J/A+A/704/A326 BHB and MS in inner Galactic halo (Culpan+, 2025)
A search for missing binaries: Blue horizontal-branch stars in binary systems
in the inner Galactic halo.
Culpan R., Dorsch M., Pelisoli I., Schaffenroth V., Geier S., Heber U.,
Kubatova B., Dawson H., Pritzkuleit M., Bhat A., Cabezas M., Maryeva O.,
Kubat J., Kurpas M., Vostretcova E., Vos J., Mattig F., Hainich R.
<Astron. Astrophys. 704, A326 (2025)>
=2025A&A...704A.326C 2025A&A...704A.326C (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Stars, double and multiple ; Radial velocities ;
Rotational velocities ; Optical
Keywords: binaries: general - stars: horizontal branch - stars: mass-loss
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. 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.
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 catalogue of 22336 BHB candidates from Gaia DR3, we analysed
radial velocity variations found in spectra (263 spectra of 89
targets) acquired using the Ondvrejov Echelle spectrograph attached to
the Perek 2m 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
SEDs and binary detection probabilities were modelled to account for
selection effects.
We find a binary fraction of <2.2% (1 sigma confidence), far lower
than the rates for their main-sequence (MS) and red-giant branch (RGB)
progenitors 30 to 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.
Description:
In this work we carried out the first systematic, multi-technique
search for binarity among BHB stars in the inner Galactic halo,
combining spectroscopy, astrometry, and photometry. Starting from the
C24 catalogue of 22336 BHB candidates from Gaia DR3, we obtained 263
spectra of 89 targets with the Ondrejov Echelle Spectrograph and
supplemented these with archival UVES and FEROS data, yielding a total
of 319 high-resolution spectra for radial velocity monitoring. Our
spectroscopic analysis probes binarity up to orbital periods of 5000
days. Our observations are supported by crossmatching the C24
catalogue of BHB candidates with Gaia DR3 eclipsing binaries, and ZTF
datasets where binary light curve variability probes the short period
of the order of a few days.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 77 252 Spectral analysis results, per-spectrum RVs
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Byte-by-byte Description of file:tablea1.dat
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Bytes Format Units Label Explanations
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1- 16 A16 --- Name Simbad ID
17- 20 A4 --- Type Star type (BHB or MS)
21 I1 --- NOES Number of OES spectra acquired
23 I1 --- NUVES/FEROS Number of UVES and FEROS spectra used
25- 40 F16.8 d BJD Barycentric Julian Date (1)
42- 48 F7.2 km/s RV ? Radial velocity from the
individual spectrum
50- 54 F5.2 km/s e_RV ? Standard error on the radial velocity
from the individual spectrum
56- 61 F6.2 km/s Vrotmean Weighted mean rotational velocity
plus minus one standard error
63- 65 F3.1 km/s e_Vrotmean Standard error on the rotational velocity
from the individual spectrum
67- 73 F7.2 km/s RVmean Weighted mean rotational velocity
plus minus one standard error
75- 77 F3.1 km/s e_RVmean Standard error on the rotational velocity
from the individual spectrum
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Note (1): The Barycentric Julian Dates (BJD) were calculated using the latitude,
longitude and elevation of the observatory, the RA and dec of the target,
as well as the date and time of the observation taken from the respective FITS
files. The barycentric light-travel times along the line of sight were then
computed with the Astropy light travel time method. This procedure follows
current community standards for precise time-stamping of astronomical
observations and ensures sub-second accuracy for typical ground-based
observatories.
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Acknowledgements:
Richard Culpan, rick(at)culpan.de
(End) Patricia Vannier [CDS] 10-Nov-2025