Context. The study of stellar activity cycles is crucial to understand the underlying dynamo and how it causes magnetic activity signatures such as dark spots and bright faculae. Having knowledge ...about the dominant source of surface activity might allow us to draw conclusions about the stellar age and magnetic field topology, and to put the solar cycle in context. Aims. We investigate the underlying process that causes magnetic activity by studying the appearance of activity signatures in contemporaneous photometric and chromospheric time series. Methods. Lomb-Scargle periodograms are used to search for cycle periods present in the photometric and chromospheric time series. To emphasize the signature of the activity cycle we account for rotation-induced scatter in both data sets by fitting a quasi-periodic Gaussian process model to each observing season. After subtracting the rotational variability, cycle amplitudes and the phase difference between the two time series are obtained by fitting both time series simultaneously using the same cycle period. Results. We find cycle periods in 27 of the 30 stars in our sample. The phase difference between the two time series reveals that the variability in fast-rotating active stars is usually in anti-phase, while the variability of slowly rotating inactive stars is in phase. The photometric cycle amplitudes are on average six times larger for the active stars. The phase and amplitude information demonstrates that active stars are dominated by dark spots, whereas less-active stars are dominated by bright faculae. We find the transition from spot to faculae domination to be at the Vaughan–Preston gap, and around a Rossby number equal to one. Conclusions. We conclude that faculae are the dominant ingredient of stellar activity cycles at ages ≳2.55 Gyr. The data further suggest that the Vaughan–Preston gap cannot explain the previously detected dearth of Kepler rotation periods between 15 and 25 days. Nevertheless, our results led us to propose an explanation for the lack of rotation periods to be due to the non-detection of periodicity caused by the cancelation of dark spots and bright faculae at ∼800 Myr.
Over the course of its history, the Milky Way has ingested multiple smaller satellite galaxies. While these accreted stellar populations can be forensically identified as kinematically distinct ...structures within the Galaxy, it is difficult in general to precisely date the age at which any one merger occurred. Recent results have revealed a population of stars that were accreted via the collision of a dwarf galaxy, called \textit{Gaia}-Enceladus, leading to a substantial pollution of the chemical and dynamical properties of the Milky Way. Here, we identify the very bright, naked-eye star \(\nu\)\,Indi as a probe of the age of the early in situ population of the Galaxy. We combine asteroseismic, spectroscopic, astrometric, and kinematic observations to show that this metal-poor, alpha-element-rich star was an indigenous member of the halo, and we measure its age to be \(11.0 \pm 0.7\) (stat) \(\pm 0.8\) (sys)\(\,\rm Gyr\). The star bears hallmarks consistent with it having been kinematically heated by the \textit{Gaia}-Enceladus collision. Its age implies that the earliest the merger could have begun was 11.6 and 13.2 Gyr ago at 68 and 95% confidence, respectively. Input from computations based on hierarchical cosmological models tightens (i.e. reduces) slightly the above limits.
This thesis focuses on the application of asteroseismology to red giants observed with Kepler alongside searching for solar g-modes using the Birmingham Solar Oscillations Network (BiSON). In the ...case of the Sun, solar gravity modes are highly sought after because they can shed light on the inner rotation profile of the Sun. This thesis contains work showing how the low frequency regime of BiSON data has been cleaned enabling the search to be made in BiSON data without instrumental artefacts. Moving onwards along the stars evolution, thanks to space mission such as Kepler and CoRoT tens of thousands of red giant stars have been observed allowing huge ensemble investigations. The ability to use high-quality, long datasets as constraints to shorter and noiser datasets has been investigated through fitting the background power of 6000 Kepler red giants. Red giants also offer the opportunity to study the inclination angle distribution of stars to confirm that the distribution conforms to the expected isotropy used in many simulations. This can be extended to inferring the obliquity through asteroseismology, as applied to a red-giant, M-dwarf eclipsing binary. This offering a means to probe obliquity distributions in in a different regime to that using traditional spectroscopic techniques.
The internal working of low-mass stars is of great significance to both the study of stellar structure and the history of the Milky Way. Asteroseismology has the power to directly sense the internal ...structure of stars and allows for the determination of the evolutionary state -- i.e. has helium burning commenced or is the energy generated only by the fusion in the hydrogen-burning shell? We use observational data from red-giant stars in a combination (known as APOKASC) of asteroseismology (from the \textit{Kepler} mission) and spectroscopy (from SDSS/APOGEE). The new feature of the analysis is that the APOKASC evolutionary state determination is based on the comparison of diverse approaches to the investigation of the frequency-power spectrum. The high level of agreement between the methods is a strong validation of the approaches. Stars for which there is not a consensus view are readily identified. The comparison also facilitates the identification of unusual stars including those that show evidence for very strong coupling between p and g cavities. The comparison between the classification based on the spectroscopic data and asteroseismic data have led to a new value for the statistical uncertainty in APOGEE temperatures. These consensus evolutionary states will be used as an input for methods that derive masses and ages for these stars based on comparison of observables with stellar evolutionary models (`grid-based modeling') and as a training set for machine-learning and other data-driven methods of evolutionary state determination
This thesis focuses on the application of asteroseismology to red giants observed with Kepler alongside searching for solar g-modes using the Birmingham Solar Oscillations Network (BiSON). In the ...case of the Sun, solar gravity modes are highly sought after because they can shed light on the inner rotation profile of the Sun. This thesis contains work showing how the low frequency regime of BiSON data has been cleaned enabling the search to be made in BiSON data without instrumental artefacts. Moving onwards along the stars evolution, thanks to space mission such as Kepler and CoRoT tens of thousands of red giant stars have been observed allowing huge ensemble investigations. The ability to use high-quality, long datasets as constraints to shorter and noiser datasets has been investigated through fitting the background power of 6000 Kepler red giants. Red giants also offer the opportunity to study the inclination angle distribution of stars to confirm that the distribution conforms to the expected isotropy used in many simulations. This can be extended to inferring the obliquity through asteroseismology, as applied to a red-giant, M-dwarf eclipsing binary. This offering a means to probe obliquity distributions in in a different regime to that using traditional spectroscopic techniques.
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