The Distribution of Collisionally Induced Blue Stragglers in the Color‐Magnitude Diagram

Abstract

A primary production mechanism for blue stragglers in globular clusters is thought to be collisionally induced mergers, perhaps mediated by dynamical encounters involving binary stars. We model the formation and evolution of such blue stragglers and produce theoretical distributions of them in the color-magnitude diagram. We use a crude representation of cluster dynamics and detailed binary-single star encounter simulations to produce cross sections and rates for a variety of collisions. The results of the collisions are determined on the basis of smoothed-particle hydrodynamics simulations of realistic star models. The evolution of the collision products are then followed in detail using the Yale stellar evolution code. We present our results in the form of distributions in the observed color-magnitude diagram. We use our results to explore the effects of a variety of input assumptions on the number and kind of blue stragglers created by collisions. In particular, we describe the changes in the blue straggler distribution that result from using realistic collision products rather than the fully mixed assumption, and from changes in assumptions about the number and orbital-period distribution of the primordial binary population. We then apply our models to existing data from the core of M3, where the large blue straggler population is thought to be dominated by collision products. We find that we have difficulty successfully modeling the observed blue stragglers under a single consistent set of assumptions. However, if three particularly bright blue stragglers are considered to be part of a different observed population (as has been suggested previously for other clusters) and left out of the population, the remainder can be successfully modeled using realistic encounter products and assuming a 20% binary fraction with a plausible period distribution. Finally, we suggest a variety of routes toward a more comprehensive understanding of the blue straggler phenomenon.