beta Cas: the first delta 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 Square Deconvolution and Zeeman Doppler Imaging techniques. Results. β Cas shows only three independent p-mode frequencies down to the few ppm-level; its highest amplitude frequency is suggested to be a n = 3, = 2, m = 0 mode. Its magnetic field structure is quite complex and almost certainly of a dynamo origin. β Cas' atmosphere 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, of the transition region between fossil and dynamo fields, and the interaction between pulsations and magnetic field.

Towards the true number of flaring giant stars in the Kepler field. Are there flaring specialities associated with the giant nature?

“…Just recently however, Zwintz et al (2020) showed that β Cas, an F2III δ Sct type pulsator with only three p-mode frequencies of low amplitude, has a complex magnetic field very possibly of dynamo origin. β Cas is a terminal-age mainsequence (TAMS) star with log g = 3.53±0.58 and T eff = 6920±140 K, rotating very fast with P rot = 0.898 d which probably helps to maintain the dynamo even at such high temperature (Zwintz et al 2020). Thus, KIC 5080290 could be another example of a pulsating star with magnetic activity.…”