Atmospheric parameters of red giants in the Kepler field

Context. Accurate fundamental parameters of stars are mandatory for the asteroseismic investigation of the Kepler mission to succeed. Aims. We determine the atmospheric parameters for a sample of six well-studied bright K giants to confirm that our method produces reliable results. We then apply the same method to 14 K giants that are targets of the Kepler mission. Methods. We used high-resolution, high signal-to-noise ratio spectra acquired using the FIES spectrograph on the Nordic Optical Telescope. We applied the iterative spectral synthesis method VWA to derive the fundamental parameters from carefully selected high-quality iron lines and pressure-sensitive Calcium lines. Results. We find good agreement with parameters from the literature for the six bright giants. We compared the spectroscopic values with parameters based on photometric indices in the Kepler Input Catalogue (KIC). We identify serious problems with the KIC values for [Fe/H] and find a large RMS scatter of 0.5 dex. The log g values in KIC agree reasonably well with the spectroscopic values displaying a scatter of 0.25 dex after excluding two low-metallicity giants. The T-eff values from VWA and KIC agree well with a scatter of about 85K. We also find good agreement with log g and T-eff derived from asteroseismic analyses for seven Kepler giant targets. Conclusions. We determine accurate fundamental parameters of 14 giants using spectroscopic data. The large discrepancies between photometric and spectroscopic values of [Fe/H] emphasize the need for further detailed spectroscopic follow-up of the Kepler targets. This will be mandatory to be able to produce reliable constraints for detailed asteroseismic analyses and interpretation of possible exo-planet candidates found around giant stars.