![]() An obvious drawback of this approach is the lack of robustness of the results and severe reproducibility issues. In these cases, the classification of a star into a given group, and the number of identified populations itself, is based on subjective judgement and not on statistical methods. The MP identification is often based on judgments made by naked eye, guided by of apparent splits in the colour-magnitude diagram (CMD) or peaks in the element abundances. A widely used signature to identify MPs inside a GC is the distribution of key abundance indexes such as or CN-CH (e.g., Carretta et al. A typical feature of these MPs is an abundance variation in light elements with little or no dispersion in heavy elements, with significant element abundance anti-correlations. 2020), a clear understanding of the MP formation is still lacking (see e.g., Bastian & Lardo 2018 and references therein). Despite important theoretical efforts, such as the recent suggestion of the role of stellar mergers in the MP origin ( Wang et al. The presence of multiple stellar populations (MPs) is considered today a distinctive feature of almost all globular clusters (GCs). Key words: methods: statistical / stars: evolution / stars: abundances / globular clusters: general / globular clusters: individual: NGC 2808 ![]() The adoption of statistically grounded methods, which adopt all the available information to split the data into subsets and explicitly address the problem of data uncertainty, is of paramount importance to present more robust and reproducible research. By means of Monte Carlo simulations and multimodality statistical tests, we show that the often adopted study of the histogram of the differences in some key elements is prone to multiple false-positive findings.Ĭonclusions. The discrepancy with literature claims can be explained with the different methods that were adopted to characterise MPs. ![]() The silhouette analysis suggests that the population substructure is reliable for high-resolution spectroscopy data, while the actual existence of MP is questionable for the low-resolution spectroscopy data. Two MPs are detected for both data sets, while the literature reports five and four MPs from high- and low-resolution spectroscopy, respectively. For both data sets the results of the statistical pipeline are at odds with those reported in the literature. The results of the clustering algorithms were subjected to a silhouette width analysis to compare the performance of the split into different numbers of MPs. We explicitly addressed the usually neglected problem of measurement errors, for which we relied on techniques that were recently introduced in the statistical literature. We adopted hierarchical clustering and partition methods. We focus our analysis on the red giant branch phase, where two data sets are available in the recent literature for low- and high-resolution spectroscopy. Using NGC 2808 as test case, we show the use of well-established statistical clustering methods. ![]() However, while data availability and quality have improved in the past decades, this is not always true for the techniques that are adopted to analyse them, which creates problems of objectivity for the claims and reproducibility.Īims. The finding of multiple stellar populations (MPs), which are defined by patterns in the stellar element abundances, is considered today a distinctive feature of globular clusters. Tognelli 2ĭipartimento di Fisica “Enrico Fermi”, Università di Pisa, Largo Pontecorvo 3, 56127 Pisa, ItalyĮ-mail: Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21 Praha 8, Czech RepublicĬontext.
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