The binary fraction of Blue Horizontal-Branch (BHB) Stars

TL;DR

This study estimates the intrinsic binary fraction of Blue Horizontal-Branch stars, revealing differences based on kinematic and temperature properties, which inform their formation and evolution in the Galactic halo.

Contribution

It provides the first detailed measurement of the binary fraction of BHB stars, accounting for observational biases and exploring dependencies on metallicity, kinematics, and temperature.

Findings

Halo-like BHBs have a lower binary fraction than disk-like BHBs.

Bluer BHB stars show a higher binary fraction than redder BHB stars.

Intrinsic binary fraction of BHBs is approximately 30-50%, depending on classification.

Abstract

Blue horizontal-branch (BHB) stars are old, low-mass, metal-poor stars that serve as important tracers of the Galactic halo structure, kinematics, and evolution.Understanding their binary properties provides key insights into their formation channels and the role of binary interactions in the evolution of horizontal branch stars. We intend to investigate the intrinsic binary fraction $f_{\rm b}^{\rm in}$ of BHB stars and its dependencies on metallicity, kinematics, and effective temperature. We collect \GG{299} BHB stars from LAMOST with multiple radial velocity (RV) measurements and classify the sample into halo-like and disk-like BHBs based on their kinematics and metallicity, as well as into bluer and redder BHBs based on their \G{effective temperature}. We then investigate the observed binary fraction for each group based on the radial velocity variations and apply a set of Monte Carlo simulations, assuming distributions of $f(P) \propto P^\pi$ and $f(q) \propto q^\kappa$, to correct the observed binary fraction for observational biases and derive the intrinsic binary fraction. After correcting for observational biases, the intrinsic binary fraction increases to 31% for n > 2 and 32% for n > 3. A clear contrast is observed between halo-like and disk-like BHB stars, with halo-like BHBs exhibiting a lower intrinsic binary fraction (28% for n > 2 and 29% for n> 3) compared to disk-like BHBs (46% and 51%, respectively), indicating different formation pathways. Additionally, we find that bluer BHB stars exhibit a significantly higher binary fraction (42% for n > 2 and 45% for n> 3) than redder BHB stars (24% and 23%, respectively), which suggests a possible link between binarity and the effective temperature, although more samples are required to confirm this.