Insights into the chemical composition of the metal-poor Milky Way halo globular cluster NGC 6426^⋆,⋆⋆

¹ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
² Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
e-mail: mhanke@ari.uni-heidelberg.de
³ Department of Physics, Lancaster University, Bailrigg, Lancaster LA1 4YB, UK
⁴ Dark Cosmology Centre, The Niels Bohr Institute, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁵ Carnegie Observatories, 813 Santa Barbara St., Pasadena, CA 91101, USA

Received: 5 September 2016
Accepted: 6 December 2016

Abstract

We present our detailed spectroscopic analysis of the chemical composition of four red giant stars in the halo globular cluster NGC 6426. We obtained high-resolution spectra using the Magellan2/MIKE spectrograph, from which we derived equivalent widths and subsequently computed abundances of 24 species of 22 chemical elements. For the purpose of measuring equivalent widths, we developed a new semi-automated tool, called EWCODE. We report a mean Fe content of [Fe/H] =−2.34 ± 0.05 dex (stat.) in accordance with previous studies. At a mean α-abundance of [(Mg, Si, Ca)/3 Fe] = 0.39 ± 0.03 dex, NGC 6426 falls on the trend drawn by the Milky Way halo and other globular clusters at comparably low metallicities. The distribution of the lighter α-elements as well as the enhanced ratio [Zn/Fe] = 0.39 dex could originate from hypernova enrichment of the pre-cluster medium. We find tentative evidence for a spread in the elements Mg, Si, and Zn, indicating an enrichment scenario, where ejecta of evolved massive stars of a slightly older population have polluted a newly born younger one. The heavy element abundances in this cluster fit well into the picture of metal-poor globular clusters, which in that respect appear to be remarkably homogeneous. The pattern of the neutron-capture elements heavier than Zn points toward an enrichment history governed by the r-process with little, if any, sign of s-process contributions. This finding is supported by the striking similarity of our program stars to the metal-poor field star HD 108317.