The role of the time step and overshooting in the modelling of PMS evolution: The case of EK Cephei
1
Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
2
Grupo de Astrofísica da Universidade de Coimbra, Observatório Astronómico da Universidade de Coimbra, Santa Clara, Coimbra, Portugal
3
Departamento de Matemática da FCTUC, Coimbra, Portugal
4
Departamento de Matemática Aplicada, Faculdade de Ciências da Universidade do Porto, Portugal
Corresponding author: J. P. Marques, jmarques@astro.up.pt
Received:
14
November
2003
Accepted:
6
April
2004
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 and a precise determination
of the metallicity. Most importantly for our work, its low mass
(1.12 
) component appears to be in the pre-main sequence (PMS) phase. We have
produced detailed evolutionary models of the binary EK Cep using the
CESAM stellar evolution code (Morel [CITE]). A 
-minimisation was
performed to derive the most reliable set of modelling parameters (age,
, 
and Yi). We have found that an
evolutionary age of about 26.8 Myr fits 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. Our revised
calibration shows clearly that EK Cep A is in
the beginning of the main
sequence, while EK Cep B is indeed a PMS star.
Such a combination allows for a
precise age determination of the binary, and provides a strict test of the
modelling. In particular we have found that the definition of the time step in
calculating the PMS evolution is crucial to reproduce the observations. A
discussion of the optimal time step for calculating PMS evolution is
presented.
The fitting to the radii of both components is a more difficult task; although
we managed to do it for EK Cep B, EK Cep A has a lower radius than our
best models. We further studied the effect of the inclusion of a
moderate convective overshooting; the calibration of
the binary is not significantly altered, but the effect of the inclusion of
overshooting can be dramatic in the approach to the
main sequence of stars with masses high enough to burn hydrogen
through the CNO cycle on the main sequence.
Key words: stars: pre-main sequence / stars: evolution / stars: fundamental parameters / stars: individual: EK Cephei
© ESO, 2004