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

• Published in: Astronomy and astrophysics (Berlin) 2020-11, Vol.643, p.A110
• Main Authors: Zwintz, K., Neiner, C., Kochukhov, O., Ryabchikova, T., Pigulski, A., Müllner, M., Steindl, T., Kuschnig, R., Handler, G., Moffat, A. F. J., Pablo, H., Popowicz, A., Wade, G. A.
• Format: Article
• Language: English
• Subjects: Amplitudes; Astrophysics; Constellations; F stars; Frequency analysis; Imaging techniques; Low noise; Magnetic fields; Magnetic properties; Magnetism; Noise levels; Physics; Pulsation; Space missions; Stability analysis; Stellar magnetic fields; Stellar rotation; Variable stars
• ISSN: 0004-6361; 1432-0746; 1432-0756
• DOI: 10.1051/0004-6361/202038210

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.