The Binary Fraction of B-type Runaway Stars from LAMOST DR8

TL;DR

This study estimates the intrinsic binary fraction of Galactic B-type runaway stars using LAMOST DR8 data, revealing a significant binary presence that informs the relative roles of binary-supernova and dynamical ejection formation mechanisms.

Contribution

It provides the first robust estimate of the intrinsic binary fraction of B-type runaway stars and analyzes their period and mass ratio distributions, offering insights into their formation scenarios.

Findings

Observed binary fraction is 5.4% with radial velocity variations.

Corrected intrinsic binary fraction is 27%.

Both formation mechanisms contribute similarly to B-type runaway stars.

Abstract

Runaway stars are defined as stars that depart from their birth clusters at high peculiar velocities. There are two main mechanisms for the formation of runaway stars, i.e., the binary-supernova scenario (BSS) and the dynamical ejection scenario (DES). Investigating the binary fraction of runaway stars is an important step in further exploring the relative significance of the two mechanisms. We analyzed the binary fraction of 203 Galactic B-type runaway stars identified in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope Data Release 8 database. Our analysis of radial velocity variations in the runaway star sample reveals an observed spectroscopic binary fraction of $5.4\%\pm 1.6\%$, representing the proportion of objects that exhibit statistically significant variations in radial velocity with amplitudes larger than $\rm 16~km~s^{-1}$. We employed a Monte Carlo method to correct for observational biases and determined an intrinsic binary fraction of $27\%\pm 8\%$. The period and mass ratio distributions that best reproduce the observation are $f(P)\propto P^{-5.7}$ for $1\leq P\leq 1000$ days, and $f(q)\propto q^{-3.6}$ for $0.1\leq q\leq 1.0$, indicating a preference for binaries with shorter periods and less massive companions compared to a uniform distribution. The intrinsic binary fraction, in combination with previous studies on the binary fractions of runaway stars formed by the BSS and the DES, implies that both scenarios contribute comparably to the formation of Galactic B-type runaway stars, where the ratio of the BSS to the DES is 0.86.