Search for exoplanets around pulsating stars of A–F type in Kepler short-cadence data and the case of KIC 8197761

Abstract

We searched for extrasolar planets around pulsating stars by examining Kepler data for transit-like events hidden in the intrinsic variability. All Short Cadence observations for targets with 6000 K < T eff < 8500 K were visually inspected for transit-like events following the removal of pulsational signals by sinusoidal fits. Clear transit-like events were detected in KIC 5613330 and KIC 8197761. KIC 5613330 is a confirmed exoplanet host (Kepler-635b), where the transit period determined here is consistent with the literature value. KIC 8197761 is a γ Doradus -δ Scuti star exhibiting eclipses/transits occurring every 9.8686667(27) d, having durations of 8.37 h, and causing brightness drops ∆F F = 0.00629(29). The star's pulsation spectrum contains several mode doublets and triplets, identified as l = 1, with a mean spacing of 0.001659(15) d −1 , implying an internal rotation period of 301 ± 3 d. Trials to calculate the size of the light travel time effect (LTTE) from the pulsations to constrain the companion's mass ended inconclusive. Finding planets around γ Doradus stars from the pulsational LTTE, therefore, is concluded to be unrealistic. Spectroscopic monitoring of KIC 8197761 revealed sinusoidal radial velocity variations with a semi-amplitude of 19.75 ± 0.32 km/s, while individual spectra present rotational broadening consistent with v sin i = 9 ± 1 km/s. This suggests that the stellar surface rotation is synchronized with the orbit, whereas the stellar core rotates ∼30 times slower. Combining the observed radial velocity variability with the transit photometry, constrains the companion's mass to be ≈ 0.28 M , ruling out an exoplanet hypothesis.

Sensitivity of gravito-inertial modes to differential rotation in intermediate-mass main-sequence stars

“…A second limitation of our approach is that we require the surface rotation rate and the near-core rotation rate to agree within 50%. This assumption would be unjustified in (very) slowly rotating stars, such as KIC 8197761 (Sowicka et al 2017). However, the stars in our sample rotate significantly faster (Van Reeth et al 2016).…”

Slowing the spins of stellar cores

“…For rotation rates of about 100 days, equation (47) predicts tAM ∼ 10 6 yr, which may be longer than the timescale for shear to develop due to other effects such as internal gravity waves (Rogers et al 2013;Fuller et al 2014;Townsend et al 2018). Hence, differential rotation can persist in slowly rotating stars, and this could explain why some very slowly rotating stars (see Kurtz et al 2014;Saio et al 2015;Triana et al 2015;Kallinger et al 2017;Sowicka et al 2017) appear to exhibit some degree of differential rotation, while more rapidly rotating stars main sequence stars appear to be nearly rigidly rotating (Aerts et al 2018). The short AM transport time for main sequence stars may seemingly contradict observations of rotational evolution of young ≈ 1.0 M stars, for which several works (e.g., Denissenkov et al 2010;Gallet & Bouvier 2015;Lanzafame & Spada 2015) find evidence for core-envelope coupling times in the range tAM ∼ 10 − 100 Myr.…”

The effect of tides on near-core rotation: analysis of 35 Kepler γ Doradus stars in eclipsing and spectroscopic binaries

“…The period spacings are almost identical, implying an extremely slow rotation rate, and we obtained a near-core rotation rate of 0.0053 ± 0.0015 d −1 (see a recent study in Zhang et al 2020). The last one is KIC 8197761, whose near-core rotation rate is 0.00336 ± 0.00013 d −1 , which was measured by rotational splittings by Sowicka et al (2017) and Li et al (2019a), hence shows a smaller uncertainty. Sowicka et al (2017) made a spectroscopic observation of KIC 8197761 and found that the stellar surface is synchronized with the orbit, but the stellar core rotates ∼30 times slower.…”

“…The last one is KIC 8197761, whose near-core rotation rate is 0.00336 ± 0.00013 d −1 , which was measured by rotational splittings by Sowicka et al (2017) and Li et al (2019a), hence shows a smaller uncertainty. Sowicka et al (2017) made a spectroscopic observation of KIC 8197761 and found that the stellar surface is synchronized with the orbit, but the stellar core rotates ∼30 times slower. The strong differential rotation is unexpected (e.g.…”