Chemical Complexity in the Eu-enhanced Monometallic Globular Cluster 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 if 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
(sigma = 7.44 km s^-1). We derived a mean metallicity of [Fe/H] = -1.54 dex
(sigma = 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 (sigma = 0.08 dex), which is among the highest ratios
detected in a Galactic cluster. NGC 5986 exhibits classical globular cluster
characteristics, such as uniformly enhanced [alpha/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 4-5 populations with distinct light element compositions, and the
presence of a clear Mg-Al anti-correlation 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. [abridged]
