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Magnetic and activity cycles of cool stars

dc.contributor.advisorJeffers, Sandra V. Dr.
dc.contributor.authorBoro Saikia, Sudeshna
dc.titleMagnetic and activity cycles of cool starsde
dc.contributor.refereeJeffers, Sandra V. Dr.
dc.subject.gokPhysik (PPN621336750)de
dc.description.abstractengCool stars are known to exhibit weak to strong magnetic activity. Our nearest cool star, the Sun, is a middle aged and moderately active star with a cyclic generation of its magnetic field, which results in a 22 year magnetic cycle and a 11 year activity cycle. The physical process behind the solar cycles is called the dynamo. The aim of this thesis is to investigate the magnetic and activity cycles in cool stars over a range of stellar parameters, and to understand the dynamo mechanism and its dependence on basic stellar properties such as rotation and mass. The relationship between magnetic geometry and activity cycle was investigated. The large-scale magnetic field of three cool stars, HN Peg, 61 Cyg A and Epsilon Eridani is reconstructed using Zeeman Doppler imaging (ZDI). These stars vary in mass, rotation and age. The large-scale magnetic field of HN Peg is complex and highly variable, with a strong toroidal component, which is not seen in the Sun. The magnetic geometry of 61 Cyg A is strongly poloidal, which is a simple dipole during minimum activity and more complex during maximum activity, similar to the solar case. The large-scale field also flips polarity from one minimum to the next, indicating a solar-like magnetic cycle. This is the first detection of a solar-like magnetic cycle in any cool star other than the Sun. Epsilon Eridani shows a non axisymmetric complex magnetic field geometry during minimum chromospheric activity, with both poloidal and toroidal components. The toroidal percentage increases gradually in time, which is different from our Sun's axisymmetric poloidal dipolar magnetic geometry at activity minimum. Magnetic proxies such as chromospheric activity were also used to provide information on the activity cycles in cool stars, although they do not provide information about the magnetic field geometry evolution or the magnetic cycle of the star. A cool star chromospheric activity catalogue of Ca II H and K activity was developed. For the entire  cool star range, we use a recently developed calibration to obtain accurate log R’HK measurements, the physical representation of chromospheric activity. For stars with long-term observations, the activity cycle periods were determined, using a period search algorithm, and their dependence on stellar rotation was investigated. For slowly rotating stars the chromospheric activity cycle periods show a possible dependence on rotation. This dependence has previously been shown and called the `Inactive' branch of stellar cycles. On the other hand, and in contrast to previous work, we show that rapidly rotating stars exhibit a more random distribution on the cycle period-rotation plane, indicating an `Active' region rather than an 'Active' branch. Furthermore, rapidly rotating stars also show the presence of multiple cycles. The multiple cycles might be caused by their complex magnetic field geometry, which can be completely different from the solar case. This thesis shows that cool stars exhibit a range of magnetic geometry variations during their activity cycle, which does not necessarily have to be solar-like. This result is important for understanding the dynamo processes acting in cool stars including our own
dc.contributor.coRefereeDreizler, Stefan Prof. Dr.
dc.contributor.thirdRefereeSchmitt, Jürgen Prof. Dr.
dc.subject.engmagnetic fieldsde
dc.affiliation.instituteFakultät für Physikde

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