The multiplicity fraction in 324 open clusters from Gaia

Abstract

This Master’s thesis is devoted to the estimation of the multiplicity fraction of high-mass ratio main-sequence multiple systems in Galactic open clusters. The main study consists in estimating in an automated fashion the unresolved multiplicity fraction through the location of the unresolved multiple systems in colour-magnitude diagrams, using Gaia DR2 and EDR3 data. Regarding this study, this thesis is a continuation of the previous preliminary work of my bachelor thesis (Donada, 2021), which explored the possibility of applying a Gaussian mixture model algorithm. Now we revise and improve this method, and also develop an alternative implementation using a Markov chain Monte Carlo method. The unresolved multiplicity fractions obtained through both methods are compared and characterized using custom realistic Gaia-like open cluster simulations performed with the Gaia Object Generator (Luri et al., 2014). Using these simulations we estimate the effective limiting mass ratio above which the Markov chain Monte Carlo method is able to detect the presence of a secondary companion (i.e., the high mass ratio range which our estimated unresolved multiplicity fraction comprises). The simulations further enable us to correct for the presence of resolved multiple systems. So, as a second part of the study, we estimate the total high mass ratio multiplicity fraction of the open clusters’ main sequences. Finally, we compare our results to the ones estimated through ASteCA (Automated Stellar Cluster Analysis package). The main result of this work is the largest homogeneous catalogue of multiplicity fractions in open clusters to date, including the unresolved and total multiplicity fractions of main-sequence systems with mass ratio larger than 0.6+0.05 −0.15 for 324 open clusters, estimated through the Markov chain Monte Carlo method. All studied open clusters are closer than 1.5 kpc, and have ages between 3.9 Myr and 4.3 Gyr. Their total multiplicity fractions, between 0.05 and 0.71, are found to increase with the mass of the primary star, display an overall decreasing trend with the open cluster age up until ages about 100-320 Myr, above which the trend increases; and do not depend on the open cluster position in the Galaxy.