Blue straggler formation at core collapse

TL;DR

This study uses N-body simulations to investigate how core collapse in star clusters triggers the formation of blue straggler stars, revealing multiple formation episodes and the roles of collisions and binaries.

Contribution

It demonstrates that core collapse induces bursts of blue straggler formation, combining collisional and binary pathways, in simulated star clusters.

Findings

Blue stragglers form in bursts at core collapse onset

Multiple BSS formation episodes occur during post-core-collapse

Both collisional and binary BSSs are produced after core collapse

Abstract

Among the most striking feature of blue straggler stars (BSS) is the presence of multiple sequences of BSSs in the colour-magnitude diagrams (CMDs) of several globular clusters. It is often envisaged that such a multiple BSS sequence would arise due a recent core collapse of the host cluster, triggering a number of stellar collisions and binary mass transfers simultaneously over a brief episode of time. Here we examine this scenario using direct N-body computations of moderately massive star clusters (of order 10^4 Msun ). As a preliminary attempt, these models are initiated with approx. 8-10 Gyr old stellar population and King profiles of high concentrations, being "tuned" to undergo core collapse quickly. BSSs are indeed found to form in a "burst" at the onset of the core collapse and several of such BS-bursts occur during the post-core-collapse phase. In those models that include a few percent primordial binaries, both collisional and binary BSSs form after the onset of the (near) core-collapse. However, there is as such no clear discrimination between the two types of BSSs in the corresponding computed CMDs. We note that this may be due to the less number of BSSs formed in these less massive models than that in actual globular clusters.