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

Christian I. Johnson, R. Michael Rich, Nelson Caldwell, Mario Mateo, John I. Bailey, Edward W Olszewski, Matthew G. Walker

Research output: Contribution to journalArticle

7 Citations (Scopus)

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-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.

Original languageEnglish (US)
Article number71
JournalAstronomical Journal
Volume155
Issue number2
DOIs
StatePublished - Feb 1 2018

Fingerprint

globular clusters
chemical composition
light elements
metallicity
radial velocity
pollution
luminosity
horizontal branch stars
stars
galactic bulge
heavy elements
spectrographs
velocity distribution
galaxies
distribution
high resolution

Keywords

  • globular clusters: general
  • globular clusters: individual (NGC 6569)
  • stars: abundances

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Exploring the Chemical Composition and Double Horizontal Branch of the Bulge Globular Cluster NGC 6569. / Johnson, Christian I.; Rich, R. Michael; Caldwell, Nelson; Mateo, Mario; Bailey, John I.; Olszewski, Edward W; Walker, Matthew G.

In: Astronomical Journal, Vol. 155, No. 2, 71, 01.02.2018.

Research output: Contribution to journalArticle

Johnson, Christian I. ; Rich, R. Michael ; Caldwell, Nelson ; Mateo, Mario ; Bailey, John I. ; Olszewski, Edward W ; Walker, Matthew G. / Exploring the Chemical Composition and Double Horizontal Branch of the Bulge Globular Cluster NGC 6569. In: Astronomical Journal. 2018 ; Vol. 155, No. 2.
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