The active young solar-type star HR 1817 (=HD 35850)

The active young solar-type star HR 1817 (=HD 35850)

The active F dwarf HR 1817 represents the upper temperature extreme of what are broadly termed solar-type stars - stars which have the same internal structure as the Sun, albeit in this case with a much smaller convective zone. To date, studies of the active surface features and magnetic fields of solar-type stars have been restricted to G and K dwarfs. This thesis investigates the surface and magnetic features of HR 1817 using the techniques of Doppler and Zeeman Doppler Imaging, resulting in tomographic maps of the stellar surface and magnetic field. Cooler stars than HR 1817 exhibit large polar spots, and while HR 1817 also exhibits a polar spot, it is not nearly as large as those usually seen. The lower-latitude surface features of HR 1817 are weak but well defined and cover a relatively small area of the stellar surface. Total spot coverage is relatively small (~ 1.7 - 2 per cent). Zeeman Doppler Imaging reveals that HR 1817 exhibits a richly-detailed, though weak magnetic topography. A ring of azimuthal field appears around the pole, while the radial field exhibits many well-defined and distinct bipolar mid-latitude magnetic features, perhaps indicating a more dominant interface dynamo as opposed to the posited distributed dynamo of cooler active dwarfs. Finally, a differential rotation measurement of the star indicates an extremely large rotational shear. Values for the equatorial rotation and rotational shear of 6.494 +/- 0.010 rad/d and 0.256 +/- 0.017 rad/d respectively are found. This equatorial rotation is equivalent to a rotational period for HR 1817 of ~ 0.98 days. The very high rotational shear of 0.256 rad/d is fast enough for the equator to lap the pole in approximately 23 days.