2019ApJ...884...17L

The presence of multiple populations (MPs) in almost all globular clusters (GCs) older than ∼10 Gyr has drawn lots of attention. Recently, cumulative evidence has indicated that extragalactic GCs that are older than 2 Gyr seem to also harbor MPs; however, those that are younger than 2 Gyr do not. These observations seem to imply that age is a primary property that controls the presence of MPs in star clusters. However, because of the lack of studies of intermediate-age (∼2-6 Gyr old), low-mass clusters, it is unclear if the cluster mass, in addition to age, also plays a role in the occurrence of MPs. In this work, we studied a ∼4 Gyr old, low-mass (∼23,000 M_☉) cluster, Lindsay-113, in the Small Magellanic Cloud. Using Hubble Space Telescope photometry, we found that the width of the red-giant branch in this cluster, when measured in a specific color index that is sensitive to star-to-star chemical variations, can be adequately explained by a "simple" stellar population model with some possible noises contributed from measurement uncertainty, photometric artifact, and differential reddening. The comparison of observations with predictions from synthetic spectra indicates that the internal chemical spread in nitrogen abundance, which is a signature of MPs, would not exceed 0.2 dex. Since Lindsay 113 is significantly older than other GCs with MPs, we suggest that the onset of MPs is likely determined by the combination of cluster age and mass.