The effective temperature scale of giant stars (F0-K5) I. The effective temperature determination by means of the IRFM

We have applied the InfraRed Flux Method (IRFM) to a sample of approximatel y 500 giant stars in order to derive their effective temperatures with an i nternal mean accuracy of about 1.5% and a maximum uncertainty in the zero p oint of the order of 0.9%. For the application of the IRFM, we have used a homogeneous grid of theoretical model atmosphere flux distributions develop ed by Kurucz (1993). The atmospheric parameters of the stars roughly cover the ranges: 3500 K less than or equal to T-eff less than or equal to 8000 K ; -3.0 less than or equal to [Fe/H] less than or equal to +0.5; 0.5 less th an or equal to log(g) less than or equal to 3.5. The monochromatic infrared fluxes at the continuum are based on recent photometry with errors that sa tisfy the accuracy requirements of the work. We have derived the bolometric correction of giant stars by using a new calibration which takes the effec t of metallicity into account. Direct spectroscopic determinations of metal licity have been adopted where available: although estimates based on photo metric calibrations have been considered for some stars lacking spectroscop ic ones. The adopted infrared absolute flux calibration, based on direct op tical measurements of stellar angular diameters, puts the effective tempera tures determined in this work in the same scale as those obtained by direct methods. We have derived up to four temperatures, T-J, T-H, T-K and T-L', for each s tar using the monochromatic fluxes at different infrared wavelengths in the photometric bands J, H, K and L'. They show good consistency over 4000 K, and there is no appreciable trend with wavelength, metallicity and/or tempe rature. We provide a detailed description of the steps followed for the application of the IRFM, as well as the sources of error and their effect on final tem peratures. We also provide a comparison of the results with previous work.