Formation Channels for Blue Straggler Stars

TL;DR

This paper reviews two main formation channels for blue straggler stars—stellar collisions and binary mass transfer—and discusses how their prevalence varies with cluster mass based on simulations and population studies.

Contribution

It provides a comparative analysis of blue straggler formation mechanisms and their dependence on cluster properties, integrating simulation results with observational population trends.

Findings

Stellar collisions produce relatively little mass loss, forming blue stragglers effectively.

The number of blue stragglers from collisions increases with cluster mass.

In massive clusters, binary populations contributing to blue stragglers decrease due to dynamical interactions.

Abstract

In this chapter we consider two formation channels for blue straggler stars: 1) the merger of two single stars via a collision, and 2) those produced via mass transfer within a binary. We review how computer simulations show that stellar collisions are likely to lead to relatively little mass loss and are thus effective in producing a young population of more-massive stars. The number of blue straggler stars produced by collisions will tend to increase with cluster mass. We review how the current population of blue straggler stars produced from primordial binaries decreases with increasing cluster mass. This is because exchange encounters with third, single stars in the most massive clusters tend to reduce the fraction of binaries containing a primary close to the current turn-off mass. Rather, their primaries tend to be somewhat more massive and have evolved off the main sequence, filling their Roche lobes in the past, often converting their secondaries into blue straggler stars (but more than 1 Gyr or so ago and thus they are no longer visible today as blue straggler stars).