Where are the binaries? -- Searching for blue horizontal-branch stars in binary systems in the inner Galactic Halo

Abstract

Blue horizontal-branch (BHB) stars are evolved low-mass objects that have completed their core hydrogen burning main-sequence (MS) stage and have lost significant mass during the red giant phase culminating in the helium flash. We determine the fraction of BHBs in binary systems over a wide range of separations in the inner Galactic Halo to constrain mass-loss mechanisms and evolutionary pathways. Using a catalog of 22,336 BHB candidates from Gaia DR3, we analysed radial velocity variations found in spectra (263 spectra of 89 targets) acquired using the Ondrejov Echelle spectrograph at the Astronomical Institute of the Czech Academy of Sciences together with archival spectra from the Ultraviolet and Visual Echelle Spectrograph (UVES) and Fiber-fed Extended Range Optical Spectrograph (FEROS). We searched for wide common proper motion pairs, binary candidates with enhanced astrometric noise, and binaries with astrometric orbital solutions in Gaia DR3. Archival light curves from Gaia DR3 and the Zwicky Transient Facility (ZTF) were checked for binary induced variations. Synthetic SEDs and binary detection probabilities were modelled to account for selection effects. We find a binary fraction of <2.2%, far lower than the rates for their main-sequence (MS) and red-giant branch (RGB) progenitors (30-50%). This suggests that BHBs are either not descendants of binary systems, or that existing companions do not survive the BHB formation process. The negligible binary fraction implies single-star evolution could dominate BHB formation, contrasting with EHB stars where binarity is critical. Our results challenge models of mass loss on the RGB and highlight the need for alternative mechanisms.

19 pages, 7 figures

References in corpus (48)

• A large stellar evolution database for population synthesis studies. I. Scaled solar models and isochrones
• Mind your Ps and Qs: the Interrelation between Period (P) and Mass-ratio (Q) Distributions of Binary Stars
• Common Envelope Evolution: Where we stand and how we can move forward
• Gaia Early Data Release 3: Parallax bias versus magnitude, colour, and position
• The Zwicky Transient Facility: Science Objectives
• New constraints on the chemical evolution of the solar neighbourhood and Galactic disc(s). Improved astrophysical parameters for the Geneva-Copenhagen Survey
• A large stellar evolution database for population synthesis studies. II. Stellar models and isochrones for an alpha-enhanced metal distribution
• Unresolved stellar companions with Gaia DR2 astrometry
• Hot subluminous stars
• The Updated BaSTI Stellar Evolution Models and Isochrones: I. Solar Scaled Calculations
• Horizontal Branch Stars: The Interplay between Observations and Theory, and Insights into the Formation of the Galaxy
• White Dwarf Mass Distribution in the SDSS
• The Zwicky Transient Facility Catalog of Periodic Variable Stars
• An Analysis of the Shapes of Interstellar Extinction Curves. VII. Milky Way Spectrophotometric Optical-through-Ultraviolet Extinction and Its R-Dependence
• The Close Binary Fraction of Solar-type Stars is Strongly Anti-correlated with Metallicity
• Wind Roche-lobe overflow: Application to carbon-enhanced metal-poor stars
• Chemical Abundances and Rotation Velocities of Blue Horizontal-Branch Stars in Six Globular Clusters
• Gaia Data Release 3. Summary of the variability processing and analysis
• Stellar Multiplicity Meets Stellar Evolution And Metallicity: The APOGEE View
• Mass loss on the red giant branch: the value and metallicity dependence of Reimers' η in globular clusters
• Gaia Data Release 3. The first Gaia catalogue of eclipsing binary candidates
• Chemo-kinematics of the $Gaia$ RR Lyrae: the halo and the disc
• The 40 pc sample of white dwarfs from Gaia
• The age of the Milky Way halo stars from the Sloan Digital Sky Survey
• Alone but not lonely: Observational evidence that binary interaction is always required to form hot subdwarf stars
• Hot subdwarfs in close binaries observed from space I: orbital, atmospheric, and absolute parameters and the nature of their companions
• The Origin of Weakened Magnetic Braking in Old Solar Analogs
• The role of binaries in the enrichment of the early Galactic halo. I. r-process-enhanced metal-poor stars
• Alfvén-wave driven magnetic rotator winds from low-mass stars I: rotation dependences of magnetic braking and mass-loss rate
• RR Lyrae From Binary Evolution: Abundant, Young and Metal-Rich
• White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 16
• A quantitative spectral analysis of 14 hypervelocity stars from the MMT survey
• Minimalist coupled evolution model for stellar x-ray activity, rotation, mass loss, and magnetic field
• Radial velocity variability and evolution of hot subdwarf stars
• A lack of close binaries among hot horizontal branch stars in globular clusters. II. NGC\,2808
• Fundamental properties of red-clump stars from long-baseline H-band interferometry
• Line Broadening in Field Metal-poor Red Giant and Red Horizontal Branch Stars
• SHOTGLAS I: The ultimate spectroscopic census of extreme horizontal branch stars in $\omega$ Centauri
• Ondr}ejov echelle spectrograph, ground based support facility for exoplanet missions
• Clean catalogues of blue horizontal-branch stars using Gaia EDR3
• Surface magnetism of rapidly rotating red giants: single versus close binary stars
• The most metal-rich asymptotic giant branch stars
• Red Horizontal Branch stars: an asteroseismic perspective
• 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
• Magnetic Braking at work in binaries
• The binary fraction of Blue Horizontal-Branch (BHB) Stars
• Analysis of KIC 7023917 -- spotted low-mass ratio eclipsing binary with $\delta$ Scuti pulsations