2018ApJ...865L..10D

Galactic globular clusters (GCs) show overwhelming photometric and spectroscopic evidence for the existence of multiple stellar populations. The question of whether or not there exists a GC that represents a true "simple stellar population" remains open. Here we focus on Ruprecht 106 (R106), a halo GC with [Fe/H] = -1.5 and [α/Fe] ≃0. A previous spectroscopic study found no sign of the Na-O anticorrelation among nine of its brightest red giants, which led to the conclusion that R106 is a true simple stellar population GC. Here we present new Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) photometry of R106 that, when combined with archival HST images spanning a 6 year baseline, allows us to create proper-motion cleaned color-magnitude diagrams spanning the ultraviolet (F336W) to the near-infrared (F814W). These data allow us to construct the pseudo-color C_U,B,I that is sensitive to the presence of light-element abundance spreads. We find no evidence of a split along the red giant branch (RGB) in the C_U,B,I diagram but the width of the RGB (σ_CU,B,I_=0.015) is marginally broader than expected from artificial star tests (σ_CU,B,I_=0.009). The observed spread in C_U,B,I is smaller than any other Galactic GC studied to date. Our results raise important questions about the role of formation environment and primordial chemical composition in the formation of multiple stellar populations in GCs.