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

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.

The BRITE data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/643/A110

Based on data collected by the BRITE Constellation satellite mission, designed, built, launched, operated and supported by the Austrian Research Promotion Agency (FFG), the University of Vienna, the Technical University of Graz, the University of Innsbruck, the Canadian Space Agency (CSA), the University of Toronto Institute for Aerospace Studies (UTIAS), the Foundation for Polish Science & Technology (FNiTP MNiSW), and National Science Centre (NCN). Also based on observations obtained at the Telescope Bernard Lyot (USR5026) operated by the Observatoire Midi-Pyrénées, Université de Toulouse (Paul Sabatier), Centre National de la Recherche Scientifique (CNRS) of France.