The dynamical importance of binary systems in young massive star clusters

TL;DR

This study investigates how binary star systems in young massive star clusters are affected by dynamical processes, revealing that binary disruption can alter radial distributions and complicate mass segregation signatures.

Contribution

The paper combines Hubble observations and N-body simulations to show that binary disruption significantly influences binary fractions in cluster centers, challenging traditional mass segregation interpretations.

Findings

Binary disruption occurs rapidly in dense cluster cores.

Radial binary fractions can initially decrease towards the center.

Disruption effects can mask mass segregation signatures.

Abstract

Characterization of the binary fractions in star clusters is of fundamental importance for many fields in astrophysics. Observations indicate that the majority of stars are found in binary systems, while most stars with masses greater than $0.5 M_\odot$ are formed in star clusters. In addition, since binaries are on average more massive than single stars, in resolved star clusters these systems are thought to be good tracers of (dynamical) mass segregation. Over time, dynamical evolution through two-body relaxation will cause the most massive objects to migrate to the cluster center, while the relatively lower-mass objects remain in or migrate to orbits at greater radii. This process will globally dominate a cluster's stellar distribution. However, close encounters involving binary systems may disrupt `soft' binaries. This process will occur more frequently in a cluster's central, dense region than in its periphery, which may mask the effects of mass segregation. Using high resolution Hubble Space Telescope observations, combined with sophisticated $N$-body simulations, we investigate the radial distributions of the main-sequence binary fractions in massive young Large Magellanic Cloud star clusters. We show that binary disruption may play an important role on very short timescales, depending on the environmental conditions in the cluster cores. This may lead to radial binary fractions that initially decline in the cluster centers, which is contrary to the effects expected from dynamical mass segregation.