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

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$\alpha$ emission. We have analysed the TESS light curve, and find that it is a $\beta$ 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 $⟨B_z⟩$ about $+1.5$ kG as compared to $⟨B_z⟩\sim -0.8$ 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$\alpha$ emission appears to originate around HD 156424B. Using H$\alpha$, 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.

16 pages, 13 figures, 4 tables, accepted for publication in MNRAS

References in corpus (27)

• The Transiting Exoplanet Survey Satellite
• A Grid of NLTE Line-Blanketed Model Atmospheres of Early B-type Stars
• Large-scale magnetic topologies of early M dwarfs
• Dynamical Simulations of Magnetically Channeled Line-Driven Stellar Winds: II. The Effects of Field-Aligned Rotation
• The MiMeS survey of magnetism in massive stars: Magnetic analysis of the O-type stars
• The evolution of surface magnetic fields in young solar-type stars II: The early main sequence (250-650 Myr)
• Searching for links between magnetic fields and stellar evolution III. Measurement of magnetic fields in open cluster Ap stars with ESPaDOnS
• Variability of OB stars from TESS southern Sectors 1-13 and high-resolution IACOB and OWN spectroscopy
• The Magnetic Early B-type Stars I: Magnetometry and Rotation
• A naked-eye triple system with a nonaccreting black hole in the inner binary
• Diverse Variability of O and B Stars Revealed from 2-minute Cadence Light Curves in Sectors 1 and 2 of the TESS Mission: Selection of an Asteroseismic Sample
• The effects of surface fossil magnetic fields on massive star evolution: I. Magnetic field evolution, mass-loss quenching and magnetic braking
• MOBSTER - II. Identification of rotationally variable A stars observed with \emph{TESS} in Sectors 1 to 4
• The Magnetic Early B-type Stars IV: Breakout or Leakage? H$\alpha$ emission as a diagnostic of plasma transport in centrifugal magnetospheres
• The effects of surface fossil magnetic fields on massive star evolution: II. Implementation of magnetic braking in MESA and implications for the evolution of surface rotation in OB stars
• Magnetic OB[A] Stars with TESS: probing their Evolutionary and Rotational properties (MOBSTER) - I. First-light observations of known magnetic B and A stars
• Evidence of magnetic field decay in massive main-sequence stars
• Discovery of new magnetic early-B stars within the MiMeS HARPSpol survey
• How the breakout-limited mass in B-star centrifugal magnetospheres controls their circumstellar H-alpha emission
• The Magnetic Early B-type Stars II: stellar atmospheric parameters in the era of Gaia
• HD 156324: a tidally locked magnetic triple spectroscopic binary with a disrupted magnetosphere
• The magnetic field and spectral variability of the He-weak star HR 2949
• B field in OB stars (BOB): The outstandingly strong magnetic field in the evolved He-strong star CPD-62 2124
• The pulsating magnetosphere of the extremely slowly rotating magnetic $\beta$ Cep star ${\xi }^1$ CMa
• $\epsilon$ Lupi: measuring the heartbeat of a doubly-magnetic massive binary with BRITE-Constellation
• A JVLA survey of the high frequency radio emission of the massive magnetic B- and O-type stars
• The origin of magnetic fields in hot stars

Cited by in corpus (10)

• MOBSTER -- VI. The crucial influence of rotation on the radio magnetospheres of hot stars
• TESS survey of rotational and pulsational variability of mercury-manganese stars
• The effects of surface fossil magnetic fields on massive star evolution: IV. Grids of models at Solar, LMC, and SMC metallicities
• Testing the fossil field hypothesis: could strongly magnetised OB stars produce all known magnetars?
• The Polstar High Resolution Spectropolarimetry MIDEX Mission
• Detection of an extremely strong magnetic field in the double-degenerate binary merger product HD 144941
• Discovery of Magnetospheric Interactions in the Doubly-Magnetic Hot Binary $\epsilon$ Lupi
• Discovery of extraordinary X-ray emission from magnetospheric interaction in the unique binary stellar system $\epsilon$ Lupi
• HD 34736: An intensely magnetised double-lined spectroscopic binary with rapidly-rotating chemically peculiar B-type components
• MOBSTER -- VII. Using light curves to infer magnetic and rotational properties of stars with centrifugal magnetospheres