Binary Frequencies in Globular Clusters

TL;DR

This study measures binary star fractions in 35 globular clusters using Hubble data, revealing a broad initial distribution, dynamical evolution effects, and a correlation between binary fraction and cluster age.

Contribution

It provides new binary fraction measurements for multiple clusters and analyzes their variation with cluster age and dynamical state, highlighting discrepancies with previous studies.

Findings

Binary fractions vary nearly an order of magnitude across clusters.

Core binary fractions decrease with cluster age.

Radial gradients in binary fractions are linked to dynamical interactions.

Abstract

Binary stars are predicted to have an important role in the evolution of globular clusters, so we obtained binary fractions for 35 globular clusters that were imaged in the F606W and F814W with the ACS on the Hubble Space Telescope. When compared to the values of prior efforts (Sollima et al. 2007; Milone et al. 2012), we find significant discrepancies, despite each group correcting for contamination effects and having performed the appropriate reliability tests. The most reliable binary fractions are obtained when restricting the binary fraction to q > 0.5. Our analysis indicates that the range of the binary fractions is nearly an order of magnitude for the lowest dynamical ages, suggesting that there is a broad distribution in the binary fraction at globular cluster formation. Dynamical effects also appears to decrease the core binary fractions by a factor of two over a Hubble time, but this is a weak relationship. We confirm a correlation from previous work that the binary fraction within the core radius decreases with cluster age, indicating that younger clusters formed with higher binary fractions. The strong radial gradient in the binary fraction with cluster radius appears to be a consequence of dynamical interactions. It is often not present in dynamically young clusters but nearly always present in dynamically old clusters.