Magnetic Fields of F-Type Stars

Magnetic Fields of F-Type Stars

Magnetic fields are an observable feature of many stars, with warm, F-type stars of interest in searching for the transition from the fossil fields in hot stars to dynamos in cool stars driven by convection zones. Thus, this thesis presents an observational survey of F-type stars close to the expected transition between fossil and dynamo magnetic fields. A magnetic snapshot survey of 55 targets resulted in 14 magnetic field detections, across every spectral class from F3V-F9V ranging in strength from 0.3±0.1G (36 UMa, F8V) to 8.3±0.9 G (h Dra, F8V). These results are followed up with detailed mapping using Zeeman Doppler Imaging (ZDI) of the large-scale surface magnetic fields for four of these stars, β CrB, σ Boo, θ Dra, and β Vir. On the hot side of the suspected transition zone, β CrB (A9) shows a relatively complex, but stable topology which is not common for a fossil field and is more complex than a dipole. On the cool side of the suspected transition zone, the surface magnetic field of σ Boo (F3V) shows a relatively simple field topology with a dominant dipole structure. This is the first magnetic map for an F3 star and provides an opportunity to study magnetism in a star with an outer convection zone thickness less than three per cent of the stellar radius. ZDI maps are also presented for two late F-stars, θ Dra (F8IV) and β Vir (F9V). These two late F-stars are contrasted with σ Boo, and show a more complex magnetic topology. There are differences in complexity of the magnetic field between θ Dra and β Vir, which is possibly related to an increased rotation rate for θ Dra which shows more detail due to a greater surface resolution. The ZDI maps are also used in modelling the stellar winds. While definitive evidence of the transition from fossil to dynamo field is not clear from the single-epoch magnetic map results obtained, this work suggests additional future maps obtained across different epochs will provide empirical evidence for how stellar mass, rotation and age drive the emergence of dynamo fields in stars with shallow convection zones.

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