Dotter, AaronMilone, Antonino P.Conroy, CharlieMarino, AnnaSarajedini, Ata2025-12-142025-12-142041-8205https://hdl.handle.net/1885/733794775Galactic 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 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 diagram but the width of the RGB () is marginally broader than expected from artificial star tests (). The observed spread in 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.Support for this work was provided by NASA through grant number HST-GO-14726.004 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555. A.P.M. acknowledges support by the European Research Council through the ERC-StG 2016, project 716082 “GALFOR” and by the MIUR through the FARE project R164RM93XW “SEMPLICE.” C.C. acknowledges support from the Packard foundation. A.F.M. acknowledges support by the Australian Research Council through Discovery Early Career Researcher Award DE160100851. Facilities: HST(WFC3), HST(ACS).application/pdfen-AU© 2018 The American Astronomical Society201810.3847/2041-8213/aae08f2023-10-22