The binary fraction of blue horizontal branch stars

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 investigated the intrinsic binary fraction f_ ̊m b ̊m in of BHB stars and its dependences on metallicity, kinematics, and effective temperature We collected BHB stars from LAMOST with multiple radial velocity (RV) measurements and classified the sample into halo-like and disk-like BHBs based on their kinematics and metallicity, and into bluer and redder BHBs based on their effective temperature . We then investigated the observed binary fraction for each group based on the radial velocity variations and applied a set of Monte Carlo simulations, assuming distributions of f(P) ∝ P^π and f(q) ∝ q^ąppa, to correct the observed binary fraction for observational biases and to derive the intrinsic binary fraction. The observed binary fraction of BHB stars is $18%±2%$ for cases with n ≥ 2 and $21%±3%$ for cases with n ≥ 3, where n represents the number of observation times. After correcting for observational biases, the intrinsic binary fraction increases to $31%±3%$ for n ≥ 2 and $32%±3%$ 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 ( $28%±3%$ for n ≥ 2 and $29%±3%$ for n ≥ 3) compared to disk-like BHBs ( $46%±11%$ and $51%±11%$ respectively), indicating different formation pathways. In particular, halo-like BHB stars are more likely to have formed via a single-star evolution channel, whereas disk-like BHB stars may predominantly result from binary evolution processes. Additionally, we find that bluer BHB stars exhibit a significantly higher binary fraction ( $42%±6%$ for n ≥ 2 and $45%±6%$ for n ≥ 3) than redder BHB stars ( $24%±5%$ and $23%±5%$ respectively), which suggests a possible link between binarity and the effective temperature although more samples are required to confirm this. No correlation is found between π (ąppa) and metallicity or kinematics, nor between π (ąppa) and the effective temperature of BHB stars.