A research team from the Yunnan Observatories of the Chinese Academy of Sciences has conducted a study on the binary fraction of Blue horizontal-branch (BHB) stars. Their findings, recently published in Astronomy & Astrophysics, provide the intrinsic binary fraction and reveal its relationship with metallicity, kinematics, and effective temperature.
BHB stars are metal-poor Population II stars, typically found in the Galactic halo and thought to originate from low-mass red giant branch (RGB) stars. However, their origin may also involve binary interactions. Research on this binary channel has been limited, leaving the evolutionary channels unclear and the binary fraction uncertain.
To address this challenge, the team compiled a sample of 299 BHB stars with multiple radial velocity measurements and reliable atmospheric parameters, based on data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST).
Using empirical relationships among age, metallicity, and Galactic rotation, the researchers divided the sample into disk-like BHB stars (exhibiting higher metallicity and Galactic rotation) and halo-like BHB stars (with lower metallicity and Galactic rotation). The sample was further subdivided by effective temperature into bluer (hotter) and redder (cooler) groups.
After correcting the observed binary fractions via Monte Carlo simulations, the team found that the intrinsic binary fraction of the full BHB sample is approximately 32%. Disk-like BHB stars exhibit a notably high intrinsic binary fraction of 51%, whereas halo-like BHB stars show only 29%. This suggests that disk-like BHB stars may primarily form through binary interactions, while halo-like BHB stars are more likely to arise from single-star evolution.
Moreover, the bluer BHB subsample has a binary fraction of 45%-nearly twice that of the redder subsample (23%)-indicating a possible link between binary interactions and effective temperature.
These findings provide observational support for the environmental dependence of the BHB binary fraction and offer new evidence for the formation and evolution of BHB stars.
