Evolution of the Binary Fraction in Dense Stellar Systems

TL;DR

This study uses an improved Monte Carlo code to analyze how binary star fractions evolve in dense globular clusters, revealing key processes like mass segregation and tidal stripping, and highlighting the complexity of observational measurements.

Contribution

The paper introduces an enhanced Monte Carlo simulation incorporating stellar evolution to study binary fraction evolution in globular clusters, aligning with previous N-body results.

Findings

Core binary fraction tends to increase over time for initial fractions below 90%.

Mass segregation and tidal stripping are key processes affecting binary evolution.

The observed binary fraction near the half-mass radius reflects the primordial binary fraction.

Abstract

Using our recently improved Monte Carlo evolution code, we study the evolution of the binary fraction in globular clusters. In agreement with previous N-body simulations, we find generally that the hard binary fraction in the core tends to increase with time over a range of initial cluster central densities for initial binary fractions <~ 90%. The dominant processes driving the evolution of the core binary fraction are mass segregation of binaries into the cluster core and preferential destruction of binaries there. On a global scale, these effects and the preferential tidal stripping of single stars tend to roughly balance, leading to overall cluster binary fractions that are roughly constant with time. Our findings suggest that the current hard binary fraction near the half-mass radius is a good indicator of the hard primordial binary fraction. However, the relationship between the true binary fraction and the fraction of main-sequence stars in binaries (which is typically what observers measure) is non-linear and rather complicated. We also consider the importance of soft binaries, which not only modify the evolution of the binary fraction, but can drastically change the evolution of the cluster as a whole. Finally, we describe in some detail the recent addition of single and binary stellar evolution to our cluster evolution code.