Gaia FGK Benchmark stars: Opening the black box of stellar element
  abundance determination
  Gaia and its complementary spectroscopic surveys combined will yield the most
comprehensive database of kinematic and chemical information of stars in the
Milky Way. The Gaia FGK benchmark stars play a central role in this matter as
they are calibration pillars for the atmospheric parameters and chemical
abundances for various surveys. The spectroscopic analyses of the benchmark
stars are done by combining different methods, and the results will be affected
by the systematic uncertainties inherent in each method. In this paper we
explore some of these systematic uncertainties. We determined line abundances
of Ca, Cr, Mn and Co for four benchmark stars using six different methods. We
changed the default input parameters of the different codes in a systematic way
and found in some cases significant differences between the results. Since
there is no consensus on the correct values for many of these default
parameters, we urge the community to raise discussions towards standard input
parameters that could alleviate the difference in abundances obtained by
different methods. In this work we provide quantitative estimates of
uncertainties in elemental abundances due to the effect of differing technical
assumptions in spectrum modelling.
