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

¹ Institute for Astro- and Particle Physics, Universität Innsbruck, Technikerstrasse 25, 6020 Innsbruck, Austria
e-mail: konstanze.zwintz@uibk.ac.at
² LESIA, Paris Observatory, PSL University, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
³ Department of Physics and Astronomy, Uppsala University, Box 516, 75120 Uppsala, Sweden
⁴ Institute of Astronomy, Russian Academy of Sciences (RAS), Pyatnitskaya 48, 119017 Moscow, Russia
⁵ Instytut Astronomiczny, Uniwersytet Wroclawski, ul. Kopernika 11, 51-622 Wroclaw, Poland
⁶ Institut für Kommunikationsnetze und Satellitenkommunikation, Technical University Graz, Inffeldgasse 12, 8010 Graz, Austria
⁷ Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
⁸ Département de physique, Université de Montréal, CP 6128, Succursale Centre-Ville, Montréal, Québec H3C 3J7, Canada
⁹ Centre de Recherche en Astrophysique du Québec (CRAQ), Montréal, Québec H3C 3J7, Canada
¹⁰ American Association of Variable Star Observers, 49 Bay State Road, Cambridge, MA 02138, USA
¹¹ Silesian University of Technology, Department of Electronics, Electrical Engineering and Microelectronics, Akademicka 15, 44-100 Gliwice, Poland
¹² Department of Physics and Space Science, Royal Military College of Canada, PO Box 17000, Stn Forces, Kingston, K7K 7B4 Ontario, Canada

Received: 20 April 2020
Accepted: 10 September 2020

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.