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 [ \alpha /Fe ] \simeq 0 .
A previous spectroscopic study found no sign of the Na-O anticorrelation among 9 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 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 ( \sigma _ { C _ { U,B,I } } = 0.015 ) is marginally broader than expected from artificial star tests ( \sigma _ { C _ { U,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 rôle of formation environment and primordial chemical composition in the formation of multiple stellar populations in GCs .