Absolute dimensions of eclipsing binaries. XXV. U Ophiuchi and the evolution and composition of 5 M⊙ stars

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 {M}_⊙.
Methods: We determine masses, radii, log g, log T_eff, 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 {M}_⊙ and 4.74 ± 0.07 {M}_⊙, radii of 3.48 ± 0.02 {R}_⊙ and 3.11 ± 0.03 {R}_⊙; we argue that systematic errors are negligible. The apsidal motion period is exceptionally short, U ∼21 yr, while the light-time orbit has P₃ ∼ 38.4 yr. The precise log g values for DI Her, U Oph, V760 Sco, and MU Cas, all within 10% of 5 {M}_⊙, place them successively from the ZAMS to the TAMS, at ages of 5-100×10⁶ 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 {M}_⊙. 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.

Based on observations collected at the European Southern Observatory, La Silla, Chile. Table [see full text] is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/469/285