2 min in Sectors 57 and 58. We used the Presearch Data Conditioning Simple Aperture Photometry (PDCSAP) light curves, which were included in the SPOC datasets and corrected for systematic trends using other sources on the TESS detector. Subsequently, we excluded the small discontinuities and flux ramps due to the momentum dumps, and the dimming parts due to the secondary eclipse. Figure 1 shows the light curves of TOI-1518 obtained from TESS data. We created light-curve models of TOI-1518 using PyTransit (Parviainen 2015) by a supersampling method to calculate an accurate model of the transit light curve. As the eccen.0047 tricity is negligible (e = 0.0031+0 −0.0022 ; Cabot et al. 2021), this orbit is represented by a circular orbit. Then we used a Matern 3/2 kernel for the Gaussian process to fit the wavelet-shaped features using celerite (Foreman-Mackey et al. 2017). We fitted the light curves in two epochs to the models with the following 12 parameters using Markov Chain Monte Carlo (MCMC): impact parameter b in each year, two hy2 (...truncated)