Deriving temperature, mass, and age of evolved stars from high-resolution spectra^*

Application to field stars and the open cluster IC 4651

¹ INAF - Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy e-mail: katia.biazzo@oact.inaf.it
² ESO - European Southern Observatory, Karl-Schwarzschild-Str. 3, 85748 Garching bei München, Germany
³ INAF - Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
⁴ Observatório Nacional, R. Gal. José Cristino 77, 20921-400 São Cristóvão, Rio de Janeiro, Brazil
⁵ Max Planck-Institute für Astronomie, Heidelberg, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Thüringer Landessternwarte, Tauterburg, Sternwarte 5, 07778 Tauterburg, Germany

Received: 28 February 2007
Accepted: 3 October 2007

Abstract

Aims.We test our capability of deriving stellar physical parameters of giant stars by analysing a sample of field stars and the well studied open cluster IC 4651 with different spectroscopic methods.

Methods.The use of a technique based on line-depth ratios (LDRs) allows us to determine with high precision the effective temperature of the stars and to compare the results with those obtained with a classical LTE abundance analysis.

Results.(i) For the field stars we find that the temperatures derived by means of the LDR method are in excellent agreement with those found by the spectral synthesis. This result is extremely encouraging because it shows that spectra can be used to firmly derive population characteristics (e.g., mass and age) of the observed stars. (ii) For the IC 4651 stars we use the determined effective temperature to derive the following results. a) The reddening of the cluster is , largely independent of the color-temperature calibration used. b) The age of the cluster is  Gyr. c) The typical mass of the analysed giant stars is . Moreover, we find a systematic difference of about 0.2 dex in between spectroscopic and evolutionary values.

Conclusions.We conclude that, in spite of known limitations, a classical spectroscopic analysis of giant stars may indeed result in very reliable stellar parameters. We caution that the quality of the agreement, on the other hand, depends on the details of the adopted spectroscopic analysis.