A search for missing binaries: Blue horizontal-branch stars in binary systems in the inner Galactic halo

¹ Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam-Golm, Germany
² Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁴ Astronomical Institute AS CR, Fričova 298, 251 65 Ondřejov, Czech Republic
⁵ Dr. Remeis-Sternwarte & ECAP, Astronomical Institute, FAU Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 August 2025
Accepted: 4 November 2025

Abstract

Context. Blue horizontal-branch (BHB) stars are evolved low-mass objects that have completed their core hydrogen burning main-sequence (MS) stage and lost significant mass during the red giant phase culminating in the helium flash. They are, hence, very old objects that can be used as markers in studying galactic structure and formation history. Their formation requires significant mass loss during the red-giant phase, but the role of stellar interactions in this process remains unclear. Knowing the fraction of BHBs that exist in binary or higher multiple systems where mass transfer may occur will enhance our understanding of their stellar evolution.

Aims. 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.

Methods. Using a catalogue of 22 336 BHB candidates from Gaia DR3, we analysed radial velocity variations found in spectra (263 spectra of 89 targets) acquired using the Ondřejov Echelle spectrograph attached to the Perek 2 m telescope 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 spectral energy distributions and binary detection probabilities were modelled to account for selection effects.

Results. We find a binary fraction of < 2.2% (1σ confidence), far lower than the rates for their MS and red-giant branch (RGB) progenitors (30–50%). This suggests that either BHBs are not descendants of binary systems, or that existing companions do not survive the BHB formation process.

Conclusions. The negligible binary fraction implies that 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.