The search for multiple populations in Magellanic Cloud clusters – I. Two stellar populations in the Small Magellanic Cloud globular cluster NGC 121

2016/09/06 by Florian Niederhofer, F. Niederhofer, N. Bastian +19 · 1 citation
Physics and Astronomy · #Astronomy #Astronomy and Astrophysical Research #Astrophysics #Astrophysics and Star Formation Studies #Galaxy #Globular cluster #Horizontal branch #Hubble space telescope #Large Magellanic Cloud #Milky Way #Physics #Population #Small Magellanic Cloud #Star cluster #Star formation #Stars #Stellar population #Stellar, planetary, and galactic studies #astro-ph.GA #astro-ph.SR

paper · pdf · doi:10.1093/mnras/stw2269

12 pages, 9 figures, accepted for publication in MNRAS

arxiv created 2016/09/06 · openalex publication_date 2016/09/08 · arxiv updated 2016/09/21 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06

Abstract

We started a photometric survey using the WFC3/UVIS instrument onboard the Hubble Space
\nTelescope to search for multiple populations within Magellanic Cloud star clusters at various
\nages. In this paper, we introduce this survey. As first results of this programme, we also present
\nmultiband photometric observations of NGC 121 in different filters taken with the WFC3/UVIS
\nand ACS/WFC instruments. We analyse the colour–magnitude diagram (CMD) of NGC 121,
\nwhich is the only ‘classical’ globular cluster within the Small Magellanic Cloud. Thereby,
\nwe use the pseudo-colour CF336W,F438W,F343N = (F336W − F438W) − (F438W − F343N)
\nto separate populations with different C and N abundances. We show that the red giant
\nbranch splits up in two distinct populations when using this colour combination. NGC 121
\nthus appears to be similar to Galactic globular clusters in hosting multiple populations. The
\nfraction of enriched stars (N rich, C poor) in NGC 121 is about 32 per cent ± 3 per cent,
\nwhich is lower than the median fraction found in Milky Way globular clusters. The enriched
\npopulation seems to be more centrally concentrated compared to the primordial one. These
\nresults are consistent with the recent results by Dalessandro et al. The morphology of the
\nhorizontal branch in a CMD using the optical filters F555W and F814W is best produced by a
\npopulation with a spread in helium of Y = 0.025±0.005.

Citations

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