A differential chemical element analysis of the metal poor Globular
  Cluster NGC 6397
  We present chemical abundances in three red giants and two turn-off stars in
the metal poor Galactic globular cluster (GC) NGC 6397 based on spectroscopy
obtained with the MIKE high resolution spectrograph on the Magellan 6.5-m Clay
telescope. Our results are based on a line-by-line differential abundance
analysis relative to the well-studied red giant Arcturus and the Galactic halo
field star Hip 66815. At a mean of -2.10 +/- 0.02 (stat.) +/- 0.07 (sys.) the
differential iron abundance is in good agreement with other studies in the
literature based on gf-values. As in previous, differential works we find a
distinct departure from ionization equilibrium in that the abundances of Fe I
and Fe II differ by ~0.1 dex, with opposite sign for the RGB and TO stars. The
alpha-element ratios are enhanced to 0.4 (RGB) and 0.3 dex (TO), respectively,
and we also confirm strong variations in the O, Na, and Al/Fe abundance ratios.
Accordingly, the light-element abundance patterns in one of the red giants can
be attributed to pollution by an early generation of massive SNe II. TO and RGB
abundances are not significantly different, with the possible exception of Mg
and Ti, which is, however, amplified by the patterns in one TO star,
additionally belonging to this early generation of GC stars. We discuss
interrelations of these light elements as a function of the GC metallicity.
