Multiple populations in the old and massive Small Magellanic Cloud globular cluster NGC121

TL;DR

This study reveals multiple stellar populations in the old, massive SMC globular cluster NGC121, showing a unique dominance of first-generation stars, challenging previous assumptions about cluster evolution and stellar mass-loss.

Contribution

First detection of multiple stellar populations in an SMC cluster, highlighting differences in star population ratios compared to Galactic GCs.

Findings

NGC121 hosts multiple stellar populations.

Second-generation stars are more centrally concentrated.

NGC121 is dominated by first-generation stars, unlike typical Galactic GCs.

Abstract

We used a combination of optical and near-UV Hubble Space Telescope photometry and FLAMES/ESO-VLT high-resolution spectroscopy to characterize the stellar content of the old and massive globular cluster (GC) NGC121 in the Small Magellanic Cloud (SMC). We report on the detection of multiple stellar populations, the first case in the SMC stellar cluster system. This result enforces the emerging scenario in which the presence of multiple stellar populations is a distinctive-feature of old and massive GCs regardless of the environment, as far as the light element distribution is concerned. We find that second population (SG) stars are more centrally concentrated than first (FG) ones. More interestingly, at odds with what typically observed in Galactic GCs, we find that NGC121 is the only cluster so far to be dominated by FG stars that account for more than 65% of the total cluster mass. In the framework where GCs were born with a 90-95% of FG stars, this observational finding would suggest that either NGC121 experienced a milder stellar mass-loss with respect to Galactic GCs or it formed a smaller fraction of SG stars.