Stellar Collisions and Blue Straggler Stars in Dense Globular Clusters

TL;DR

This study uses advanced simulations to analyze blue straggler star formation in globular clusters, revealing the dominant formation channels depend on cluster density and highlighting the weak correlation with traditional collision rate estimates.

Contribution

The paper introduces comprehensive Monte Carlo models that incorporate all relevant physical processes, providing new insights into BSS formation channels and their dependence on cluster properties.

Findings

Stellar collisions dominate BSS formation in dense clusters.

Mass transfer in binaries significantly contributes in lower density clusters.

BSS numbers correlate with binary fraction, not collision rate.

Abstract

Blue straggler stars (BSS) are abundantly observed in all Galactic globular clusters (GGC) where data exist. However, observations alone cannot reveal the relative importance of various formation channels or the typical formation times for this well studied population of anomalous stars. Using a state-of-the-art H\'enon-type Monte Carlo code that includes all relevant physical processes, we create 128 models with properties typical of the observed GGCs. These models include realistic numbers of single and binary stars, use observationally motivated initial conditions, and span large ranges in central density, concentration, binary fraction, and mass. Their properties can be directly compared with those of observed GGCs. We can easily identify the BSSs in our models and determine their formation channels and birth times. We find that for central densities above ~10^3 Msun/pc^3 the dominant formation channel is stellar collisions while for lower density clusters, mass transfer in binaries provides a significant contribution (up to ~ 60% in our models). The majority of these collisions are binary-mediated, occurring during 3-body and 4-body interactions. As a result a strong correlation between the specific frequency of BSSs and the binary fraction in a cluster can be seen in our models. We find that the number of BSSs in the core shows only a weak correlation with the collision rate estimator \Gamma traditionally used by observers, in agreement with the latest Hubble Space Telescope (ACS) data. Using an idealized "full mixing" prescription for collision products, our models indicate that the BSSs observed today may have formed several Gyrs ago. However, denser clusters tend to have younger (~1 Gyr) BSSs.