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

Abstract

In the past, SDSS J160429.12+100002.2 was spectroscopically classified as a blue horizontal branch (BHB) star. Assuming a luminosity that is characteristic of BHB stars, the object's radial velocity and proper motions from Gaia Early Data Release 3 would imply that its Galactic rest-frame velocity exceeds its local escape velocity. Consequently, the object would be considered a hypervelocity star, which would prove particularly interesting because its Galactic trajectory points in our direction. However, based on the spectroscopic analysis of follow-up observations, we show that the object is actually a short-period ($P\approx 3.4$ h) single-lined spectroscopic binary system with a visible B-type star (effective temperature $T_{\mathrm{eff}}=15840\pm 160$ K and surface gravity $\log (g)=4.86\pm 0.04$) that is less luminous than typical BHB stars. Accordingly, the distance of the system is lower than originally thought, which renders its Galactic orbit bound to the Galaxy. Nevertheless, it is still an extreme halo object on a highly retrograde orbit. The abundances of He, C, N, O, Ne, Mg, Al, Si, S, and Ca are subsolar by factors from 3 to more than 100, while Fe is enriched by a factor of about 6. This peculiar chemical composition pattern is most likely caused by atomic diffusion processes. Combining constraints from astrometry, orbital motion, photometry, and spectroscopy, we conclude that the visible component is an unevolved proto-helium white dwarf with a thin hydrogen envelope that was stripped by a substellar companion through common-envelope ejection. Its unique configuration renders the binary system an interesting test bed for stellar binary evolution in general and common-envelope evolution in particular.

arXiv commentAccepted for publication in A&A (Astronomy and Astrophysics)

References in corpus (52)

• Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds
• The chemical composition of the Sun
• Measuring Reddening with SDSS Stellar Spectra and Recalibrating SFD
• The Eleventh and Twelfth Data Releases of the Sloan Digital Sky Survey: Final Data from SDSS-III
• Estimating distances from parallaxes IV: Distances to 1.33 billion stars in Gaia Data Release 2
• Evolutionary models for cool brown dwarfs and extrasolar giant planets. The case of HD 20945
• First Results from the Catalina Real-time Transient Survey
• Gaia Early Data Release 3: The astrometric solution
• X-shooter, the new wide band intermediate resolution spectrograph at the ESO Very Large Telescope
• The Multi-Object, Fiber-Fed Spectrographs for SDSS and the Baryon Oscillation Spectroscopic Survey
• Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position
• The Tenth Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-III Apache Point Observatory Galactic Evolution Experiment
• Gaia Early Data Release 3: Photometric content and validation
• Ultraviolet Radiation from Evolved Stellar Populations -- I. Models
• The unWISE Catalog: Two Billion Infrared Sources from Five Years of WISE Imaging
• ESI, a new Keck Observatory echellette spectrograph and imager
• Spectroscopic Analysis of DA White Dwarfs: Stark Broadening of Hydrogen Lines Including Non-Ideal Effects
• An Analysis of the Shapes of Interstellar Extinction Curves. VII. Milky Way Spectrophotometric Optical-through-Ultraviolet Extinction and Its R-Dependence
• New evolutionary sequences for extremely low mass white dwarfs: Homogeneous mass and age determinations, and asteroseismic prospects
• New white dwarf stars in the Sloan Digital Sky Survey Data Release 10
• Milky Way Mass Models for Orbit Calculations
• US 708 - An unbound hyper-velocity subluminous O star
• Models of low-mass helium white dwarfs including gravitational settling, thermal and chemical diffusion, and rotational mixing
• The Solar Neighborhood XXXII. The Hydrogen Burning Limit
• A Comparative Study of the Mass Distribution of Extreme Ultraviolet-Selected White Dwarfs
• The fastest unbound star in our Galaxy ejected by a thermonuclear supernova
• The ELM Survey. VIII. 98 Double White Dwarf Binaries
• The formation of low-mass helium white dwarfs orbiting pulsars: Evolution of low-mass X-ray binaries below the bifurcation period
• EL CVn-type binaries - Discovery of 17 helium white dwarf precursors in bright eclipsing binary star systems
• Formation of Extremely Low-mass White Dwarfs in Double Degenerates
• The timescale of low-mass proto-helium white dwarf evolution
• A Metal-Rich Low-Gravity Companion to a Massive Millisecond Pulsar
• Near-Infrared Photometry of Globular Clusters Towards the Galactic Bulge: Observations and Photometric Metallicity Indicators
• The catalogue of radial velocity variable hot subluminous stars from the MUCHFUSS project
• Cooling curves and initial models for low-mass white dwarfs (<0.25 Msun) with helium core
• Two white dwarfs in ultrashort binaries with detached, eclipsing, likely substellar companions detected by K2
• Discovery of a Bright, Extremely Low-Mass White Dwarf in a Close Double Degenerate System
• Hypervelocity stars in the Gaia era: Runaway B stars beyond the velocity limit of classical ejection mechanisms
• A new method for an objective, ${\chi }^2$-based spectroscopic analysis of early-type stars
• The hot horizontal-branch stars in omega Centauri
• The Hyper-MUCHFUSS project: probing the Galactic halo with sdB stars
• Spectroscopy of horizontal branch stars in NGC6752 - Anomalous results on atmospheric parameters and masses
• A quantitative spectral analysis of 14 hypervelocity stars from the MMT survey
• SDSS J074511.56+194926.5: Discovery of a Metal-Rich and Tidally Distorted Extremely Low Mass White Dwarf
• Heavy metals in a light white dwarf: Abundances of the metal-rich, extremely low-mass GALEX J1717+6757
• The coolest extremely low-mass white dwarfs
• The population of hot subdwarf stars studied with Gaia I. The catalogue of known hot subdwarf stars
• Spectroscopic twin to the hypervelocity sdO star US 708 and three fast sdB stars from the Hyper-MUCHFUSS project
• An extremely fast halo hot subdwarf star in a wide binary system
• Flying far and fast: the distribution of distant hypervelocity star candidates from Gaia DR2 data
• A Model of the Pulsating Extremely Low-Mass White Dwarf Precursor WASP 0247-25B
• Quantitative spectral analysis of the sdB star HD 188112: a helium-core white dwarf progenitor

