Chemical Complexity in the Eu-enhanced Monometallic Globular NGC 5986

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

Research output: Contribution to journalArticle

19 Scopus citations

Abstract

NGC 5986 is a poorly studied but relatively massive Galactic globular cluster that shares several physical and morphological characteristics with "iron-complex" clusters known to exhibit significant metallicity and heavy-element dispersions. In order to determine whether NGC 5986 joins the iron-complex cluster class, we investigated the chemical composition of 25 red giant branch and asymptotic giant branch cluster stars using high-resolution spectra obtained with the Magellan-M2FS instrument. Cluster membership was verified using a combination of radial velocity and [Fe/H] measurements, and we found the cluster to have a mean heliocentric radial velocity of +99.76 km s-1 (σ = 7.44 km s-1). We derived a mean metallicity of [Fe/H] = -1.54 dex (σ = 0.08 dex), but the cluster's small dispersion in [Fe/H] and low [La/Eu] abundance preclude it from being an iron-complex cluster. NGC 5986 has <[Eu/Fe]> = +0.76 dex (σ = 0.08 dex), which is among the highest ratios detected in a Galactic cluster, but the small [Eu/Fe] dispersion is puzzling because such high values near [Fe/H] ∼ -1.5 are typically only found in dwarf galaxies exhibiting large [Eu/Fe] variations. NGC 5986 exhibits classical globular cluster characteristics, such as uniformly enhanced [α/Fe] ratios, a small dispersion in Fe-peak abundances, and (anti)correlated light-element variations. Similar to NGC 2808, we find evidence that NGC 5986 may host at least four to five populations with distinct light-element compositions, and the presence of a clear Mg-Al anticorrelation along with an Al-Si correlation suggests that the cluster gas experienced processing at temperatures 65-70 MK. However, the current data do not support burning temperatures exceeding ∼100 MK. We find some evidence that the first- and second-generation stars in NGC 5986 may be fully spatially mixed, which could indicate that the cluster has lost a significant fraction of its original mass.

Original languageEnglish (US)
Article number24
JournalAstrophysical Journal
Volume842
Issue number1
DOIs
StatePublished - Jun 10 2017

Keywords

  • globular clusters: general
  • globular clusters: individual (NGC 5986)
  • stars: abundances

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Johnson, C. I., Caldwell, N., Rich, R. M., Mateo, M., Bailey, J. I., Olszewski, E. W., & Walker, M. G. (2017). Chemical Complexity in the Eu-enhanced Monometallic Globular NGC 5986. Astrophysical Journal, 842(1), [24]. https://doi.org/10.3847/1538-4357/aa7414