Astronomy and Astrophysics, volume 511, A22-22 (2010/2-2)
Absolute dimensions of solar-type eclipsing binaries. III. EW Orionis: stellar evolutionary models tested by a G0 V system.
CLAUSEN J.V., BRUNTT H., OLSEN E.H., HELT B.E. and CLARET A.
Abstract (from CDS):
Recent studies of inactive and active solar-type binaries suggest that chromospheric activity, and its effect on envelope convection, is likely to cause significant radius and temperature discrepancies. Accurate mass, radius, and abundance determinations from additional solar-type binaries exhibiting various levels of activity are needed for a better insight into the structure and evolution of these stars. We aim to determine absolute dimensions and abundances for the G0 V detached eclipsing binary EW Ori, and to perform a detailed comparison with results from recent stellar evolutionary models. uvby light curves and uvbyβ standard photometry were obtained with the Stroemgren Automatic Telescope, published radial velocity observations from the CORAVEL spectrometer were reanalysed, and high-resolution spectra were observed at the FEROS spectrograph; all are/were ESO, La Silla facilities. State-of-the-art methods were applied for the photometric and spectroscopic analyses. Masses and radii that are precise to 0.9% and 0.5%, respectively, have been established for both components of EW Ori. The 1.12M☉ secondary component reveals weak CaII H and K emission and is probably mildly active; no signs of activity are seen for the 1.17M☉ primary. We derive an [Fe/H] abundance of +0.05±0.09 and similar abundances for Si, Ca, Sc, Ti, Cr, and Ni. Yonsai-Yale and Granada solar-scaled evolutionary models for the observed metal abundance reproduce the components fairly well at an age of ≃2Gyr. Perfect agreement is, however, obtained at an age of 2.3Gyr for a combination of a) a slight downwards adjustment of the envelope mixing length parameter for the secondary, as seen for other active solar-type stars; and b) a slightly lower helium content than prescribed by the Y-Z relations adopted for the standard model grids. The orbit is eccentric (e=0.0758±0.0020), and apsidal motion with a 62% relativistic contribution has been detected. The apsidal motion period is U=16300±3900yr, and the inferred mean central density concentration coefficient, log(k2)=-1.66±0.30, agrees marginally with model predictions. The measured rotational velocities, 9.0±0.7 (primary) and 8.8±0.6 (secondary) km/s, are in agreement with both the synchronous velocities and the theoretically predicted pseudo-synchronous velocities. Finally, the distance (175±7pc), age, and center-of mass velocity (6km/s) exclude suggested membership of the open cluster Collinder 70. EW Ori now belongs to the small group of solar-type eclipsing binaries with well-established astrophysical properties.