Ruprecht 106: A riddle, wrapped in a mystery, inside an enigma

TL;DR

This study investigates Ruprecht 106, a unique globular cluster with minimal evidence of multiple stellar populations, using advanced HST photometry to analyze its chemical composition and stellar population characteristics.

Contribution

The paper provides new high-precision photometric data and analysis of Ruprecht 106, challenging the assumption that all globular clusters host multiple stellar populations.

Findings

No evidence of multiple populations in the RGB of R106.

The RGB width in R106 is smaller than in other Galactic GCs.

R106's properties question the link between formation environment and multiple populations.

Abstract

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.