Cited by in corpus (22)

• Hot subdwarfs in close binaries observed from space I: orbital, atmospheric, and absolute parameters and the nature of their companions
• Birth of the ELMs: a ZTF survey for evolved cataclysmic variables turning into extremely low-mass white dwarfs
• The ELM Survey South. II. Two dozen new low mass white dwarf binaries
• Discovery of a highly magnetic He-sdO star from a double-degenerate binary merger
• Hot subdwarfs in close binaries observed from space II: Analysis of the light curves
• The bright blue side of the night sky: Spectroscopic survey of bright and hot (pre-) white dwarfs
• Mass distribution for single-lined hot subdwarf stars in LAMOST
• SHOTGLAS II. MUSE spectroscopy of blue horizontal branch stars in the core of $\omega$ Centauri and NGC 6752
• A spectroscopic and kinematic survey of fast hot subdwarfs
• Post-common Envelope Evolution of Helium-core White Dwarfs
• The first massive compact companion in a wide orbit around a hot subdwarf star
• Spectroscopic analysis of BPS CS 22940-0009: connecting evolved helium stars
• Pulse Timing Discovery of a Three-Day Companion to the Hot Subdwarf BPM 36430
• J0526+5934: a peculiar ultra-short period double white dwarf
• Hot subdwarf stars from the Hamburg Quasar Survey
• How an overweight and rapidly rotating PG 1159 star in the Galactic halo challenges evolutionary models
• Onset of common envelope evolution during a core helium flash by rapid envelope expansion
• Abundance analysis of a nitrogen-rich extreme-helium hot subdwarf from the SALT survey
• Ancient 'ghost' planetary nebulae discovered with amateur telescopes
• Discovery of the first outbursting hot subdwarf binary: ZTF J0007+4804
• Discovery of a double white dwarf in the Galactic globular cluster NGC 6397
• Two hot pre-white dwarfs inside the red-giant-branch planetary nebula Pa 13 -- Double core evolution or common envelope-induced rejuvenation?