High-precision effective temperatures of 215 FGK giants from line-depth ratios

We present precise effective temperatures (T-eff) of 215 FGK giants determined using the method of line-depth ratios. For each star, we have measured the line depths and equivalent widths of a large number of spectral lines of low and high excitation potentials and established similar to 100 relations between T-eff and their ratios. Starting with an initial value T-eff, the relations are then self-calibrated by an iterative process. Our final estimates have been compared with very accurate (1 per cent) published temperatures and show a good agreement. Using our calibrations, we derive precise temperatures for 215 giants with near-solar metallicity, from high-resolution (R = 42 000) and high signal-to-noise ratio (S/N = 100-250) echelle spectra, obtained with the ELODIE spectrometer at the Observatoire de Haute Provence (OHP). The range of application of the method is 3500-5700 K (G0 III-K4 III). The internal error for a single calibration is less than 95 K, while the combination of all 100 calibrations reduces the uncertainty to only 5-20 K (1 sigma). A big advantage of the line ratio method is its independence of interstellar reddening, and its modest sensitivity to spectral resolution, abundance, macroturbulence and other factors.