Blue Stragglers in Globular Clusters: Observations, Statistics and Physics

TL;DR

This paper investigates the formation of blue stragglers in globular clusters by analyzing their observed statistics, revealing a strong correlation with cluster mass rather than collision rates, suggesting a binary formation channel.

Contribution

It provides new insights into blue straggler formation by demonstrating the correlation with cluster mass and binary fractions, challenging previous assumptions about dynamical encounters.

Findings

Blue straggler numbers correlate strongly with total cluster mass.

Little evidence links blue straggler numbers to stellar collision rates.

Binary fractions may depend mainly on core mass, explaining observed correlations.

Abstract

This chapter explores how we might use the observed {\em statistics} of blue stragglers in globular clusters to shed light on their formation. This means we will touch on topics also discussed elsewhere in this book, such as the discovery and implications of bimodal radial distributions and the "double sequences" of blue stragglers that have recently been found in some clusters. However, we will focus particularly on the search for a "smoking gun" correlation between the number of blue stragglers in a given globular cluster and a physical cluster parameter that would point towards a particular formation channel. As we shall see, there is little evidence for an intrinsic correlation between blue straggler numbers and stellar collision rates, even in dense cluster cores. On the other hand, there is a clear correlation between blue straggler numbers and the total (core) mass of the cluster. This would seem to point towards a formation channel involving binaries, rather than dynamical encounters. However, the correlation between blue straggler numbers and actual binary numbers - which relies on recently determined empirical binary fractions - is actually weaker than that with core mass. We explain how this surprising result may be reconciled with a binary formation channel if binary fractions depend almost uniquely on core mass. If this is actually the case, it would have significant implications for globular cluster dynamics more generally.