Blue straggler stars beyond the Milky Way. III. Detection of evolved blue straggler candidates in Large Magellanic Cloud clusters

TL;DR

This study uses Hubble observations of Large Magellanic Cloud clusters to identify evolved blue straggler star candidates, revealing bright evolved stars unlikely explained by artifacts or standard models, suggesting they are evolved blue stragglers.

Contribution

First detection and analysis of evolved blue straggler candidates in LMC clusters, expanding understanding of their properties and distribution beyond the Milky Way.

Findings

Bright evolved stars cannot be explained by artifacts or reddening.

Spatial distribution indicates these stars are cluster members.

Mass estimates suggest they are evolved blue stragglers.

Abstract

We analyze {\sl Hubble Space Telescope} observations of nine Large Magellanic Cloud star clusters with ages of 1--2 Gyr to search for evolved counterparts of blue straggler stars. Near the red clump regions in the clusters' color--magnitude diagrams, we find branches of evolved stars that are much brighter than normal evolved stars. We examine the effects of photometric artifacts, differential reddening, and field contamination. We conclude that these bright evolved stars cannot be explained by any of these effects. Our statistical tests show that the contributions of photometric uncertainties and crowding effects, as well as that owing to differential reddening, to these bright evolved stars are insufficient to fully explain the presence of these bright evolved stars. Based on isochrone fitting, we also ruled out the possibility that these bright evolved stars could be reproduced by an internal chemical abundance spread. The spatial distributions of the bright evolved stars exhibit clear concentrations that cannot be explained by homogeneously distributed field stars. This is further confirmed based on Monte Carlo-based tests. By comparing our observations with stellar evolution models, we find that the masses of most of bright evolved stars do not exceed twice the average mass of normal evolved stars. We suggest that these bright evolved stars are, in fact, evolved blue straggler stars.