Blue straggler evolution caught in the act in the Large Magellanic Cloud globular cluster Hodge 11

TL;DR

This study uses high-resolution Hubble data to analyze blue straggler stars in the Large Magellanic Cloud cluster Hodge 11, revealing bimodal distribution patterns that support multiple formation channels and providing insights into their evolutionary history.

Contribution

It provides the first observational evidence of bimodal blue straggler distributions in an extragalactic cluster, supporting theories of different formation mechanisms based on stellar density.

Findings

Blue straggler stars show bimodal radial distribution.

Inner BSs align with collision-based formation models.

Outer BSs are consistent with binary evolution processes.

Abstract

High-resolution {\sl Hubble Space Telescope} imaging observations show that the radial distribution of the field-decontaminated sample of 162 'blue straggler' stars (BSs) in the $11.7^{+0.2}_{-0.1}$ Gyr-old Large Magellanic Cloud cluster Hodge 11 exhibits a clear bimodality. In combination with their distinct loci in color--magnitude space, this offers new evidence in support of theoretical expectations that suggest different BS formation channels as a function of stellar density. In the cluster's color--magnitude diagram, the BSs in the inner 15$"$ (roughly corresponding to the cluster's core radius) are located more closely to the theoretical sequence resulting from stellar collisions, while those in the periphery (at radii between 85$"$ and 100$"$) are preferentially found in the region expected to contain objects formed through binary mass transfer or coalescence. In addition, the objects' distribution in color--magntiude space provides us with the rare opportunity in an extragalactic environment to quantify the evolution of the cluster's collisionally induced BS population and the likely period that has elapsed since their formation epoch, which we estimate to have occurred $\sim$4--5 Gyr ago.