A binary-related origin mediated by environmental conditions for blue straggler stars

TL;DR

This study analyzes over 3000 blue straggler stars in 48 globular clusters, revealing that their abundance is inversely related to cluster density and collision rate, supporting a binary evolution origin influenced by environmental factors.

Contribution

It provides the first large-scale statistical evidence that blue straggler formation is primarily driven by binary evolution in low-density environments, challenging collision-based formation theories.

Findings

Blue straggler numbers anti-correlate with cluster density.

Binary fraction shows similar anti-correlation.

Low-density regions favor binary-related blue straggler formation.

Abstract

Blue stragglers are anomalously massive core hydrogen-burning stars that, according to the theory of single star evolution, should not exist. They are suspected to form in mass-enhancement processes, involving binary evolution or stellar collisions. In dynamically active systems like globular clusters, the number of blue stragglers originated by collisions is expected to increase with the local density and the rate of stellar encounters. Here we analyse more than 3000 blue stragglers in 48 Galactic globular clusters with different structures, finding that their number normalized to the sampled luminosity anti-correlates (instead of correlating) with the central density, collision rate, and dynamical age of the parent cluster. Similar trends are also found for the cluster binary fraction. Once inserted in the context of the current knowledge of the BSS phenomenon, these correlations indicate that low-density regions (possibly because of a higher binary production/survival rate) are the natural habitat of both BSSs and binary systems, and the observed BSSs mostly have a binary-related origin mediated by the environmental conditions.