Multiple Populations in the Old and Massive Small Magellanic Cloud Globular Cluster NGC 121*

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) NGC 121 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 NGC 121 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 NGC 121 experienced a milder stellar mass-loss with respect to Galactic GCs or it formed a smaller fraction of SG stars.

Age as a major factor in the onset of multiple populations in stellar clusters

“…The CUBI pseudo-colour (defined as (U − B)−(B − I)) has been shown to be very effective at unveiling multiple sequences on the RGB (Dalessandro et al 2016;Monelli et al 2013). Here, we take advantage of a similar pseudo-colour, known as CUnBI ≡(U n−B)−(B−I), to provide a direct and quantifiable comparison between all the clusters in our survey.…”

“…While some GC stars have the same C, N, O, Na abundances as field stars with the same metallicity (i.e., iron content), a significant fraction of cluster members show systematically enhanced N and Na along with depleted C and O (Cannon et al 1998;Carretta et al 2009). Almost all old (> 10 Gyr) and massive (above a few times 10 4 M ) GCs in the Milky Way (Piotto et al 2015) and nearby galaxies (Mucciarelli et al 2009;Dalessandro et al 2016;Niederhofer et al 2017b;Larsen, Brodie & Strader 2012) have been found to host star-to-star chemical anomalies.…”

NGC 6535: the lowest mass Milky Way globular cluster with a Na-O anti-correlation?

“…This was the first SMC cluster to display the presence of MPs; the discovery was confirmed by Niederhofer et al 2017a using a similar HST filter set. Interestingly, Dalessandro et al (2016) found a dominant FG (about 65%), which could be due either to a smaller formation of SG stars or to a smaller loss of FG stars than in MW GCs of similar mass and metallicity. These authors also acquired highresolution spectra of five giants, but they seem to belong only to FG, according to their Na, O and Al, Mg abundances.…”

“…When however younger ages are considered, Mucciarelli et al (2008Mucciarelli et al ( , 2011Mucciarelli et al ( , 2014 did not find MPs in intermediate-age LMC clusters, seemingly reproducing what we see in the MW (but see below). Dalessandro et al (2016) discovered the presence of MPs in NGC 121, the only SMC cluster as old as the classical MW GCs, using mostly near-UV HST photometry. This was the first SMC cluster to display the presence of MPs; the discovery was confirmed by Niederhofer et al 2017a using a similar HST filter set.…”

“…(2006); Mucciarelli et al (2009) for the Large Magellanic Cloud (LMC), and Dalessandro et al (2016) for the Small Magellanic Cloud (SMC), or the Fornax dwarf spheroidal (Fnx dSph; Letarte et al 2006). Low-resolution spectroscopy and photometry have been employed to study younger clusters in the MCs or the Fnx dSph; see e.g.…”