The magnetic fields, activity and hidden companions of solar-type stars

The magnetic fields, activity and hidden companions of solar-type stars

The magnetic fields of the Sun and other solar-type stars manifest in a range of observable surface phenomena that provide insight into the magnetic dynamos operating within, but also hamper the detection of exoplanets. This thesis uses spectropolarimetric observations to study the connection between magnetic fields and surface activity for solar-type stars, learn about solar and stellar dynamos, their dependence on stellar properties and evolution. It also determines realistic detection thresholds for exoplanets orbiting the most magnetically active, young solar analogues. The body of work includes the largest survey to date of contemporaneous magnetic field strengths and chromospheric activity measurements for∼ 1000 late F- to early M-type dwarf stars. For F, K and M stars the mean chromospheric activity level declines fairly smoothly with decreasing mean magnetic field strength, whereas G stars show a more complex relationship suggestive of a change in dynamo properties around the middle of the main sequence. For two stars, the mature F7V star HD 75332 and the young, extremely-active G2V star V889 Her, long-term variations in the large-scale magnetic field structure and surface activity are analysed in detail. A rapid 1.06yr solar-like magnetic cycle is observed for HD 75332, whereas V889 Her shows possible 3−4yr solar-like magnetic fluctuations but no solar-like magnetic cycle. These results show for the first time that organised magnetic dynamos can operate within young solar-type stars, as well as in mature stars. Using V889 Her as a test case for the radial velocity detection of exoplanets around extremely active solar-mass stars, we determine that Jupiter-mass planets could be detected in a ≤ 8 d orbit if Doppler Imaging is used to filter the impacts of stellar activity. Overall, this thesis provides new observational constraints on the magnetic dynamos operating within both young and mature solar-type stars, and insight into the possible early history of the Sun’s dynamo by proxy. Practical recommendations, such as an ideal observing strategy of nightly observations spanning a few months, are provided to improve the capability of detecting exoplanets orbiting young solar analogues, which is a step toward the discovery of young planetary systems similar to our own solar system during its infancy.

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