NGC 5694: another foster son of the Galactic Halo

TL;DR

This study analyzes high-resolution spectra of six stars in the globular cluster NGC 5694, revealing its extragalactic origin and unique chemical composition, with implications for understanding its formation and evolution.

Contribution

It provides the first detailed chemical abundance analysis of NGC 5694, confirming its extragalactic origin and revealing unusual abundance patterns compared to typical halo stars.

Findings

NGC 5694 has nearly solar alpha-element abundances, unlike typical halo stars.

Y, Ba, La, Eu are significantly lower than in similar metallicity field stars.

Evidence of early self-enrichment through Na-O and Mg-Al anti-correlations.

Abstract

We present the results of the analysis of high-resolution spectra obtained with UVES-FLAMES@VLT for six red giant branch stars in the outer-halo metal-poor ([Fe/H]I=-1.98 and [Fe/H]II=-1.83) Galactic globular cluster NGC 5694, which has been suggested as a possible incomer by Lee et al. (2006) based on the anomalous chemical composition of a single cluster giant. We obtain accurate abundances for a large number of elements and we find that: (a) the six target stars have the same chemical composition within the uncertainties, except for Na and Al; (b) the average cluster abundance of \alpha\ elements (with the only exception of Si) is nearly solar, at odds with typical halo stars and globular clusters of similar metallicity; (c) Y, Ba, La and Eu abundances are also significantly lower than in Galactic field stars and star clusters of similar metallicity. Hence we confirm the Lee et al. classification of NGC 5694 as a cluster of extra-galactic origin. We provide the first insight on the Na-O and Mg-Al anti-correlations in this cluster: all the considered stars have very similar abundance ratios for these elements, except one that has significantly lower [Na/Fe] and [Al/Fe] ratios, suggesting that some degree of early self-enrichment has occurred also in this cluster.