A research team from the Yunnan Observatories of the Chinese Academy of Sciences has recently published a study on the binary fraction of Blue Horizontal-branch (BHB) stars in Astronomy & Astrophysics, providing the intrinsic binary fraction and revealing its relation to 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 into this binary channel has been limited, leaving the evolutionary channels unclear and the binary fraction uncertain, which makes this new research critical for understanding their formation mechanisms.

Based on data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), the study compiled a sample of 299 BHB stars with multiple radial velocity measurements and reliable atmospheric parameters.

Using empirical relations 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 with Monte Carlo simulations, the team found the intrinsic binary fraction of the full BHB sample is about 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 an environmental dependence of the BHB binary fraction and offer new evidence for the formation and evolution of BHB stars.

Contact:
GUO Yanjun
Yunnan Observatories, CAS
E-mail:guoyanjun@ynao.ac.cn