Determinations of high-precision effective temperatures for giants based on spectroscopic criteria

Spectral lines with high and low excitation potentials respond differently to changes of the effective temperature (Teff), making the ratio of their depths (or equivalent widths) a very sensitive temperature indicator. We derive a set of 100 equations relating Teff to the line-depth ratios, calibrated against accurate (to within 1%) published temperature determinations for giants. These relations are used to determine very accurate temperatures for a sample of 110 giants with nearly solar metallicities based on high-resolution (R = 42 000) echelle spectra with high signal-to-noise ratios (SNRs). The calibration relations are valid for temperatures of 4000–7000 K (F2III–K4III). The internal errors of each of the calibration relations are below 95 K, and applying all these relations together to spectra with SNR = 100 reduces the errors to 5–25 K (1 σ). A major advantage of this technique is that it is independent from interstellar reddening, spectroscopic resolution, and line broadening due to rotation and microturbulence.