Stellar pulsations interfering with the transit light curve: Configurations with false positive misalignment

Abstract

Aims. Asymmetric features in exoplanet transit light curves are often interpreted as an effect of gravity darkening, especially if there is spectroscopic evidence of a spin-orbit misalignment. Since other processes can also lead to light-curve asymmetries, this may lead to inaccurate gravity darkening parameters. Here, we investigate the case of non-radial pulsations as possible sources of asymmetry and likely source of misinterpreted parameters through simulations. Methods. We obtained a series of simulated transit light curves of a hypothetical exoplanet-star system constructed to study the phenomenon: a host star with no gravity darkening exhibiting small amplitude pulsations and a typical hot Jupiter in a circular, edge-on orbit. Multiple scenarios of single- or multi-periodic, radial-, or non-radial pulsations of various amplitudes have been considered, and a proper account of the obscuring effect of transits on all the surface intensity components was carried out. The magnitude of amplitude and phase modulations of non-radial pulsations during transits was also also investigated. We then fit both a non-gravity-darkened and a gravity-darkened free spin-orbit axis model on the data. The Akaike and Bayesian information criteria were used for an objective selection of the most plausible model. We then explored the dependence of the parameter deviations on the pulsation properties to identify the specific configurations that may lead to falsely misaligned solutions. Results. The modulation of amplitudes of non-radial pulsations during transits have extremely low levels, so that the non-radial nature of pulsations can be safely ignored. Low-amplitude pulsations generally do not affect the determination of the system parameters beyond their noise properties. However, frequencies close to multiples of the orbital frequency (n * forb) are found to cause distortions, leading to solutions that involve a side tilted stellar rotational axis; therefore, it is preferable to clean them beforehand for the sake of a improved accuracy. Additionally, in cases with higher amplitude pulsations, a preprocessing and cleaning of the pulsations before the analysis is recommended.