Magnetic characterization and variability study of the magnetic SPB star o Lupi

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: bram.buysschaert@obspm.fr
² Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ Department of Physics, California Lutheran University, 60 West Olsen Road # 3700, Thousand Oaks, CA 91360, USA
⁴ Dept. of Astrophysics, IMAPP, Radboud University Nijmegen, 6500 GL Nijmegen, The Netherlands
⁵ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France

Received: 7 September 2017
Accepted: 7 August 2018

Abstract

Thanks to large dedicated surveys, large-scale magnetic fields have been detected for about 10% of early-type stars. We aim to precisely characterize the large-scale magnetic field of the magnetic component of the wide binary o Lupi, by using high-resolution ESPaDOnS and HARPSpol spectropolarimetry to analyze the variability of the measured longitudinal magnetic field. In addition, we have investigated the periodic variability using space-based photometry collected with the BRITE-Constellation by means of iterative prewhitening. The rotational variability of the longitudinal magnetic field indicates a rotation period P_rot = 2.95333(2) d and that the large-scale magnetic field is dipolar, but with a significant quadrupolar contribution. Strong differences in the strength of the measured magnetic field occur for various chemical elements as well as rotational modulation for Fe and Si absorption lines, suggesting a inhomogeneous surface distribution of chemical elements. Estimates of the geometry of the large-scale magnetic field indicate i = 27 ± 10°, β = 74₋₉^+7°, and a polar field strength of at least 5.25 kG. The BRITE photometry reveals the rotation frequency and several of its harmonics, as well as two gravity mode pulsation frequencies. The high-amplitude g-mode pulsation at f = 1.1057 d⁻¹ dominates the line-profile variability of the majority of the spectroscopic absorption lines. We do not find direct observational evidence of the secondary in the spectroscopy. Therefore, we attribute the pulsations and the large-scale magnetic field to the B5IV primary of the o Lupi system, but we discuss the implications should the secondary contribute to or cause the observed variability.