Discovery of a highly magnetic He-sdO star from a double-degenerate binary merger

Abstract

Helium-rich hot subdwarf stars of spectral type O (He-sdO) are considered prime candidates for stellar merger remnants. Such events should lead to the generation of strong magnetic fields. However, no magnetic He-sdO has yet been unambiguously discovered despite the high magnetic rate (20%) among white dwarf stars, the progeny of hot subdwarfs. Here we present the discovery of a strong magnetic field (B = 353 $\pm$ 10 kG) from Zeeman-split hydrogen, helium, and metal lines in the optical X-SHOOTER spectrum of an He-sdO and present the first spectroscopic analysis of any magnetic hot subdwarf. For this we used line-blanketed Tlusty non-local thermodynamic equilibrium models and assumed a simple homogeneous magnetic field. The derived atmospheric parameters $T_{\mathrm{eff}}$ = 44900 $\pm$ 1000 K and log g = 5.93 $\pm$ 0.15 are typical for He-sdO stars, while the star is less hydrogen-poor than most He-sdOs at log n(He)/n(H) = +0.28 $\pm$ 0.10. The star is a slow rotator ($v_{\mathrm{rot}}\sin i$ < 40 km s${}^{-1}$). Its chemical composition is N-rich and C- and O-poor, and the Si and S abundances are close to solar. Combining the atmospheric parameters with Gaia parallax and photometry, the stellar radius and luminosity are found to be typical for He-sdOs and place the star on the helium main sequence in the Hertzsprung-Russell diagram. Its mass of $0.93_{-0.30}^{+0.44}$ $M_{\odot }$, although uncertain, appears to be remarkably high. The strong magnetic field along with the atmospheric parameters and metal abundances provide overwhelming evidence for the double-degenerate merger scenario.

arXiv commentAccepted for publication as a letter in A&A

References in corpus (48)

• The Gaia mission
• Estimating distances from parallaxes IV: Distances to 1.33 billion stars in Gaia Data Release 2
• Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position
• Gaia Early Data Release 3: Photometric content and validation
• The Origin of Subdwarf B Star (I): the Formation Channels
• The origin of sdB stars (II)
• A million binaries from Gaia eDR3: sample selection and validation of Gaia parallax uncertainties
• Hot subluminous stars
• Pan-STARRS Photometric and Astrometric Calibration
• An Analysis of the Shapes of Interstellar Extinction Curves. VII. Milky Way Spectrophotometric Optical-through-Ultraviolet Extinction and Its R-Dependence
• How mergers magnetise massive stars
• Planetary nebulae seen with TESS: Discovery of new binary central star candidates from Cycle 1
• Remnant evolution after a carbon-oxygen white dwarf merger
• SkyMapper Southern Survey: Second Data Release (DR2)
• eleanor: An open-source tool for extracting light curves from the TESS Full-Frame Images
• Spectral models for binary products: Unifying Subdwarfs and Wolf-Rayet stars as a sequence of stripped-envelope stars
• Double degenerate mergers as progenitors of high-field magnetic white dwarfs
• The DECam Plane Survey: Optical photometry of two billion objects in the southern Galactic plane
• Hot subdwarfs from the ESO Supernova Ia Progenitor Survey: II. Atmospheric parameters of subdwarf O stars
• The population of hot subdwarf stars studied with Gaia II. The Gaia DR2 catalogue of hot subluminous stars
• The DECam Local Volume Exploration Survey: Overview and First Data Release
• Flux calibration of medium-resolution spectra from 300 nm to 2500 nm: Model reference spectra and telluric correction
• The Post-Merger Magnetized Evolution of White Dwarf Binaries: The Double-Degenerate Channel of Sub-Chandrasekhar Type Ia Supernovae and the Formation of Magnetized White Dwarfs
• On The Evolution of Magnetic White Dwarfs
• New insight into the magnetism of degenerate stars from the analysis of a volume limited sample of white dwarfs
• A brief introductory guide to TLUSTY and SYNSPEC
• The FORS1 catalogue of stellar magnetic field measurements
• On the formation of single and binary helium-rich sdO stars
• The catalogue of radial velocity variable hot subluminous stars from the MUCHFUSS project
• Alone but not lonely: Observational evidence that binary interaction is always required to form hot subdwarf stars
• Rotational modulation in TESS B stars
• Hot subdwarf stars in close-up view - II. Rotational properties of single and wide binary subdwarf B stars
• Hot subdwarfs formed from the merger of two He white dwarfs
• Magnetized Moving Mesh Merger of a Carbon-Oxygen White Dwarf Binary
• Discovery of six new strongly magnetic white dwarfs in the 20 pc local population
• A proto-helium white dwarf stripped by a substellar companion via common-envelope ejection -- Uncovering the true nature of a candidate hypervelocity B-type star
• Hydrogen in hot subdwarfs formed by double helium white dwarf mergers
• The enigmatic He-sdB pulsator LS IV$-$14${}^{\circ }$116: new insights from the VLT
• Heavy metals in intermediate He-rich hot subdwarfs: The chemical composition of HZ44 and HD127493
• The cool magnetic DAZ white dwarf NLTT 10480
• The magnetic fields of hot subdwarf stars
• A Plague of Magnetic Spots Among the Hot Stars of Globular Clusters
• Heavy-metal enrichment of intermediate He-sdOB stars: the pulsators Feige 46 and LS IV -14 116 revisited
• KIC 10449976: discovery of an extreme-helium subdwarf in the Kepler field
• A quantitative NLTE analysis of visual and ultraviolet spectra of four helium-rich subdwarf O stars
• A search for magnetic fields in cool sdB stars
• Looking into the cradle of the grave: J22564-5910, a young post-merger hot subdwarf?
• Hot subluminous Stars: Highlights from the MUCHFUSS and Kepler missions

Cited by in corpus (10)

• The population of hot subdwarf stars studied with Gaia -- IV. Catalogues of hot subluminous stars based on Gaia EDR3
• A massive helium star with a sufficiently strong magnetic field to form a magnetar
• The bright blue side of the night sky: Spectroscopic survey of bright and hot (pre-) white dwarfs
• Large-scale ordered magnetic fields generated in mergers of helium white dwarfs
• A spectroscopic and kinematic survey of fast hot subdwarfs
• Discovery of three magnetic helium-rich hot subdwarfs with SALT
• Stars on the ascending helium giant branch I. From white dwarf merger to helium giant: the evolutionary state of the rapidly rotating hot subdwarf HE 1518-0948
• Discovery and analysis of three magnetic hot subdwarf stars: evidence for merger-induced magnetic fields
• Two hot pre-white dwarfs inside the red-giant-branch planetary nebula Pa 13 -- Double core evolution or common envelope-induced rejuvenation?
• I Can Do It With A Broken Planetesimal: Characterising the planetary debris in heavily polluted cool white dwarfs