Fewer Companions in the Crowd: The Low Close Binary Fraction in Globular Clusters from Gaia RVS

TL;DR

This study uses Gaia RVS data to compare close binary fractions in globular clusters and field stars, revealing a significantly lower binary presence in clusters regardless of metallicity, and discussing dynamical and evolutionary implications.

Contribution

It provides the largest multi-epoch spectroscopic analysis of GCs' close binary fractions, demonstrating their lower prevalence compared to field stars with a homogeneous method.

Findings

Globular clusters have significantly lower close binary fractions than field stars.

No clear correlation between binary fraction and cluster metallicity.

Dynamical interactions and evolutionary processes may explain the low binary fractions.

Abstract

In dense environments like globular clusters (GCs), dynamical interactions can disrupt or harden close binaries, nonetheless, detailed comparisons with field binary fractions remain limited. Here, we present an analysis of the close binary fraction in a carefully selected sample of field stars and 10 GCs using Gaia Radial Velocity Spectrometer (RVS) data, which is among the largest samples of GCs analysed using multi-epoch spectroscopy to date. By assessing the peak-to-peak variations of the sources' radial velocity (RV), we estimate the close binary fractions through a method that fits the distribution as the product of two Gaussian distributions. By applying the same RV-variability method to both cluster members and field stars, we ensure a homogeneous and inclusive comparison between the two environments. Despite matching stellar parameters between the field and GC samples, our findings confirm that GCs possess a significantly lower close binary fraction than field stars. Interestingly, we do not detect any clear trend of binary fraction with cluster metallicity; metal-rich and metal-poor GCs are uniformly binary-poor (within uncertainties). We discuss possible interpretations, including dynamical hardening in dense environments and the effects of common envelope evolution, which may lead to companion accretion or merger events.