NGC 6139: a normal massive globular cluster, or a first-generation dominated cluster? Clues from the light elements ^{⋆,⋆⋆,⋆⋆⋆}

¹ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: angela.bragaglia@oabo.inaf.it
² Dipartimento di Fisica e Astronomia, Università di Bologna, viale Berti Pichat 6, 40127 Bologna, Italy
³ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Monash Centre for Astrophysics, School of Physics and Astronomy, Monash University, Melbourne, VIC 3800, Australia
⁵ Department of Physics and Astronomy, Macquarie University, Sydney, NSW 2109, Australia
⁶ Department of Astronomy and McDonald Observatory, The University of Texas, Austin, TX 78712, USA

Received: 25 May 2015
Accepted: 30 July 2015

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 investigate the anti-correlations 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, has 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) anti correlation. NGC 6139 behaves as expected from its mass and contains a large portion (about two thirds) of second-generation stars.