Classification of stellar populations in globular clusters

Yue Wang1,2, Gang Zhao1,2 and Hai-Ning Li1   2017-05-29 00:20:14

1 Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China; gzhao@nao.cas.cn

2 School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 100049, China

Received 2016 November 29; accepted 2017 February 14

Abstract Possessing multiple stellar populations has been accepted as a common feature of globular clusters (GCs). Different stellar populations manifest themselves with different chemical features, e.g. the well-known O−Na anti-correlation. Generally, thefirst (primordial) population has O and Na abundances consistent with those offield stars with similar metallicity; while the second (polluted) population is identified by their Na overabundance and O deficiency. The fraction of the populations is an important constraint on the GC formation scenario. Several methods have been proposed for the classi-fication of GC populations. Here we examine a criterion derived based on the distribution of Galacticfield stars, which relies on Na abundance as a function of [Fe/H], to distinguishfirst and second stellar populations in GCs. By comparing thefirst population fractions of 17 GCs estimated by thefield star criterion with those in the literature derived by methods related to individual GCs, wefind that thefield star criterion tends to overestimate the first population fractions. The population separation methods, which are related to an individual GC sample, are recommended because the diversity of GCs can be taken into consideration. Currently, more caution should be exercised if one wants to regardfield stars as a reference for the identification of a GC population. However, further study on the connection betweenfield stars and GCs populations is still needed.

  Key words: stars: abundances—globular clusters: general

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