Insights into the chemical composition of the metal-poor Milky Way halo
  globular cluster NGC 6426
  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$\pm$0.05 dex (stat.) in accordance with previous studies. At
a mean $\alpha$-abundance of [(Mg,Si,Ca)/3 Fe] = 0.39$\pm$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 $\alpha$-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 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 point towards an enrichment history governed by the r-process with only
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.
