Abstract :

Context :  It is well known that massive O stars are frequently (if not always) found in binary or higher-order multiple systems, but this fact has been less robustly investigated for the lower mass range of the massive stars, represented by B-type stars. Obtaining the binary fraction and orbital parameter distributions of B-type stars is crucial to understand the impact of multiplicity on the archetypal progenitor of core-collapse supernovae as well as to properly investigate formation channels for gravitational wave progenitors.
Aims : This work aims to characterise the multiplicity of the B star population of the young open cluster NGC 6231 through multi-epoch optical spectroscopy of 80 B-type stars.
Methods: We analyse 31 FLAMES/GIRAFFE observations of 80 B-type stars, monitoring their radial velocities (RVs) and performing a least-squares spectral analysis (Lomb-Scargle) to search for periodicity in those stars with statistically significant variability in their RVs.
Results : We constrained an observed spectroscopic binary fraction of 33 ± 5% for the B-type stars of NGC 6231, with a first order bias correction giving a true spectroscopic binary fraction of 52 ± 8%. Out of 27 B-type binary candidates, we obtained orbital solutions for 20 systems: 15 single-lined (SB1) and five double-lined spectroscopic binaries (SB2s). We present these orbital solutions and the orbital parameter distributions associated with them.
Conclusions : Our results indicate that Galactic B-type stars are less frequently found in binary systems than their more massive O-type counterparts, but their orbital properties generally resemble those of B- and O-type stars in both the Galaxy and Large Magellanic Cloud.

Full Tables C.1-C.5 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/658/A69

Based on observations collected at the ESO Paranal observatory under ESO program 099.D-0895 and 0101.D-0163.

Publication : Astronomy & Astrophysics, Volume 658, id.A69, 28 pp.
DOI : 10.1051/0004-6361/202141037 
Bibcode : 2022A&A...658A..69B 
Keywords : binaries: spectroscopic; stars: early-type; stars: massive; open clusters and associations: individual: NGC 6231; Astrophysics - Solar and Stellar Astrophysics; Astrophysics - Astrophysics of Galaxies