Chemical Complexity in the Eu-enhanced Monometallic Globular NGC 5986
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-comp...
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Veröffentlicht in: | The Astrophysical journal 2017-06, Vol.842 (1), p.24 |
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Sprache: | eng |
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Zusammenfassung: | 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. |
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ISSN: | 0004-637X 1538-4357 |
DOI: | 10.3847/1538-4357/aa7414 |