Astronomy & Astrophysics

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Issue		A&A Volume 399, Number 3, March I 2003
Page(s)		1115 - 1119
Section		Stellar atmospheres
DOI		10.1051/0004-6361:20021857

A&A 399, 1115-1119 (2003)
DOI: 10.1051/0004-6361:20021857

The massive and evolved EBS V380 Cygni: A case of critical evolution

A. Claret

Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain
(Received 5 April 2002 / Accepted 27 November 2002 )

Abstract
The evolution and the internal structure of the double-lined eclipsing binary V380 Cyg showing apsidal motion is investigated. The system consists of an evolved massive primary with a mass of 11.1 $\pm$ 0.5 $M_\odot$ and a secondary which is still close to the ZAMS and has a mass of 6.95 $\pm$ 0.25 $M_\odot$ . The core overshooting parameter $\alpha_{{\rm ov}}$ was recently determined for this star as 0.6 $\pm$ 0.1. However, such a value was derived using error bars in the effective temperature which seem to be unrealistically small. In addition, there is no independent support for this value of $\alpha_{{\rm ov}}$ either from Colour-Magnitude Diagrams of clusters or from double-lined eclipsing binaries. Therefore, we decided to compute stellar models with several values of core-overshooting parameter in order to try to disentangle this apparent disagreement. In order to compare the observations with the theoretical predictions, we decided to use the ratio of the effective temperatures, ${\rm TR}=T_{{\rm eff2}}$ / $T_{{\rm eff1}}$ , rather than the effective temperatures themselves to avoid problems with calibrations and/or model atmosphere dependencies. Contrary to what has been previously established, we have found acceptable common ages for $\alpha_{{\rm ov}}=$ 0.2, 0.4, and 0.6 which fit the TR, the masses, the radii and the apsidal-motion rate simultaneously. Such indetermination in $\alpha_{{\rm ov}}$ should not be interpretated as a negative result. Instead, it indicates that errors in $\alpha_{{\rm ov}}$ cannot be determined with a precision of 0.1 if realistic observational error bars are used; we are dealing with an EBS in a very critical phase of evolution. Moreover, these new inferred values of $\alpha_{{\rm ov}}$ imply that the dependency of convective core overshooting on the stellar mass, if it exists, is more uncertain and less pronounced than previous investigations suggested.

Key words: stars: binaries: close -- stars: evolution -- stars: early-type -- stars: fundamental parameters

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