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A Calibration of EK Cephei

Authors: Marques, P. J.; J. Fernandes; Monteiro, M. J. P. F. G.

Ref.: Open Issues in Local Star Formation. Contents of the CD-ROM: Poster Contributions. Proceedings of the Ouro Preto Colloquium, Brazil, April 5-10, 2003. Edited by Jacques Lépine and Jane Gregorio-Hetem. Astrophysics and Space Science Library, Volume 299. Kluwer Academic Publishers, Dordrecht. 299 (2003)

Abstract: EK Cephei (HD 206821) is a unique candidate to test predictions based on stellar evolutionary models. It is a double-lined detached eclipsing binary system with accurate absolute dimensions available, good determination of the metallicity and in particular of the lithium abundance. Most importantly, for our work, its low mass (1.12M_sun) component appears to be in the pre-main sequence (PMS) phase, being therefore the rst known solar-type PMS star with a directly determined mass. We have produced detailed evolutionary models of the binary EK Cep using the CESAM stellar evolution code (Morel 1997). Models were constructed using the OPAL equation of state, OPAL opacities and NACRE thermonuclear reaction rates. A Chi^2-minimization was performed in order to derive the most reliable set of modelling parameters (age, alpha_A, alpha_B and Y_i). We have found that an evolutionary age of about 26.7 Myrs ts both components in the same isochrone. The positions of EK Cep A and B in the HR diagram are consistent (within the observational uncertainties) with our results. In doing this we have found that a very small time step for the evolution of the models is able to t the observations much better. This indicates that an adequate definition of the time step is of great importance for modelling the PMS phase. Our results show clearly that EK Cep A is in the beginning of the main sequence, while EK Cep B is indeed a PMS star. We will complement this study by modeling the chemical evolution of the two components given the special relevance of their properties and age for studying this aspect of the evolution.

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