The binary fractions in the massive young Large Magellanic Cloud star clusters NGC 1805 and NGC 1818

TL;DR

This study analyzes the radial distribution of binary star fractions in two young Large Magellanic Cloud clusters using Hubble data, revealing contrasting trends likely driven by different dynamical processes.

Contribution

It compares two methods for measuring binary fractions and finds consistent results, providing insights into the dynamical mechanisms affecting binaries in young star clusters.

Findings

NGC 1805 shows decreasing binary fraction towards the center

NGC 1818 exhibits increasing binary fraction towards the center

Different dynamical processes likely influence the binary distributions

Abstract

Using high-resolution data sets obtained with the Hubble Space Telescope, we investigate the radial distributions of the F-type main-sequence binary fractions in the massive young Large Magellanic Cloud star clusters NGC 1805 and NGC 1818. We apply both an isochrone-fitting approach and chi^2 minimization using Monte Carlo simulations, for different mass-ratio cut-offs, q, and present a detailed comparison of the methods' performance. Both methods yield the same radial binary fraction profile for the same cluster, which therefore supports the robustness and applicability of either method to young star clusters which are as yet unaffected by the presence of multiple stellar populations. The binary fractions in these two clusters are characterized by opposite trends in their radial profiles. NGC 1805 exhibits a decreasing trend with increasing radius in the central region, followed by a slow increase to the field's binary-fraction level, while NGC 1818 shows a monotonically increasing trend. This may indicate dominance of a more complicated physical mechanism in the cluster's central region than expected a priori. Time-scale arguments imply that early dynamical mass segregation should be very efficient and, hence, likely dominates the dynamical processes in the core of NGC 1805. Meanwhile, in NGC 1818 the behavior in the core is probably dominated by disruption of soft binary systems. We speculate that this may be owing to the higher velocity dispersion in the NGC 1818 core, which creates an environment in which the efficiency of binary disruption is high compared with that in the NGC 1805 core.