β Cas: The first δ Scuti star with a dynamo magnetic field

Abstract

Context. F-type stars are characterised by several physical processes such as different pulsation mechanisms, rotation, convection, diffusion, and magnetic fields. The rapidly rotating δ Scuti star β Cas can be considered as a benchmark star to study the interaction of several of these effects. Aims. We investigate the pulsational and magnetic field properties of β Cas. We also determine the star’s apparent fundamental parameters and chemical abundances. Methods. Based on photometric time series obtained from three different space missions (BRITE-Constellation, SMEI, and TESS), we conduct a frequency analysis and investigate the stability of the pulsation amplitudes over four years of observations. We investigate the presence of a magnetic field and its properties using spectropolarimetric observations taken with the Narval instrument by applying the least-squares deconvolution and Zeeman-Doppler imaging techniques. Results. The star β Cas shows only three independent p-mode frequencies down to the few ppm-level; its highest amplitude frequency is suggested to be an n = 3, ℓ = 2, m = 0 mode. Its magnetic field structure is quite complex and almost certainly of a dynamo origin. The atmosphere of β Cas is slightly deficient in iron peak elements and slightly overabundant in C, O, and heavier elements. Conclusions. Atypically for δ Scuti stars, we can only detect three pulsation modes down to exceptionally low noise levels for β Cas. The star is also one of very few δ Scuti pulsators known to date to show a measurable magnetic field and the first δ Scuti star with a dynamo magnetic field. These characteristics make β Cas an interesting target for future studies of dynamo processes in the thin convective envelopes of F-type stars, the transition region between fossil and dynamo fields, and the interaction between pulsations and magnetic field.