Roche tomography : mapping stellar activity in close binaries

• Main Author: Hill, Colin Alastair
• Published: Queen's University Belfast 2016
• Subjects: 523.8
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.706974

The secondary stars in cataclysmic variables (CVs) and close binaries are crucial to our understanding of the origin, evolution and behaviour of these systems. Magnetic activity of the secondaries has been invoked to explain many of their observed characteristics, and is thought to influence their evolution to shorter orbital periods through magnetic braking. Studies of such binaries provide unique tests of stellar dynamo theories, allowing the impact of tidal forces on the dynamo to be assessed. In this thesis, the indirect imaging technique of Roche tomography is used to map surface features and to determine binary parameters. Differential rotation (DR) of the convective envelopes of low-mass stars is a key ingredient in stellar dynamo theory. However, observational measurements of DR rates are still greatly lacking. In this thesis, the DR rate of the secondary in the CV, AE Aqr, has been measured for the first time in a mass-transferring binary. The surface was found to be not tidally locked, overturning the assumptions held by both theoreticians and observers for decades - that tidal distortion by the primary star would suppress any DR. Comparisons of the long-term magnetic activity of different stellar types is crucial to understanding the nature of the stellar dynamo, how it evolves, and what system parameters are most important in its operation. In this thesis, the long-term magnetic activity of the secondary in AE Aqr is explored by imaging starspots on seven surface maps, spanning eight years. Variations in starspot distributions due to DR and the underlying stellar dynamo mechanism are discussed; two distinct spot bands may form part of a magnetic activity cycle. Lastly, the magnetic activity of the near fully-convective close-binary, QS Vir is explored. Long- lived starspots and large prominences are found, remaining stable on timescales of over a year. Furthermore, the secondary star is found to under-fill its Roche lobe, and the system appears to be a pre-CV, yet to have initiated mass transfer via the inner Lagrangian point.