Stellar Encounters with Multiple Star Systems and the Blue Straggler Problem

TL;DR

This paper introduces an analytic method to trace the dynamical formation scenarios of multiple star systems with blue stragglers, revealing that triple encounters significantly contribute to blue straggler populations in old clusters.

Contribution

The paper presents a novel analytic prescription for energy conservation in stellar encounters, enabling backward inference of initial orbital parameters from observed systems.

Findings

Blue straggler systems in M67 and NGC 188 can originate from dynamical encounters.

Encounters involving triples are a major pathway for blue straggler formation.

Triples significantly contribute to stellar mergers in old open clusters.

Abstract

We present a technique to identify the most probable dynamical formation scenario for an observed binary or triple system containing one or more merger products or, alternatively, to rule out the possibility of a dynamical origin. Our method relies on an analytic prescription for energy conservation during stellar encounters. With this, observations of the multiple star system containing the merger product(s) can be used to work backwards in order to constrain the initial orbital energies of any single, binary or triple systems that went into the encounter. The initial semi-major axes of the orbits provide an estimate for the collisional cross section and therefore the time-scale for the encounter to occur in its host cluster. We have applied our analytic prescription to observed binary and triple systems containing blue stragglers, in particular the triple system S1082 in M67 and the period distribution of the blue straggler binaries in NGC 188. We have shown that both S1082 and most of the blue straggler binaries in NGC 188 could have a dynamical origin, and that encounters involving triples are a significant contributor to blue straggler populations in old open clusters. In general, our results suggest that encounters involving triples could make up a significant fraction of those dynamical interactions that result in stellar mergers, in particular encounters that produce multiple star systems containing one or more blue stragglers.