The magnetic fields of β Coronae Borealis and the early F-star σ Bootis

The study of magnetism in stars close to the transition from fossil to dynamo magnetic fields is important for understanding the nature of the stellar dynamo, and dynamics of the outer atmosphere. We present surface magnetic maps for two stars which are located on opposites side of the suspected transition zone; the chemically peculiar late A-star β Coronae Borealis (A9SrEuCr), and the early F-star σ Bootis (F3V). The large scale magnetic field reconstructed at six epochs for β Coronae Borealis shows a complex fossil magnetic field, which is highly poloidal, and contains almost half the magnetic energy in higher multipoles (ℓ > 1). In contrast the single epoch magnetic map for σ Bootis contains a simple surface magnetic topology which is mostly poloidal, and predominantly dipolar, and is consistent with observations of other mature late F-stars.