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-1 (σ = 5.3 km s-1; 148 stars) and 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 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 ΔY ∼ 0.02 cannot be ruled out and may be sufficient to reproduce the double HB.
- globular clusters: general
- globular clusters: individual (NGC 6569)
- stars: abundances
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science
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Johnson, C. I. (Creator), Rich, R. M. (Creator), Caldwell, N. (Creator), Mateo, M. (Creator), Bailey, J. I. (Creator), Olszewski, E. W. (Creator), Walker, M. G. (Creator), Rich, R. M. (Creator) & Walker, M. G. (Creator), Centre de Donnees Strasbourg (CDS), 2018