Open clusters: time-scales, core collapse and blue stragglers

Félix Llorente de Andrés, Carmen Morales-Durán

Published 2022-11-20Version 1

We developed a mathematical model to derive time scales and the presence of BS stars. The model is based on the variation of mass through a circle into the cluster defined by a radius, and at a time; this mass cross is translated into a differential equation that it can be integrated for a given radius (r) and a determined time (t). From this equation we can derive the different time scales that allows us to reach conclusions like: clusters not containing blue strugglers (BS) stars disappear younger than those clusters containing BS. In clusters containing BS stars, the volume which takes up half of the cluster mass is bigger than the one corresponding to clusters without BS stars but the time to catch it up is shorter. We also studied, by means of this equation, the core collapse of stars of the cluster and the region where this concentration is stopped/retained; this region is identified by means of the relation $c/ch$, being $c=\log(rt/rc)$ and $ch=\log(rc/rh)$. Where rt and rc are the tidal and the core radius respectively, and rh is the radius where half of the cluster mass is concentrated. The model also drove us to the conclusion that the number of the blue straggler stars in a cluster follows a distribution function whose components are the ratio between relaxation time and the age, labelled as $\it f$, and a factor, named $\varpi$, which is an indicator of the origin of the BS; $\varpi$ increases as the number of BS increase but it is limited to$\sim$5.0. The mentioned distribution function is expressed as $\it NBS$ $\sim$ $\it f^3$($\frac{1}{e^{\frac{f}{\varpi}}-1}$). The validity of this function was carried out by means of matching the number of observed blue straggler (BS) stars to the number of predicted ones in the available sample of OC.