The role of the time step and overshooting in the modelling of PMS evolution: The case of EK Cephei

• Type: Article in International Scientific Journal
• Date: 2004

Authors

Marques, JP (Author) Other; Fernandes, J (Author) Other; Monteiro, MJPFG (Author) FCUP

Journal

Title: Astronomy and Astrophysics

Vol. 422

Pages: 239-245

ISSN: 0004-6361

Publisher: EDP Sciences

Indexing

ISI Web of Knowledge - 15 Citations

Scopus - 15 Citations

Scientific classification

FOS: Natural sciences > Physical sciences

Other information

Authenticus ID: P-000-9H9

DOI: 10.1051/0004-6361:20035693

Abstract (EN): 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 M-circle dot) 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 1997). A chi(2)-minimisation was performed 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 26.8 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.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 7