Light and Heavy Element Abundance Variations in the Outer Halo Globular Cluster NGC 6229

TL;DR

This study provides a detailed chemical composition analysis of NGC 6229, revealing its complex stellar populations, chemical signatures, and possible origin as a remnant of a dwarf galaxy, based on high-resolution spectroscopic data.

Contribution

First detailed high-resolution spectroscopic analysis of NGC 6229 revealing its chemical complexity and potential extragalactic origin.

Findings

NGC 6229 is a moderately metal-poor, low M/L cluster with chemical signatures similar to other complex clusters.

Presence of strongly enhanced La and Nd in some stars correlates with slight Fe increase.

The cluster shows classical light element anti-correlations and may be a remnant of a dwarf galaxy.

Abstract

NGC 6229 is a relatively massive outer halo globular cluster that is primarily known for exhibiting a peculiar bimodal horizontal branch morphology. Given the paucity of spectroscopic data on this cluster, we present a detailed chemical composition analysis of 11 red giant branch members based on high resolution (R ~ 38,000), high S/N (> 100) spectra obtained with the MMT-Hectochelle instrument. We find the cluster to have a mean heliocentric radial velocity of -138.1$_{-1.0}^{+1.0}$ km s$^{\rm -1}$, a small dispersion of 3.8$_{-0.7}^{+1.0}$ km s$^{\rm -1}$, and a relatively low (M/L$_{\rm V}$)$_{\rm \odot}$ = 0.82$_{-0.28}^{+0.49}$. The cluster is moderately metal-poor with <[Fe/H]> = -1.13 dex and a modest dispersion of 0.06 dex. However, 18% (2/11) of the stars in our sample have strongly enhanced [La,Nd/Fe] ratios that are correlated with a small (~0.05 dex) increase in [Fe/H]. NGC 6229 shares several chemical signatures with M 75, NGC 1851, and the intermediate metallicity populations of omega Cen, which lead us to conclude that NGC 6229 is a lower mass iron-complex cluster. The light elements exhibit the classical (anti-)correlations that extend up to Si, but the cluster possesses a large gap in the O-Na plane that separates first and second generation stars. NGC 6229 also has unusually low [Na,Al/Fe] abundances that are consistent with an accretion origin. A comparison with M 54 and other Sagittarius clusters suggests that NGC 6229 could also be the remnant core of a former dwarf spheroidal galaxy.