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Free keywords:
stars: horizontal-branch; planets and satellites: detection; subdwarfs; asteroseismology; techniques: photometric
Abstract:
Stars spend most of their life on the main sequence (MS). But their most substantial changes occur off the MS stage, either before on the pre-MS or beyond at the post-MS phase. Due to very complex and varied dynamical processes, the evolution of planetary systems orbiting non-MS stars significantly differs from those of MS planetary systems. This work focusses on the search for sub-stellar companions in post-MS systems and determination of the evolutionary state of their host stars, especially subdwarf B stars (sdB stars). These are stripped Helium-burning cores of red giants with a thin hydrogen atmosphere. The canonical model involves binary evolution to explain the existence of sdB stars. Formation scenarios for single sdBs are more controversially discussed and can be hard to reconcile with observational properties. Besides the merger of two helium white dwarfs or other merger processes for apparently single sdB stars, an alternative formation channel involves planetary systems. During the red giant phase, the star would develop a common envelope with a giant planet that leads to the loss of the envelope. Thus, sdB stars are laboratories to test how planets survive and influence the late phases of stellar evolution. The rapid pulsations of sdB stars can be used to detect sub-stellar companions from periodic variations in the expected arrival times of the pulsation maxima. This timing method is particularly sensitive to companions at large distances and complementary to other exoplanet detection methods because they are not efficient for stars with small radii and high gravities. Thus, the timing method opens up a new parameter range in terms of the host stars and helps to understand the formation process of single sdBs. In this work I implemented, tested and applied the pulsation timing analysis to search for sub-stellar companions in late evolutionary stage stellar systems. The method is already established in the literature but not to an extent which is capable of automatically processing long-time series of high-cadence data, i.e., from space born observations. Part I provides an introduction to extrasolar planets, and to the formation and properties of sdB stars. Part II and III describe the long-term ground-based observations of four rapidly pulsating sdB stars DW Lyn, V1636 Ori, QQ Vir and V541 Hya. The data are used to measure the secular drifts in pulsation periods. The results constrain the evolutionary state of these stars and are compared to theoretical predictions of stellar evolutionary models. Furthermore, the measurements set limits to masses and orbital periods of sub-stellar companions. In contrast to previous studies, tentative companion detections are not confirmed. Part IV describes the application of the implemented timing analysis to other pulsating stars and data sets. Compared to ground-based observatories, satellite-based telescopes offer the advantage of uninterrupted observations. Observatories like Kepler, TESS or the upcoming PLATO mission provide a large sample of targets. Besides sdB stars, delta Scuti (delta Sct) pulsators are excellent stars to apply the timing method on. delta Sct stars are evolved beyond the MS and of spectral type A. From Kepler observations, previous studies revealed a planetary companion orbiting the delta Sct star KIC 791748. The implemented timing analysis of this work is applied to these data and can recover the planetary signature, validating the implementation at hand and independently confirming the planetary companion discovery. Part V discusses the results of this thesis and provides an outlook to further applications.
