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arXiv · 2010.04221

MOBSTER -- V: Discovery of a magnetic companion star to the magnetic $β$ Cep pulsator HD 156424

M. E. Shultz·Th. Rivinius·G. A. Wade·O. Kochukhov·E. Alecian·A. David-Uraz·J. Sikora·the MiMeS Collaboration
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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 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 $\sim$years; significant differences in the frequencies determined from TESS and RV datasets 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 LSD profiles from ESPaDOnS and HARPSpol spectropolarimetry reveals the surprising presence of a strong magnetic field in the companion star, with $\langle B_z \rangle$ about $+1.5$ kG as compared to $\langle B_z \rangle \sim -0.8$ 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.

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M. E. Shultz, Th. Rivinius, G. A. Wade, O. Kochukhov, E. Alecian, A. David-Uraz, J. Sikora, the MiMeS Collaboration. 2020-10-08. MOBSTER -- V: Discovery of a magnetic companion star to the magnetic $β$ Cep pulsator HD 156424. https://doi.org/10.1093/mnras%2Fstaa3158

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