Stellar variability at the main-sequence turnoff of the intermediate-age LMC cluster NGC 1846

TL;DR

This study identifies Delta Scuti variable stars at the main-sequence turnoff of the LMC cluster NGC 1846, suggesting stellar variability may contribute to the extended MSTO phenomenon, but not fully explain it.

Contribution

First detection of a significant number of Delta Scuti variables at the MSTO in an intermediate-age LMC cluster, highlighting their potential impact on cluster CMD interpretations.

Findings

73 new variable stars identified, 55 of Delta Scuti type

Estimated 40-60 Delta Scuti members in the cluster

Variable stars unlikely to cause significant MSTO broadening

Abstract

Intermediate-age star clusters in the LMC present extended main sequence turnoffs (MSTO) that have been attributed to either multiple stellar populations or an effect of stellar rotation. Recently it has been proposed that these extended main sequences can also be produced by ill-characterized stellar variability. Here we present Gemini-S/GMOS time series observations of the intermediate-age cluster NGC 1846. Using differential image analysis, we identified 73 new variable stars, with 55 of those being of the Delta Scuti type, that is, pulsating variables close the MSTO for the cluster age. Considering completeness and background contamination effects we estimate the number of Delta Scuti belonging to the cluster between 40 and 60 members, although this number is based on the detection of a single Delta Scuti within the cluster half-light radius. This amount of variable stars at the MSTO level will not produce significant broadening of the MSTO, albeit higher resolution imaging will be needed to rule out variable stars as a major contributor to the extended MSTO phenomenon. Though modest, this amount of Delta Scuti makes NGC 1846 the star cluster with the highest number of these variables ever discovered. Lastly, our results are a cautionary tale about the adequacy of shallow variability surveys in the LMC (like OGLE) to derive properties of its Delta Scuti population.