Unveiling systematic biases in 1D LTE excitation-ionisation balance of
  Fe for FGK stars. A novel approach to determination of stellar parameters
  We present a comprehensive analysis of different techniques available for the
spectroscopic analysis of FGK stars, and provide a recommended methodology
which efficiently estimates accurate stellar atmospheric parameters for large
samples of stars. Our analysis includes a simultaneous equivalent width
analysis of Fe I and Fe II spectral lines, and for the first time, utilises
on-the-fly NLTE corrections of individual Fe I lines. We further investigate
several temperature scales, finding that estimates from Balmer line
measurements provide the most accurate effective temperatures at all
metallicites. We apply our analysis to a large sample of both dwarf and giant
stars selected from the RAVE survey. We then show that the difference between
parameters determined by our method and that by standard 1D LTE
excitation-ionisation balance of Fe reveals substantial systematic biases: up
to 400 K in effective temperature, 1.0 dex in surface gravity, and 0.4 dex in
metallicity for stars with [Fe/H] ~ -2.5. This has large implications for the
study of the stellar populations in the Milky Way.
