Probing 3D and NLTE models using APOGEE observations of globular cluster
  stars
  Hydrodynamical (or 3D) and non-local thermodynamic equilibrium (NLTE) effects
are known to affect abundance analyses. However, there are very few
observational abundance testsof 3D and NLTE models. We developed a new way of
testing the abundance predictions of 3D and NLTE models, taking advantage of
large spectroscopic survey data. We use a line-by-line analysis of the Apache
Point Observatory Galactic Evolution Experiment (APOGEE) spectra (H band) with
the Brussels Automatic Code for Characterizing High accUracy Spectra (BACCHUS).
We compute line-by-line abundances of Mg, Si, Ca, and Fe for a large number of
globular cluster K giants in the APOGEE survey. We compare this line-by-line
analysis against NLTE and 3D predictions. While the 1D-NLTE models provide
corrections in the right direction, there are quantitative discrepancies
between different models. We observe a better agreement with the data for the
models including reliable collisional cross-sections. The agreement between
data and models is not always satisfactory when the 3D spectra are computed in
LTE. However, we note that for a fair comparison, 3D corrections should be
computed with self-consistently derived stellar parameters, and not on 1D
models with identical stellar parameters. Finally, we focus on 3D and NLTE
effects on Fe lines in the H band, where we observe a systematic difference in
abundance relative to the value from the optical. Our results suggest that the
metallicities obtained from the H band are more accurate in metal-poor giants.
More atomic data and extended self-consistent 3D-NLTE computations need to be
made. The method we have developed for testing 3D and NLTE models could be
extended to other lines and elements, and is particularly suited for large
spectroscopic surveys.
