A search for missing binaries: 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 finished core hydrogen burning on the main sequence and lost a significant part of their envelope on the red giant branch, up to 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 the mass-loss mechanisms and evolutionary pathways that produce them.

Starting from a catalogue of 22,336 BHB candidates from Gaia DR3, we analysed radial velocity variations in 263 spectra of 89 targets taken with the Ondřejov Echelle spectrograph at the Astronomical Institute of the Czech Academy of Sciences, together with archival UVES and FEROS spectra. We searched for wide common proper motion pairs, for binary candidates with enhanced astrometric noise, and for binaries with astrometric orbital solutions in Gaia DR3. Archival Gaia DR3 and Zwicky Transient Facility light curves were checked for binary induced variations, and synthetic SEDs and detection probabilities were modelled to account for the selection effects.

We find a binary fraction of less than 2.2%, far below the 30 to 50% measured for the main-sequence and red giant branch progenitors. Either BHBs are not descendants of binary systems, or companions do not survive their formation. A binary fraction this low leaves single-star evolution as the dominant BHB formation channel, in sharp contrast to the extreme horizontal-branch stars, for which binarity is essential. The result challenges current models of mass loss on the red giant branch and points to the need for alternative mechanisms.