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

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 $f(P) \propto P^π$ and $f(q) \propto q^κ$, 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.

Figures (5)

• Figure 1: The distribution of the number of observational epochs for the 299 BHB stars in our sample. All stars with more than ten observations are grouped into the “>10” bin.
• Figure 2: Classification of BHB stars based on kinematics and metallicity. The magenta dashed line represents the boundary used to separate halo-like (gray) and disk-like (black) BHB stars in the plane of azimuthal velocity and metallicity.
• Figure 3: Classification of BHB stars based on their positions on the Kiel diagram. The magenta dashed line marks the boundary distinguishing redder BHB stars and bluer BHB stars in the plane of effective temperature and surface gravity. Gray dots represent halo-like BHB stars, while black dots denote disk-like BHB stars. Halo-like observed binaries (red stars) and disk-like observed binaries (cyan stars) are highlighted accordingly.
• Figure 4: Projections of the GMF onto planes defined by different pairs of $\pi$, $\kappa$, and $f_{\rm b}^{\rm in}$. The green 'x' marks the position of the absolute maximum. The red, blue, green, and black contours indicate regions of equal values corresponding to 60$\%$, 80$\%$, and 95$\%$ of the absolute maximum of the GMF, respectively.
• Figure 5: Histogram of the number of halo-like BHB (black dashed step) and disk-like BHB (black solid step) stars as a function of their $T_\mathrm{eff}$ (in kK).