NGC 6139: a normal massive globular cluster or a first-generation dominated cluster? Clues from the light elements

TL;DR

This study investigates the chemical signatures of multiple stellar populations in NGC 6139, revealing a typical globular cluster pattern with a significant second-generation star fraction, challenging previous assumptions about its formation.

Contribution

First high-resolution spectroscopic analysis of NGC 6139, demonstrating its chemical properties and second-generation star dominance consistent with its mass.

Findings

NGC 6139 has a metallicity of [Fe/H] ≈ -1.58.

The cluster exhibits a typical Na-O and Mg-Al anticorrelation.

Approximately two-thirds of stars are second-generation.

Abstract

Information on globular clusters (GC) formation mechanisms can be gathered by studying the chemical signature of the multiple populations that compose these stellar systems. In particular, we are investigating the anticorrelations among O, Na, Al, and Mg to explore the influence of cluster mass and environment on GCs in the Milky Way and in extragalactic systems. We present here the results obtained on NGC 6139 which, on the basis of its horizontal branch morphology, had been proposed to be dominated by first-generation stars. In our extensive study based on high resolution spectroscopy, the first for this cluster, we found a metallicity of [Fe/H]= -1.579 +/- 0.015 +/- 0.058 (rms=0.040 dex, 45 bona fide member stars) on the UVES scale defined by our group. The stars in NGC 6139 show a chemical pattern normal for GCs, with a rather extended Na-O (and Mg-Al) anticorrelation. NGC 6139 behaves like expected from its mass and contains a large fraction (about two thirds) of second-generation stars.