NGC 6397: The metallicity trend along the isochrone revisited
  Recent work has used spectra of about 5000 stars in NGC 6397 that were
extracted from a MUSE mosaic to determine the atmospheric parameters for these
stars by fitting the spectra against the Goettingen Spectral Library. A
significant change in metallicity between the turn off and the red giant branch
was found and was discussed as a possible manifestation of predicted effects of
atomic diffusion. However, the small amplitude of the effect and inconsistency
with earlier measurements call for more attention before this result is
interpreted. Systematic effects due to the interpolation or to the synthetic
spectra cannot be ruled out at this level of precision. We reanalyze the data
with the ELODIE and MILES reference libraries in order to assess the robustness
of the result. These empirical libraries have a finer metallicity coverage down
to approximately the cluster metalicity turn-off. We used the ULySS
full-spectrum fitting package, together with the library interpolators to
remeasure the three atmospheric parameters effective temperature, surface
gravity, and [Fe/H] metallicity. We find a very low [Fe/H] dispersion along the
isochrone (0.07 dex), consistent with our error estimate (0.05 dex). However,
the [Fe/H] trend is not reproducible. This shows that the data have the
potential to reveal patterns of the magnitude of the expected physical effects,
but the analysis methods need to be refined to cull systematic effects that
currently dominate the patterns.
