High Mass-Ratio Binary Population in Open Clusters: Segregation of Early Type Binaries and an Increasing Binary Fraction with Mass

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 (   ) 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 M ⊙ (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.

The multiplicity fraction in 202 open clusters from Gaia

“…The simulations also allow us to determine the typical minimum mass ratio to which our method is sensitive: q lim = 0.6 +0.05 −0.15 . Our q sim lim of 0.6 is similar to the value found in the recent study of Jadhav et al (2021): for their considered magnitude range (M G ∈ [1, 10] mag; comparable to our range of selected MS members), they find q = 0.6 to be the cut-off that ensures that the single MS stars are 3σ away from the thus-defined BS.…”

“…6.2). For example, our unresolved f b estimations are coherent with the values obtained by Jadhav et al (2021), but much harder to be compared to other studies, which do not specify the con-sidered mass and/or q range (see Sect. 6.1).…”

“…This is why the CMD, only requiring imaging in two filters, is still most often used for OCs as a fundamental diagnostic tool. A simple classification is usually done dividing the CMD into two regions, one associated with the SS locus (including single stars and low-q systems) and the other with the equal-mass BS (including the higher-q systems), as done in de Grijs et al ( 2013), Cordoni et al (2018), or Jadhav et al (2021. The degeneracy of metallicity and reddening, however, affects the isochrone fitting precision.…”

The cosmic DANCe of Perseus

“…Second, it is known that the fraction of binary systems in open clusters varies between 11% and 70% (Sollima et al 2010), with the fraction of unresolved binaries between 12% and 20% (e.g. Jadhav et al 2021). However, our methodologies are unable to identify and infer the mass of these possibly unresolved binaries.…”

Dynamically Forming Extremely Low-mass White Dwarf Binaries in Wide Orbits

“…Observational evidence suggests that f b is not constant for clusters of all masses. The lower end of the cluster masses, i.e., M cl /M e  10 3 − 10 4 , corresponds to open clusters with typical values of f b ∼ 0.5 (e.g., Jadhav et al 2021), while on the other extreme, i.e., for M cl /M e  10 5 , we have old globular clusters (GCs) with a typical f b ∼ 0.05 (e.g., Milone et al 2012). The progenitors of these GCs could possibly have had a higher f b (e.g., Leigh et al 2015).…”