Ultraviolet spectroscopy reveals a hot and luminous companion to the Be star+black hole candidate MWC 656

Abstract

The Galactic Be star binary MWC 656 was long considered the only known Be star+black hole (BH) system, making it a critical benchmark for models of massive binary evolution and for the expected X-ray emission of Be+BH binaries. However, recent dynamical measurements cast doubt on the presence of a BH companion. We present new multi-epoch ultraviolet spectroscopy from the Hubble Space Telescope, combined with high-resolution optical spectra, to reassess the nature of the companion. The far-ultraviolet spectra reveal high-ionisation features -- including prominent N v and He ii lines -- which are absent in the spectra of normal Be stars and are indicative of a hot, luminous companion. Spectral modelling shows that these features cannot originate from the Be star or from an accretion disc around a compact object. Instead, we find that the data are best explained by a hot ($T_{\mathrm{eff}}\approx 85$ kK), compact, hydrogen-deficient star with strong wind signatures, consistent with an intermediate-mass stripped star. Our revised orbital solution and composite spectroscopic modelling yield a companion mass of $M_2=1.48_{-0.46}^{+0.55}{\mathrm{M}}_{\odot }$, definitively ruling out a BH and disfavouring a white dwarf. MWC 656 thus joins the growing class of Be+stripped star binaries. The system's unusual properties -- including a high companion temperature and wind strength -- extend the known parameter space of such binaries. The continued absence of confirmed OBe+BH binaries in the Galaxy highlights a growing tension with population synthesis models.

21 pages, 14 figures, accepted for publication in A&A

References in corpus (54)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Gaia Data Release 3: Summary of the content and survey properties
• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• Mass loss from hot massive stars
• Modules for Experiments in Stellar Astrophysics (MESA): Time-Dependent Convection, Energy Conservation, Automatic Differentiation, and Infrastructure
• The Galactic WN stars: Spectral analyses with line-blanketed model atmospheres versus stellar evolution models with and without rotation
• A 15.65 solar mass black hole in an eclipsing binary in the nearby spiral galaxy Messier 33
• A statistical test for Nested Sampling algorithms
• The VLT-FLAMES survey of massive stars: rotation and nitrogen enrichment as the key to understanding massive star evolution
• A wide star-black-hole binary system from radial-velocity measurements
• BONNSAI: a Bayesian tool for comparing stars with stellar evolution models
• Cross-Correlation and Maximum Likelihood Analysis: A New Approach to Combine Cross-Correlation Functions
• On the nature of massive helium star winds and Wolf-Rayet-type mass loss
• orbitize!: A Comprehensive Orbit-fitting Software Package for the High-contrast Imaging Community
• On the consistent treatment of the quasi-hydrostatic layers in hot star atmospheres
• The 'hidden' companion in LB-1 unveiled by spectral disentangling
• Why binary interaction does not necessarily dominate the formation of Wolf-Rayet stars at low metallicity
• Properties of OB star-black hole systems derived from detailed binary evolution models
• Winds from stripped low-mass Helium stars and Wolf-Rayet stars
• Is HR 6819 a triple system containing a black hole? -- An alternative explanation
• An X-ray quiet black hole born with a negligible kick in a massive binary within the Large Magellanic Cloud
• A naked-eye triple system with a nonaccreting black hole in the inner binary
• The Detection and Characterization of Be+sdO Binaries from HST/STIS FUV Spectroscopy
• A catalogue of high-mass X-ray binaries in the Galaxy: from the INTEGRAL to the Gaia era
• Identifying quiescent compact objects in massive Galactic single-lined spectroscopic binaries
• Not so fast: LB-1 is unlikely to contain a 70 $M_{\odot }$ black hole
• Spectral and spatial imaging of the Be+sdO binary phi Persei
• A stripped-companion origin for Be stars: clues from the putative black holes HR 6819 and LB-1
• Partial-envelope stripping and nuclear-timescale mass transfer from evolved supergiants at low metallicity
• The Tarantula Massive Binary Monitoring VI: Characterisation of hidden companions in 51 single-lined O-type binaries, a flat mass-ratio distribution, and black-hole binary candidates
• First 3D Radiation-Hydrodynamic Simulations of Wolf-Rayet Winds
• The Hot Companion and Circumbinary Disk of the Be Star HR 2142
• X-Rays From Massive OB Stars: Thermal Emission From Radiative Shocks
• gamma Cas stars: Normal Be stars with disks impacted by the wind of a helium-star companion?
• A persistent ultraviolet outflow from an accreting neutron star binary transient
• X-Shooting ULLYSES: Massive stars at low metallicity. III. Terminal wind speeds of ULLYSES massive stars
• The First Simultaneous X-Ray/Radio Detection of the First Be/BH System MWC 656
• Dynamical masses of the primary Be star and the secondary sdB star in the single-lined binary kappa Dra (B6 IIIe)
• A partially stripped massive star in a Be binary at low metallicity: A missing link towards Be X-ray binaries and double neutron star mergers
• Gamma Cas Stars as Be + White Dwarf Binary Systems
• MWC 656 is unlikely to contain a black hole
• Forming merging double compact objects with stable mass transfer
• X-Shooting ULLYSES: Massive Stars at low metallicity VIII. Stellar and wind parameters of newly revealed stripped stars in Be binaries
• Phase-resolved spectroscopic analysis of the eclipsing black hole X-ray binary M33 X-7: System properties, accretion, and evolution
• X-Shooting ULLYSES: Massive stars at low metallicity. IV. Spectral analysis methods and exemplary results for O stars
• Comprehensive UV and Optical spectral analysis of Cygnus X-1: Stellar and wind parameters, abundances, and evolutionary implications
• Populations of evolved massive binary stars in the Small Magellanic Cloud I: Predictions from detailed evolution models
• On the evolutionary nature of puffed-up stripped star binaries and their occurrence in stellar populations
• Populations of evolved massive binary stars in the Small Magellanic Cloud II: Predictions from rapid binary evolution
• High-spatial-resolution simulations of Be star disks in binary systems: I. Structure and kinematics of coplanar disks
• An updated gamma-ray analysis of the Be-BH binary HD~215227
• Radius, rotational period and inclination of the Be stars in the Be/gamma-ray binaries MWC 148 and MWC 656
• HIP 15429: A newborn Be star on an eccentric binary orbit
• The C/N ratio from FUV spectroscopy as a constraint upon the past evolution of HS0218+3229