Examining the orbital decay targets KELT-9 b, KELT-16 b and WASP-4 b, and the TTV target HD 97658 b

Abstract

Orbital decay is suspected to occur especially for hot Jupiters, with the only observationally confirmed case of this being WASP-12b. By examining this effect, information on the properties of the host star can be obtained using the so-called stellar modified tidal quality factor Q′∗, which describes the efficiency with which kinetic energy of the planet is dissipated within the star. Using newly acquired photometric CHEOPS and TESS observations, and by re-analyzing archival data, we aim to improve current orbital decay fits and parameters for the KELT-16 and WASP-4 systems, and for the first time give an estimate for the KELT-9 system. Furthermore, we want to solve the TTVs of HD 97658 b, which prior to this work have been indicative of an increasing orbital period. Making use of the fit results obtained by fitting a stable orbit, an orbital decay, and an apsidal precession model to the transit and occultation timings, which were acquired using state-of-the-art MCMC algorithms, we compare the BIC values of the three models for each system to infer the model describing each system the best, while also accounting for the number of parameters in each fit. Additionally, for our orbital decay targets KELT-9 b, KELT-16 b, and WASP-4 b, we calculate their respective Q′∗ values based on the best-fit orbital decay parameters. We find that all examined systems are best described by a constant orbital period model, according to the resulting BIC values. However, in the case of WASP-4 b, the orbital decay model shows a significant trend (> 5σ), with the new CHEOPS data also possibly indicating a non-linear ephemeris. Likewise, in the case of the KELT-9 system, the recently obtained TESS and CHEOPS transit observations could hint towards a quadratic ephemeris, but more observations in the future are necessary to conclude these presumptions. For HD 97658 b, we solve the TTV issue and confirm a linear ephemeris, ruling out the previously suspected quadratic trend