The chemical composition of a regular halo globular cluster: NGC 5897

A Koch, A McWilliam�- Astronomy & Astrophysics, 2014 - aanda.org
A Koch, A McWilliam
Astronomy & Astrophysics, 2014aanda.org
We report for the first time on the chemical composition of the halo cluster NGC 5897 (R⊙=
12.5 kpc), based on chemical abundance ratios for 27 α-, iron-peak, and neutron-capture
elements in seven red giants. From our high-resolution, high signal-to-noise spectra
obtained with the Magellan/MIKE spectrograph, we find a mean iron abundance from the
neutral species of [Fe/H]=− 2.04�0.01 (stat.)�0.15 (sys.), which is more metal-poor than
implied by previous photometric and low-resolution spectroscopic studies. The cluster NGC�…
We report for the first time on the chemical composition of the halo cluster NGC 5897 (R = 12.5 kpc), based on chemical abundance ratios for 27 α-, iron-peak, and neutron-capture elements in seven red giants. From our high-resolution, high signal-to-noise spectra obtained with the Magellan/MIKE spectrograph, we find a mean iron abundance from the neutral species of [Fe/H] = − 2.04 � 0.01 (stat.) � 0.15 (sys.), which is more metal-poor than implied by previous photometric and low-resolution spectroscopic studies. The cluster NGC 5897 is α-enhanced (to 0.34 � 0.01 dex) and shows Fe-peak element ratios typical of other (metal-poor) halo globular clusters (GCs) with no overall, significant abundance spreads in iron or in any other heavy element. Like other GCs, NGC 5897 shows a clear Na–O anti-correlation, where we find a prominent primordial population of stars with enhanced O abundances and approximately solar Na/Fe ratios, while two stars are Na-rich, providing chemical proof of the presence of multiple populations in this cluster. Comparison of the heavy element abundances with the solar-scaled values and the metal-poor GC M15 from the literature confirms that NGC 5897 has experienced little contribution from s-process nucleosynthesis. One star of the first generation stands out in that it shows very low La and Eu abundances. Overall, NGC 5897 is a well behaved GC showing archetypical correlations and element-patterns, with little room for surprises in our data. We suggest that its lower metallicity could explain the unusually long periods of RR Lyr that were found in NGC 5897.
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