Unicorns and Giraffes in the binary zoo: stripped giants with subgiant companions

Abstract

We analyze two binary systems containing giant stars, V723 Mon ("the Unicorn") and 2M04123153+6738486 ("the Giraffe"). Both giants orbit more massive but less luminous companions, previously proposed to be mass-gap black holes. Spectral disentangling reveals luminous companions with star-like spectra in both systems. Joint modeling of the spectra, light curves, and spectral energy distributions robustly constrains the masses, temperatures, and radii of both components: the primaries are luminous, cool giants ($T_{\mathrm{eff},\mathrm{giant}}=3,800\mathrm{K}$ and $4,000\mathrm{K}$, $R_{\mathrm{giant}}=22.5R_{\odot }$ and $25R_{\odot }$) with exceptionally low masses ($M_{\mathrm{giant}}\approx 0.4M_{\odot }$) that likely fill their Roche lobes. The secondaries are only slightly warmer subgiants ($T_{\mathrm{eff},2}=5,800\mathrm{K}$ and $5,150\mathrm{K}$, $R_2=8.3R_{\odot }$ and $9R_{\odot }$) and thus are consistent with observed UV limits that would rule out main-sequence stars with similar masses ($M_2\approx 2.8M_{\odot }$ and $\approx 1.8M_{\odot }$). In the Unicorn, rapid rotation blurs the spectral lines of the subgiant, making it challenging to detect even at wavelengths where it dominates the total light. Both giants have surface abundances indicative of CNO processing and subsequent envelope stripping. The properties of both systems can be reproduced by binary evolution models in which a $1-2M_{\odot }$ primary is stripped by a companion as it ascends the giant branch. The fact that the companions are also evolved implies either that the initial mass ratio was very near unity, or that the companions are temporarily inflated due to rapid accretion. The Unicorn and Giraffe offer a window into into a rarely-observed phase of binary evolution preceding the formation of wide-orbit helium white dwarfs, and eventually, compact binaries containing two helium white dwarfs.

arXiv comment20 pages, 16 figures. Accepted to MNRAS

References in corpus (72)

• emcee: The MCMC Hammer
• The chemical composition of the Sun
• Modules for Experiments in Stellar Astrophysics (MESA)
• Modules for Experiments in Stellar Astrophysics (MESA): Giant Planets, Oscillations, Rotation, and Massive Stars
• Modules for Experiments in Stellar Astrophysics (MESA): Binaries, Pulsations, and Explosions
• MESA Isochrones and Stellar Tracks (MIST). I: Solar-Scaled Models
• Modules for Experiments in Stellar Astrophysics (MESA): Convective Boundaries, Element Diffusion, and Massive Star Explosions
• The Zwicky Transient Facility: System Overview, Performance, and First Results
• The Apache Point Observatory Galactic Evolution Experiment (APOGEE)
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• Low Temperature Opacities
• A 3D Dust Map Based on Gaia, Pan-STARRS 1 and 2MASS
• The NextGen Model Atmosphere grid for $3000\le \Teff \le 10000\K$
• Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position
• A Computational Guide to Physics of Eclipsing Binaries. Paper I. Demonstrations and Perspectives
• A standard stellar library for evolutionary synthesis: I. Calibration of theoretical spectra
• The GALAH+ Survey: Third Data Release
• A standard stellar library for evolutionary synthesis: II. The M dwarf extension
• A million binaries from Gaia eDR3: sample selection and validation of Gaia parallax uncertainties
• The Data Reduction Pipeline for the Apache Point Observatory Galactic Evolution Experiment
• The Cannon: A data-driven approach to stellar label determination
• Hot subluminous stars
• Physics Of Eclipsing Binaries. II. Towards the Increased Model Fidelity
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• The ELM Survey. I. A Complete Sample of Extremely Low Mass White Dwarfs
• A wide star-black-hole binary system from radial-velocity measurements
• Spectral models for binary products: Unifying Subdwarfs and Wolf-Rayet stars as a sequence of stripped-envelope stars
• A stellar census in globular clusters with MUSE: Binaries in NGC 3201
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• Discovery and Characterization of 3000+ Main-Sequence Binaries from APOGEE Spectra
• A systematic search of Zwicky Transient Facility data for ultracompact binary LISA-detectable gravitational-wave sources
• A Unicorn in Monoceros: the $3M_{\odot }$ dark companion to the bright, nearby red giant V723 Mon is a non-interacting, mass-gap black hole candidate
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• The timescale of low-mass proto-helium white dwarf evolution
• Signatures of unresolved binaries in stellar spectra: implications for spectral fitting
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• Evolution of Cataclysmic Variables and Related Binaries Containing a White-Dwarf
• A stripped-companion origin for Be stars: clues from the putative black holes HR 6819 and LB-1
• Clues about the scarcity of stripped-envelope stars from the evolutionary state of the sdO+Be binary system phi Persei
• Self-lensing Discovery of a $0.2M_{\odot }$ White Dwarf in an Unusually Wide Orbit Around a Sun-like Star
• A Spectroscopic Orbit for Regulus
• The Formation of EL CVn-type Binaries
• The Past and Future History of Regulus
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• Comment on "A non-interacting low-mass black hole -- giant star binary system"
• Rotation, activity, and lithium abundance in cool binary stars
• APOGEE [C/N] Abundances Across the Galaxy: Migration and Infall from Red Giant Ages
• HR 6819 is a binary system with no black hole -- revisiting the source with infrared interferometry and optical integral field spectroscopy
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• LAMOST J0140355+392651: An evolved cataclysmic variable donor transitioning to become an extremely low mass white dwarf
• The Gaia-ESO survey: Hydrogen lines in red giants directly trace stellar mass
• Tidal Effects on the Radial Velocities of V723 Mon: Additional Evidence for a Dark $3M_{\odot }$ Companion
• Spectroscopic Detection of the Pre-White Dwarf Companion of Regulus
• To be or not to be a black hole: detailed binary population models as a sanity check
• HD 63021: Chromospheric Activity and Mass Transfer in a Close Binary

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