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We test our capability of deriving stellar physical parameters of giant starsby analysing a sample of field stars and the well studied open cluster IC 4651with different spectroscopic methods. The use of a technique based online-depth ratios (LDRs) allows us to determine with high precision theeffective temperature of the stars and to compare the results with thoseobtained with a classical LTE abundance analysis. (i) For the field stars wefind that the temperatures derived by means of the LDR method are in excellentagreement with those found by the spectral synthesis. This result is extremelyencouraging because it shows that spectra can be used to firmly derivepopulation characteristics (e.g., mass and age) of the observed stars. (ii) Forthe IC 4651 stars we use the determined effective temperature to derive thefollowing results. a) The reddening E(B-V) of the cluster is 0.12+/-0.02,largely independent of the color-temperature calibration used. b) The age ofthe cluster is 1.2+/-0.2 Gyr. c) The typical mass of the analysed giant starsis 2.0+/-0.2M_sun. Moreover, we find a systematic difference of about 0.2 dexin log g between spectroscopic and evolutionary values. We conclude that, inspite of known limitations, a classical spectroscopic analysis of giant starsmay indeed result in very reliable stellar parameters. We caution that thequality of the agreement, on the other hand, depends on the details of theadopted spectroscopic analysis.

Deriving temperature, mass and age of evolved stars from high-resolution spectra. Application to field stars and the open cluster IC 4651