The observed multiplicity properties of B-type stars in the Galactic young open cluster NGC 6231
G. Banyard, H. Sana, L. Mahy, J. Bodensteiner, J. I. Villaseñor, C. J. Evans
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.