Abstract:
Binary stars play a vital role in astrophysical research, as a good fraction of stars are in binaries. Binary fraction
(BF) is known to change with stellar mass in the Galactic field, but such studies in clusters require binary
identification and membership information. Here, we estimate the total and spectral-type high-mass-ratio (HMR)
BF ( f
0.6) in 23 open clusters using unresolved binaries in color–magnitude diagrams using Gaia DR2 data. We
introduce the segregation index (SI ) parameter to trace mass segregation of HMR (total and mass) binaries and the
reference population. This study finds that in open clusters, (1) HMR BF for the mass range 0.4–3.6 Me (early M
to late B-type stars) has a range of 0.12–0.38 with a peak at 0.12–0.20; (2) older clusters have a relatively higher
HMR BF; (3) the mass-ratio distribution is unlikely to be a flat distribution and BF (total) ∼(1.5–2.5) × f
0.6; (4) a
decreasing BF (total) from late B to K-type stars, in agreement with the Galactic field stars; (5) older clusters show
radial segregation of HMR binaries; (6) B-type and A–F type HMR binaries show radial segregation in some
young clusters suggesting a primordial origin. This study will constrain the initial conditions and identify the major
mechanisms that regulate binary formation in clusters. Primordial segregation of HMR binaries could result from
massive clumps spatially segregated in the collapse phase of the molecular cloud.
