Stars on the run: escaping from stellar clusters

TL;DR

This study uses N-body simulations to analyze how stars escape from dissolving stellar clusters and contribute to the Milky Way's population, focusing on escape velocities, binary fractions, and mechanisms involved.

Contribution

It provides new insights into the escape velocities, mechanisms, and the impact of primordial binary fractions on stellar escape from clusters over 4 Gyr.

Findings

High binary fractions increase high-velocity escapers.

Most escapers have velocities below 20 km/s regardless of binary fraction.

Dissolving clusters contribute to the Galactic halo with stars up to 2.4 solar masses.

Abstract

A significant proportion of Milky Way stars are born in stellar clusters, which dissolve over time so that the members become part of the disc and halo populations of the Galaxy. In the present work we will assume that these young stellar clusters live mainly within the disc of the Galaxy and that they can have primordial binary percentages ranging from 0% to as high as 70%. We have evolved models of such clusters to an age of 4 Gyr through N-body simulations, paying attention to the stars and binaries that escape in the process. We have quantified the contribution of these escaping stars to the Galaxy population by analysing their escape velocity and evolutionary stage at the moment of escape. In this way we could analyse the mechanisms that produced these escapers, whether evaporation through weak two- body encounters, energetic close encounters or stellar evolution events, e.g. supernovae. In our models we found that the percentage of primordial binaries in a star cluster does not produce significant variations in the velocities of the stars that escape in the velocity range of 0-20 km/s. However, in the high-velocity 20-100 km/s range the number of escapers increased markedly as the primordial binary percentage increased. We could also infer that dissolving stellar clusters such as those that we have modelled can populate the Galactic halo with giant stars for which the progenitors were stars of up to 2.4 Msun. Furthermore, choices made for the velocity kicks of remnants do influence the production of hyper-velocity stars - and to a lesser extent stars in the high-velocity range - but once again the difference for the 99% of stars in the 0-20 km/s range is not significant.