Planetary candidates around the pulsating sdB star KIC 5807616 considered doubtful

Context. It has been suggested that two weak signals observed in the low frequency region of the Fourier transform amplitude spectra from the KIC 5807616 Q5-Q8 data can be interpreted as a result of the light reflection from planets orbiting the host star. Ever since the last results on KIC 5807616 were presented, the Kepler spacecraft has collected over two years of additional data, which we analysed using asteroseismological methods. Aims. To verify and improve on previous results, we used the Q 5-Q 17 Kepler data to identify pulsational modes, determine multiplet splitting, and to re-analyse the low frequency region between 33−49 μHz where two frequencies, claimed as the planetary signature, were found. Methods. Since Fourier transform amplitude spectra of the KIC 5807616 data do not show any clear multiplets, we used two stable acoustic modes to determine the theoretical width of gravity mode multiplets and their splittings. The period spacing and histograms of common multiplet component separations were used to identify pulsation modes and the observed gravity mode splittings. In the low frequency region, we analysed the amplitude variations of two planetary signature frequencies over the whole observing run. Results. We determined the rotational period of the star from the splittings. Analysis of the low frequency region shows that the amplitude and frequency change of the signals found there have similar characteristics to other gravity modes. Conclusions. New data allow for identifying gravity modes in a limited period range, as well as better rotational period estimations. We suggest that the so-called planetary signature frequencies found in previous work might instead be pulsation modes visible beyond the cut-off frequency of the star

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

$\bf{Context}$. 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 $\it{Kepler}$ field. $\bf{Aims}$. Our goal was to find characteristics and the source of weak signals that are observed in these subdwarf light curves. $\bf{Methods}$. To achieve this, we analyzed short- and long-cadence $\it{Kepler}$ 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. $\bf{Results}$. 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 $\it{Kepler}$ 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 $\mu$Hz. $\bf{Conclusions}$. 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 $\sigma$ (or higher) for a Fourier transform analysis of $\it{Kepler}$ data in low-frequency regions.Comment: 8 pages, 8 figure

Possible gravitational microlensing events in the optical lightcurve of active galaxy S5 0716+714

A well-known active galaxy of the blazar type, S5 0716+714, is characterized by a particularly high-variability duty cycle on short timescales at optical frequencies. As such, the source was subjected to numerous monitoring programs, including both ground-based as well as spaceborne telescopes. On closer inspection of the most recent accumulation of the data provided by the Transiting Exoplanet Survey Satellite, we have noticed several conspicuous events with "volcano-like" symmetric shape, all lasting for several hours, which closely resemble the achromatic events detected with the previous Whole Earth Blazar Telescope campaigns targeting the source. We propose that those peculiar features could be due to the gravitational microlensing of the innermost segments of the precessing jet in the system by a binary lens. We study the magnification pattern of the lens with the inverse-ray shooting method and the source trajectory parameters with the Python package MuLensModel. In this way, we were able to fit successfully all the selected events with a single lens, adjusting slightly only the source trajectory parameters for each lensing event. Based on the fit results, we postulate the presence of a massive binary lens containing an intermediate-mass black hole, possibly even a supermassive one, and a much less massive companion (by a factor of ≲ 0.01) located within the host galaxy of the blazar, most likely the central kiloparsec region. We discuss the major physical implications of the proposed scenario regarding the quest for the intermediate-mass and dual supermassive black holes in active galaxies

Disc light variability in the FUor star V646 Puppis as observed by TESS and from the ground

Context. We investigate small-scale light variations in V646 Pup occurring on timescales of days, weeks, and years. Aims: We aim to investigate whether this variability is similar to that observed in FU Ori. Methods: We observed V646 Pup on six occasions at the SAAO and CTIO between 2013 and 2018 with Johnson and Sloan filters, typically using a one-day cadence maintained for two to four weeks. We also utilised the public-domain 1512-day-long ASAS-SN light curve and TESS photometry obtained in 2019 over 24.1 days with a 30 min cadence. New SAAO low-resolution spectra assist in updating major disc parameters, while the archival high-resolution Keck spectra are used to search for temporal changes in the disc rotational profiles. Results: The ground-based observations confirm the constantly decreasing brightness of V646 Pup at the rate of 0.018 mag yr-1. Precise i-band sensitive TESS data show that the slight, 0.005-0.01 mag, light variations imposed on this general trend do consist of a few independent wave trains of an apparently time-coherent nature. Assuming that this is typical situation, based on an analysis of colour-magnitude diagrams obtained for earlier epochs, we were able to make a preliminarily inference that the bulk of the light changes observed could be due to the rotation of disc photosphere inhomogeneities, arising between 10-12 R☉ from the star. We do not exclude the possibility that these inhomogeneities could also manifest themselves in the rotational profiles of the disc, as obtained from the high-resolution spectra. Assuming Keplerian rotation of these inhomogeneities, we give a preliminary determination of the stellar mass at 0.7-0.9 M☉. Conclusions: Over certain weeks, at least, V646 Pup has shown time-coherent light variability pattern(s) that could be explained by the rotation of an inhomogeneous disc photosphere. These preliminary results are similar to those better established for FU Ori, which suggests a common driving mechanism(s). Tables A.1-A.8 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/644/A13

KIC 8302197 : a non-rotating or low-inclination pulsating subdwarf B star observed with the Kepler spacecraft

We present our analysis of Kepler data of a pulsating subdwarf B star, KIC 8302197. We used Q5-17 data and applied a Fourier technique to extract 30 significant pulsation modes. We searched for multiplets and period-spacing sequences to perform a mode identification and to derive a rotation period. To our surprise, KIC 8302197 does not show any multiplets. We explain the lack of multiplets by either a very slow rotation (longer than ~1000 days) or a unique (pole-on) orientation of the pulsation axis. Our mode identification relied solely on period spacing. We were successful in identifying modal degrees of most of the detected modes. An analysis of the period stability did not show any evidence of a companion to the host star. In addition to photometric data, several spectroscopic observations were collected. Our twelve radial-velocity measurements constrain a possible orbital radial-velocity amplitude to be smaller than about 10 km s-1. Furthermore, based on color indices we constrained a possible companion to be an M or later type main sequence, a compact or a substellar object. We found that the atmospheric parameters (Teff = 27 450 ± 200 K, log g = 5.438 ± 0.033 dex, and log (nHe /nH) = −2.56 ± 0.07 dex) of KIC 8302197 are consistent with other slow pulsating subdwarf B stars. From the optical spectra we derived C, N, O, Si and Fe abundances, and set an upper limit for the S abundance

Searching for low-mass companions around white dwarfs and subdwarfs from Kepler field

Knowing the late stages of the stellar evolution is crucial for understanding the fate of planets around subdwarfs and white dwarfs. Simulations by Staff et al. (2016) show that exoplanets engulfed in the extending stellar envelope will quickly spiral down onto the parent star. Therefore, we do not expect to find planets on close by orbits to the subdwarfs (Blokesz et al. 2019) or white dwarfs. However, the recent observation of planetary debris around WD 1145+017 white dwarf suggests, there might exists planets farther away from these stars. Using binarograms, O-C diagrams and Fourier transform for the Kepler space telescope data, we investigate a problem of missing planets around white dwarfs in binary systems, single white dwarfs and subdwarfs type B. The last ones, being the only stars which (due to the lack of hydrogen) go directly to the white dwarf cooling track after their red giant phase