Exploring Galactic open clusters with Gaia II. Mass Segregation and Mass Function in Fifteen Nearby Open Clusters

Abstract: Context. Mass is the most critical physical parameter in the evolution of a star. Since stars form in clusters their Initial Mass Function (IMF) is decisive in their evolution. Aims. Use Gaia DR3-based stellar masses_mass flame and the stellar members found for fifteen nearby open clusters from Paper I, to estimate their mass segregation and distribution. Methods. For each cluster, the single stars' main sequence was fitted with a moving straight line weighted fit to the Color-Magnitude Diagram, stars brighter than the residuals dispersion were taken as binaries. Single stars masses were obtained from a cubic spline fit to the mass_flame vs. G magnitude data. For binary stars, the individual masses of each component were estimated using simulated-based inference. We used the minimum spanning tree concept to measure the mass segregation of each cluster. From the stellar mass distribution, an estimate of the power-law coefficient that best describes it was used to characterize the IMF. Results. Mass segregation is visible in all the clusters, the older ones have about 50% of their most massive stars segregated, while younger ones extend from about 30% to 55%. The IMF of the studied clusters is well described by a power-law of index 2.09. Conclusions. Significant mass segregation, from one-third to one-half of its most massive population is present in open clusters as young as > 10 Myr. Mass segregation may be strong for only a few of the most massive stars or less intense but extended to a larger fraction of those, it may start as early as 0.20 of the relaxation time of the cluster and progress over time by increasing both the number of the most massive stars affected and their amount of segregation. Older open clusters show evidence of binary disruption as time progresses.