Photometric binaries, mass functions, and structural parameters of 78 Galactic open clusters

TL;DR

This study analyzes 78 Galactic open clusters using Gaia DR3 data to investigate their binary star populations, mass functions, and structural parameters, revealing consistent mass function trends and correlations with cluster density and age.

Contribution

It provides a homogeneous analysis of binary fractions, mass functions, and structural parameters across 78 open clusters, highlighting new correlations with cluster density and age.

Findings

Mass functions follow a two-slope power-law with a break at 1 solar mass.

Binary fraction varies from 15% to over 60%, with no strong dependence on mass or age.

Radial distribution of binaries depends on cluster age, showing different patterns in young versus older clusters.

Abstract

Binary stars play a crucial role in our understanding of the formation and evolution of star clusters and their stellar populations. We use Gaia Data Release 3 to homogeneously analyze 78 Galactic open clusters and the unresolved binary systems they host, each composed of two main sequence (MS) stars. We first investigated the structural parameters of these clusters, such as the core radius and the central density, and determined the cluster mass function (MF) and total mass by interpolating the density profile of each cluster. We measured the fraction of binaries with a large mass ratio and the fraction of blue straggler stars (BSSs), and finally investigated possible connections between the populations of binary stars and BSSs with the main parameters of the host cluster. {Remarkably, we find that the MFs of 78 analyzed open clusters follow a similar trend and are well reproduced by two single power-law functions, with a change in slope around masses of 1$M_{\odot}$. The fraction of binary stars ranges from $\sim$15\% to more than $\sim$60\% without significant correlation with the mass and the age of the host cluster. Moreover, we detect hints of a correlation between the total fraction of binary stars and the central density of the host cluster. We compared the fraction of binary stars with that of BSSs, finding that clusters with high and low central density exhibit different trends. The fraction of binaries does not significantly change with the mass of the primary star and the mass ratio. The radial distribution of binary stars depends on cluster age. The binaries of clusters younger than $\sim$800\,Myr typically show a flat radial distribution, with some hints of a double peak. In contrast, the binaries of the remaining clusters are more centrally concentrated than the single stars, which is similar to what is observed in globular clusters.