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 \langle [ Eu/Fe ] \rangle = + 0.76 dex ( \sigma = 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 ] \sim –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 [ \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 \gtrsim 65–70 MK .
However , the current data do not support burning temperatures exceeding \sim 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 .