The overlooked role of stellar variability in the extended main sequence of LMC intermediate-age clusters

TL;DR

This study demonstrates that Delta Scuti variable stars can significantly contribute to the extended main sequence turn offs observed in intermediate-age star clusters in the Large Magellanic Cloud, affecting interpretations of their stellar populations.

Contribution

It introduces the impact of variable stars, specifically Delta Scuti, into the analysis of extended MSTOs, highlighting their role in broadening the turn off region in synthetic CMDs.

Findings

Delta Scuti variables cause broadening of the MSTO in synthetic CMDs.

The effect is age-dependent, prominent in 1-3 Gyr clusters.

Variable stars do not explain bifurcated MSTOs or extended MSTOs in young clusters.

Abstract

Intermediate-age star clusters in the Large Magellanic Cloud show extended main sequence turn offs (MSTOs), which are not consistent with a canonical single stellar population. These broad turn offs have been interpreted as evidence for extended star formation and/or stellar rotation. Since most of these studies use single frames per filter to do the photometry, the presence of variable stars near the MSTO in these clusters has remained unnoticed and their impact totally ignored. We model the influence of Delta Scuti using synthetic CMDs, adding variable stars following different levels of incidence and amplitude distributions. We show that Delta Scuti observed at a single phase will produce a broadening of the MSTO without affecting other areas of a CMD like the upper MS or the red clump; furthermore, the amount of spread introduced correlates with cluster age as observed. This broadening is constrained to ages ~ 1-3 Gyr when the MSTO area crosses the instability strip, which is also consistent with observations. Variable stars cannot explain bifurcated MSTOs or the extended MSTOs seen in some young clusters, but they can make an important contribution to the extended MSTOs in intermediate-age clusters.