XXV. U Ophiuchi and the evolution and composition of 5 M stars
Departamento de Física, ICEX–UFMG, C.P. 702, BR-30.123-970 Belo Horizonte, MG, Brazil e-mail: email@example.com
2 The Niels Bohr Institute, Astronomy, Juliane Maries Vej 30, 2100 Copenhagen, Denmark e-mail: firstname.lastname@example.org
3 Nordic Optical Telescope Scientific Association, Apartado 474, 38 700 Santa Cruz de La Palma, Spain
4 Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18 080 Granada, Spain e-mail: email@example.com
Accepted: 29 March 2007
Context.Precise stellar masses and radii provide unique information on stellar evolution. In a Galactic context, they may also provide information on the evolution of the Solar neighbourhood.
Aims.We aim to determine absolute dimensions for the mid B-type eclipsing binary U Ophiuchi and compare the inferred ages and chemical compositions to those of other binary stars with masses near 5 .
Methods.We determine masses, radii, , , and luminosities for the stars in U Oph from new radial velocities and uvby light curves. By improving the Wilson-Devinney code, we also derive precise apsidal-motion and light-time orbits of this triple system, using 353 times of minimum over 120 years. Finally, we compare the data for U Oph and three similar systems with the predictions of stellar models.
Results.The stars in U Oph have masses of 5.27 ± 0.09 and 4.74 ± 0.07 , radii of 3.48 ± 0.02 and 3.11 ± 0.03 ; we argue that systematic errors are negligible. The apsidal motion period is exceptionally short, yr, while the light-time orbit has 38.4 yr. The precise values for DI Her, U Oph, V760 Sco, and MU Cas, all within 10% of 5 , place them successively from the ZAMS to the TAMS, at ages of 5-100106 yr. Current stellar evolution models fit both stars in each system very well if the (otherwise unconstrained) metal abundance Z is adjusted in each case.
Conclusions.More accurate data and/or binary systems with larger mass ratios are needed to actually test stellar models near 5 . The different Z values found for these young, nearby systems suggest that disk stars form with a range of metal abundances even today, but more data are needed to confirm this result.
Key words: stars: binaries: eclipsing / stars: fundamental parameters / stars: evolution / Galaxy: evolution
© ESO, 2007