Exploring the Chemical Composition and Double Horizontal Branch of the Bulge Globular Cluster NGC 6569

TL;DR

This study investigates the chemical composition and double horizontal branch of the globular cluster NGC 6569, revealing multiple populations and potential helium variation as the cause of the HB split.

Contribution

It provides detailed chemical abundances and radial velocities for NGC 6569, and explores the possible helium variation responsible for its double horizontal branch.

Findings

NGC 6569 has a mean [Fe/H] of -0.87 dex with little metallicity spread.

The cluster shows light- and heavy-element abundance patterns typical of old bulge clusters.

Helium variation, rather than metallicity, may explain the double horizontal branch.

Abstract

Photometric and spectroscopic analyses have shown that the Galactic bulge cluster Terzan 5 hosts several populations with different metallicities and ages that manifest as a double red horizontal branch (HB). A recent investigation of the massive bulge cluster NGC 6569 revealed a similar, though less extended, HB luminosity split, but little is known about the cluster's detailed chemical composition. Therefore, we have used high-resolution spectra from the Magellan-M2FS and VLT-FLAMES spectrographs to investigate the chemical compositions and radial velocity distributions of red giant branch and HB stars in NGC 6569. We found the cluster to have a mean heliocentric radial velocity of -48.8 km/s (sigma = 5.3 km/s; 148 stars) and a mean [Fe/H] =-0.87 dex (19 stars), but the cluster's 0.05 dex [Fe/H] dispersion precludes a significant metallicity spread. NGC 6569 exhibits light- and heavy-element distributions that are common among old bulge/inner Galaxy globular clusters, including clear (anti)correlations between [O/Fe], [Na/Fe], and [Al/Fe]. The light-element data suggest that NGC 6569 may be composed of at least two distinct populations, and the cluster's low mean [La/Eu] = -0.11 dex indicates significant pollution with r-process material. We confirm that both HBs contain cluster members, but metallicity and light-element variations are largely ruled out as sources for the luminosity difference. However, He mass fraction differences as small as delta Y ~ 0.02 cannot be ruled out and may be sufficient to reproduce the double HB.