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

¹ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Univ. di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
e-mail: giacomo.cordoni@unipd.it
² Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Istituto Nazionale di Astrofisica – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi, 5, Firenze 50125, Italy
⁴ Department of Astronomy, Indiana University, Bloomington, IN 47401, USA
⁵ Center for Galaxy Evolution Research and Department of Astronomy,Yonsei University, Seoul 03722, Korea

Received: 14 November 2022
Accepted: 29 January 2023

Abstract

Context. Binary stars play a crucial role in our understanding of the formation and evolution of star clusters and their stellar populations.

Aims. 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.

Methods. 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.

Results. 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_⊙. The fraction of binary stars ranges from ∼15% to more than ∼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 ∼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.