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Spectral matching for abundances and clustering analysis of stars on the giant branches of ω Centauri

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E-mail: jeffrey.simpson@pg.canterbury.ac.nz

ABSTRACT

We have determined stellar parameters and abundances for 221 giant branch stars in the globular cluster ω Centauri. A combination of photometry and lower resolution spectroscopy was used to determine temperature, gravity, metallicity, [C/Fe], [N/Fe] and [Ba/Fe]. These abundances agree well with those found by previous researchers and expand the analysed sample of the cluster. k-means clustering analysis was used to group the stars into four homogeneous groups based upon these abundances.

These stars show the expected anticorrelation between [C/Fe] and [N/Fe]. We investigated the distribution of CN-weak/CN-strong stars on the colour–magnitude diagram. Asymptotic giant branch stars, which were selected from their position on the colour–magnitude diagram, were almost all CN weak. This is in contrast to the red giant branch where a large minority were CN strong. The results were also compared with cluster formation and evolution models. Overall, this study shows that statistically significant elemental and evolutionary conclusions can be obtained from lower resolution spectroscopy.

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