Globular cluster chemistry in fast rotating dwarf stars belonging to intermediate age open clusters

TL;DR

This study identifies peculiar chemical abundance patterns, including bimodality and anticorrelations, in fast rotating dwarf stars of intermediate age open clusters, hinting at a possible link to the globular cluster multiple population phenomenon.

Contribution

It reports the discovery of chemical abundance bimodality and Na-O anticorrelation in fast rotating dwarf stars within open clusters, a phenomenon previously mainly associated with globular clusters.

Findings

Bimodality in [Na/Fe] and [O/Fe] observed in fast rotating stars.

Na-O anticorrelation detected in some clusters.

Large [Mg/Fe] variation among high [Na/Fe] stars, similar to globular clusters.

Abstract

(shortened) The peculiar chemistry observed in the multiple populations of Galactic globular clusters is not generally found in other systems like dwarf galaxies and open clusters, and at the moment no model can fully explain its presence. Exploring the boundaries of the multiple population phenomenon and the variation of its extent in the space of cluster mass, age, metallicity, and compactness has proven to be a fruitful line of investigation. In the framework of a larger project to search for multiple population in open clusters, based on literature and survey data, I found peculiar chemical abundance patterns in a sample of intermediate age open clusters with publicly available data. More specifically, fast rotating dwarf stars (v sini > 50km/s) belonging to four clusters display a bimodality in either [Na/Fe], [O/Fe] or both. Additionally, two clusters show a Na-O anticorrelation in the fast rotating stars and one cluster shows a large [Mg/Fe] variation among the stars with high [Na/Fe], reaching the extreme Mg depletion observed in NGC 2808. Even considering that the sample sizes are small, these patterns call for attention in the light of a possible connection with the multiple population phenomenon of globular clusters. The specific chemistry observed in these fast rotating dwarf stars is thought to be produced by a complex interplay of different diffusion and mixing mechanisms, like rotational mixing and mass loss, in turn influenced by metallicity, binarity, mass, age, variability, and so on. However, with the sample in hand, it was not possible to identify which stellar parameters cause the observed Na and O bimodality and Na-O anticorrelation, suggesting that other stellar properties might be important besides stellar rotation.