Magnetic fields on young, solar-type stars

This thesis describes an investigation aimed at using spectropolarimetric observations of the magnetic fields of young solar-type stars to improve the understanding of the early development of solar and stellar dynamos and activity cycles. The emergence of stellar dynamos is important in understanding the evolution of young stars (and activity effects on any attendant emerging planetary systems). Stars were selected on the basis of their rapid rotation and activity. HD 106506, HD 76298, HD 35256, HD 29613 and EK Draconis formed the primary targets. Spectropolarimetric data were obtained with the Anglo-Australian Telescope, the Canada-France-Hawaii Telescope and the T´elescope Bernard Lyot with supporting broadband photometric data from the Mount Kent Observatory. Reduction and analysis of the data were performed to produce magnetic field detections, spot occupancy and magnetic field maps, differential rotation values, chromospheric flux estimates, starspot light curves and other measurements. Several results were obtained, as follows: (1) A distributed dynamo fundamentally different to the modern Sun’s interface-layer dynamo appears widespread in young solartype stars, with all targets displaying significant surface azimuthal fields consistent with the dynamo operating throughout the convection zone. (2) All stars showed significant enhancement in chromospheric activity that appears to be primarily a function of rapid stellar rotation (3) Multi-epoch observations of the young solar analogue EK Draconis demonstrated a rapidly evolving magnetic field that could not be explained due to differential rotation alone, and hints at the presence of temporal variability due to the rapid evolution from a strongly toroidal magnetic field ( approx 80%) to a more balanced poloidal-toroidal configuration in three months of observations. (4) The differential rotation of HD 29615, with a rotational shear delta omega = 0.58+ 0.14 −0.12 rad d−1 is one of the largest yet observed through x-squared minimisation method, making this star a useful test case for the theory and modelling of dynamos in young solar-type stars.