MOBSTER – V. Discovery of a magnetic companion star to the magnetic β Cep pulsator HD 156424. Issue 4 (1st April 2021)

• Record Type: Journal Article
• Authors: Shultz, M E; Rivinius, Th; Wade, G A; Kochukhov, O; Alecian, E; David-Uraz, A; Sikora, J
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 504:Issue 4(2021)
• Page Start: 4850
• Page End: 4864
• Publication Date: 2021-04-01
• Subjects: binaries: spectroscopic -- stars: early-type -- stars: individual: HD 156424 -- stars: magnetic field -- stars: oscillations; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/staa3158
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: HD 156424 (B2 V) is a little-studied magnetic hot star in the Sco OB4 association, previously noted to display both high-frequency radial velocity (RV) variability and magnetospheric H α emission. We have analysed the Transiting Exoplanet Survery Satellite ( TESS ) light curve, and find that it is a β Cep pulsator with 11 detectable frequencies, 4 of which are independent p -modes. The strongest frequency is also detectable in RVs from ground-based high-resolution spectroscopy. RVs also show a long-term variation, suggestive of orbital motion with a period of ∼years; significant differences in the frequencies determined from TESS and RV data sets are consistent with a light-time effect from orbital motion. Close examination of the star's spectrum reveals the presence of a spectroscopic companion, however, as its RV is not variable it cannot be responsible for the orbital motion and we therefore infer that the system is a hierarchical triple with a so-far undetected third star. Reanalysis of least-squares deconvolution profiles from ESPaDOnS and HARPSpol spectropolarimetry reveals the surprising presence of a strong magnetic field in the companion star, with 〈 Bz 〉 about +1.5 kG as compared to 〈 Bz 〉 ∼−0.8 kG for the primary. HD 156424 is thus the second hot binary with two magnetic stars. We are unable to identify a rotational period for HD 156424A. The magnetospheric H α emission appears to originate around HD 156424B. Using H α, as well as other variable spectral lines, we determine a period of about 0.52 d, making HD 156424B one of the most rapidly rotating magnetic hot stars.