Deriving Stellar Effective Temperatures of Metal-Poor Stars with the
  Excitation Potential Method
  It is well established that stellar effective temperatures determined from
photometry and spectroscopy yield systematically different results. We describe
a new, simple method to correct spectroscopically derived temperatures
("excitation temperatures") of metal-poor stars based on a literature sample
with -3.3<[Fe/H]<-2.5. Excitation temperatures were determined from FeI line
abundances in high-resolution optical spectra in the wavelength range of ~3700
to ~7000A, although shorter wavelength ranges, up to 4750 to 6800A, can also be
employed, and compared with photometric literature temperatures. Our adjustment
scheme increases the temperatures up to several hundred degrees for cool red
giants, while leaving the near-main-sequence stars mostly unchanged. Hence, it
brings the excitation temperatures in good agreement with photometrically
derived values. The modified temperature also influences other stellar
parameters, as the FeI-FeII ionization balance is simultaneously used to
determine the surface gravity, while also forcing no abundance trend on the
absorption line strengths to obtain the microturbulent velocity. As a result of
increasing the temperature, the often too low gravities and too high
microturbulent velocities in red giants become higher and lower, respectively.
Our adjustment scheme thus continues to build on the advantage of deriving
temperatures from spectroscopy alone, independent of reddening, while at the
same time producing stellar chemical abundances that are more straightforwardly
comparable to studies based on photometrically derived temperatures. Hence, our
method may prove beneficial for comparing different studies in the literature
as well as the many high-resolution stellar spectroscopic surveys that are or
will be carried out in the next few years.
