Ruprecht 106: A riddle, wrapped in a mystery, inside an enigma
  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]~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_CUBI = 0.015) is marginally broader than expected from artificial star
tests (sigma_CUBI = 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.
