A benchmark rapidly oscillating chemically peculiar (roAp) star: α Cir

¹ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
e-mail: thomas.kallinger@univie.ac.at
² Institute of Physics, University of Graz, 8020 Graz, Austria
³ Department of Physics and Astronomy, Vanderbilt University, Nashville, TN 37235, USA

Received: 19 May 2023
Accepted: 12 April 2024

Abstract

Context. The brightest chemically peculiar magnetic (mCP) star, α Cir, is also pulsating. Precise photometric and spectroscopic data, preferably with a long time base, are needed to investigate its evolutionary aspects as well. The present investigation of α Cir offers a space-based high-precision photometry study with high time resolution, covering 20+ years and supplemented by high resolution spectroscopy from the ground.

Aims. We discuss the controversial rotation periods that have been recently reported and we consider new determinations of the actual values. We process the complex pulsation frequency spectrum, considering the implications in modelling the structure of α Cir.

Methods. We developed an automated Bayesian algorithm to consistently search for periodic signals in the WIRE, SMEI, TESS, and BRITE space photometric datasets, complemented by radial velocity data from HARPS.

Results. New observations in 2021 and 2023 from TESS and BRITE indicate a detection of α Cir as a triple system. The rotation period of α Cir_A has been determined as 4.4792890 ± 0.0000018 d. The TESS data show a rich frequency spectrum including three l = 0, six l = 1, two l = 2, and one l = 3 modes. Of these, five are shown to be rotationally split. The dipole modes show significant curvature in the echelle diagram, probably due to the strong magnetic field of α Cir.

Conclusions. Overall, α Cir continues to be a cornerstone of mCP stars. A confirmation of the triple system requires additional space photometry and/or high-resolution spectroscopy to increase the time base. These data are also needed to improve the quality of the pulsation frequency spectrum and to investigate the evolutionary effects at play. A detailed seismic modelling study that considers the effects of a magnetic field on pulsation is subsequently recommended.