Analysis of putative exoplanetary signatures found in light curves of two sdBV stars observed by Kepler

Abstract

Context. We investigate the validity of the claim that invokes two extreme exoplanetary system candidates around the pulsating B-type subdwarfs KIC 10001893 and KIC 5807616 from the primary Kepler field. Aims. Our goal was to find characteristics and the source of weak signals that are observed in these subdwarf light curves. Methods. To achieve this, we analyzed short-and long-cadence Kepler data of the two stars by means of a Fourier transform and compared the results to Fourier transforms of simulated light curves to which we added exoplanetary signals. The long-cadence data of KIC 10001893 were extracted from CCD images of a nearby star, KIC 10001898, using a point spread function reduction technique.Results. It appears that the amplitudes of the Fourier transform signals that were found in the low-frequency region depend on the methods that are used to extract and prepare Kepler data. We demonstrate that using a comparison star for space telescope data can significantly reduce artifacts. Our simulations also show that a weak signal of constant amplitude and frequency, added to a stellar light curve, conserves its frequency in Fourier transform amplitude spectra to within 0.03 µHz. Conclusions. Based on our simulations, we conclude that the two low-frequency Fourier transform signals found in KIC 5807616 are likely the combined frequencies of the lower amplitude pulsating modes of the star. In the case of KIC 10001893, the signal amplitudes that are visible in the light curve depend on the data set and reduction methods. The strongest signal decreases significantly in amplitude when KIC 10001898 is used as a comparison star. Finally, we recommend that the signal detection threshold is increased to 5 σ (or higher) for a Fourier transform analysis of Kepler data in low-frequency regions.

Asteroseismic Observations of Hot Subdwarfs

“…Krzesinski then concludes that the existence of two planets orbiting KPD 1943 + 4058, as Charpinet et al (2011a) claim, must be in doubt because the two planetary signature frequencies could instead be g-mode pulsations. Blokesz et al (2019) demonstrate that the use of a comparison star to provide a local determination of the point-spread function, when extracting Kepler satellite photometry, can significantly reduce artifacts. It then appears that amplitudes of Fourier transform signals found in the low-frequency regions for KPD 1943 + 4058 (Charpinet et al, 2011a) and KIC 10001893 (Silvotti et al, 2014) depend on methods used to extract Kepler data.…”

A search for transiting planets around hot subdwarfs

“…Based on weak signals that were interpreted as reflection and thermal re-emission in Kepler light curves, five very close-in (with orbital periods of a few hours) Earth-sized planets have been claimed to orbit KIC 05807616 (Charpinet et al 2011) and KIC 10001893 (Silvotti et al 2014). However, the attribution of these signals to exoplanets is debatable (Krzesinski 2015;Blokesz et al 2019). Using the RV method, Geier et al (2009) announced the discovery of a close-in (P orb = 2.4 days) planet of several Jupiter masses around HD 149382, but this was ruled out by high-precision RV measurements obtained with the Hobby-Eberly Telescope spectrograph, which excluded the presence of almost any substellar companion with P orb < 28 days and M sin i 1M Jup (Norris et al 2011).…”

A transit survey to search for planets around hot subdwarfs: I. methods and performance tests on light curves from Kepler, K2, TESS, and CHEOPS

“…Based on weak signals interpreted as reflection and thermal re-emission in Kepler light curves, five very close-in (with orbital periods of a few hours) Earth-sized planets have been claimed to orbit KIC 05807616 (Charpinet et al 2011) andKIC 10001893 (Silvotti et al 2014). However, the attribution of such signals to exoplanets is debatable (Krzesinski 2015;Blokesz et al 2019). Using the RV method, Geier et al (2009) announced the discovery of a close-in (P orb " 2.4 days) planet of several Jupiter masses around HD 149382, but it was ruled out by high-precision RV measurements obtained with the Hobby-Eberly Telescope spectrograph, excluding the presence of almost any substellar companion with P orb ă 28 days and M sin i Á 1M Jup (Norris et al 2011).…”