Facing the problems on the determination of stellar temperatures and
  gravities: Galactic globular clusters
  We analysed red giant branch stars in 16 Galactic globular clusters,
computing their atmospheric parameters both from the photometry and from
excitation and ionisation balances. The spectroscopic parameters are lower than
the photometric ones and this discrepancy increases decreasing the metallicity,
reaching, at [Fe/H]~-2.5 dex, differences of ~350 K in effective temperature
and ~1 dex in surface gravity. We demonstrate that the spectroscopic parameters
are inconsistent with the position of the stars in the colour-magnitude
diagram, providing too low temperatures and gravities, and predicting that the
stars are up to about 2.5 magnitudes brighter than the observed magnitudes. The
parameter discrepancy is likely due to the inadequacies of the adopted physics,
in particular the assumption of 1-dimensional geometry can be the origin of the
observed slope between iron abundances and excitation potential that leads to
low temperatures. However, the current modelling of 3D/NLTE radiative transfer
for giant stars seems to be not able to totally erase this slope. We conclude
that the spectroscopic parameters are wrong for metallicity lower than -1.5 dex
and for these red giant stars photometric temperatures and gravities should be
adopted. We provide a simple relation to correct the spectroscopic temperatures
in order to put them onto a photometric scale.
