Detailed chemical abundance analysis of the thick disk star cluster Gaia 1

TL;DR

This study provides a detailed chemical analysis of Gaia 1, confirming its association with the Milky Way's thick disk and clarifying its metallicity, elemental abundances, and cluster classification.

Contribution

It offers the first high-resolution spectroscopic analysis of Gaia 1, establishing its chemical properties and confirming its status as a thick disk open cluster.

Findings

Gaia 1 has a mean Fe abundance of -0.62 dex.

Gaia 1 shows alpha-element enhancement typical of thick disk stars.

No Na-O or Mg-Al anti-correlations were observed, indicating it is not a globular cluster.

Abstract

Star clusters, particularly those objects in the disk-bulge-halo interface are as of yet poorly charted, albeit carrying important information about the formation and the structure of the Milky Way. Here, we present a detailed chemical abundance study of the recently discovered object Gaia 1. Photometry has previously suggested it as an intermediate-age, moderately metal-rich system, although the exact values for its age and metallicity remained ambiguous in the literature. We measured detailed chemical abundances of 14 elements in four red giant members, from high-resolution (R=25000) spectra that firmly establish Gaia 1 as an object associated with the thick disk. The resulting mean Fe abundance is $-0.62\pm$0.03(stat.)$\pm$0.10(sys.) dex, which is more metal-poor than indicated by previous spectroscopy from the literature, but it is fully in line with values from isochrone fitting. We find that Gaia 1 is moderately enhanced in the $\alpha$-elements, which allowed us to consolidate its membership with the thick disk via chemical tagging. The cluster's Fe-peak and neutron-capture elements are similar to those found across the metal-rich disks, where the latter indicate some level of $s$-process activity. No significant spread in iron nor in other heavy elements was detected, whereas we find evidence of light-element variations in Na, Mg, and Al. Nonetheless, the traditional Na-O and Mg-Al (anti-)correlations, typically seen in old globular clusters, are not seen in our data. This confirms that Gaia 1 is rather a massive and luminous open cluster than a low-mass globular cluster. Finally, orbital computations of the target stars bolster our chemical findings of Gaia 1's present-day membership with the thick disk, even though it remains unclear, which mechanisms put it in that place.