Absolute dimensions of detached eclipsing binaries - I. The metallic-lined system WW Aurigae

Abstract

WW Aurigae is a detached eclipsing binary composed of two metallic-lined A-type stars orbiting each other every 2.5 d. We have determined the masses and radii of both components to accuracies of 0.4 and 0.6 per cent, respectively. From a cross-correlation analysis of high-resolution spectra we find masses of 1.964 +/- 0.007 M_solar for the primary star and 1.814 +/- 0.007 M_solar for the secondary star. From an analysis of photoelectric uvby and UBV light curves we find the radii of the stars to be 1.927 +/- 0.011 R_solar and 1.841 +/- 0.011 R_solar, where the uncertainties have been calculated using a Monte Carlo algorithm. Fundamental effective temperatures of the two stars have been derived, using the Hipparcos parallax of WW Aur and published ultraviolet, optical and infrared fluxes; these are 7960 +/- 420 and 7670 +/- 410 K. The masses, radii and effective temperatures of WW Aur are only matched by theoretical evolutionary models for a fractional initial metal abundance, Z, of approximately 0.06 and an age of roughly 90 Myr. This seems to be the highest metal abundance inferred for a well-studied detached eclipsing binary, but we find no evidence that it is related to the metallic-lined nature of the stars. The circular orbit of WW Aur is in conflict with the circularization time-scales of both the Tassoul and the Zahn tidal theories and we suggest that this is due to pre-main-sequence evolution or the presence of a circular orbit when the stars were formed.