High Mass-ratio Binary Population in Open Clusters: Segregation of early type binaries and an increasing binary fraction with mass (original) (raw)

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-wise high mass-ratio (HMR) BF (f) 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-wise) binaries and the reference population. This study finds that in open clusters, (1) HMR BF for the mass range 0.4–3.6 M (early M to late B type) has a range of 0.12 to 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 to 2.5) ×f, (4) a decreasing BF(total) from late B-type to K-type, in agreement with the Galactic field s...