1994A&A...291..505C

The quadruple system ξ UMa is an interesting test case for the numerous problems encountered when studying the evolutionary status, chromospheric activity and lithium depletion of solar-type stars. We have carried out a detailed analysis of the major components A and B of ξ UMa from high resolution, high signal-to-noise CFHT spectra. We determine accurate temperatures for each of the principal components, neither disturbed by their invisible, much less massive companion, and find that the two differ in temperature by 300 K, one being somewhat hotter than the Sun, the other cooler. The metal abundances derived from iron and a few other elements are identical for the two stars, and the system is moderately metal-deficient, by a factor of two with respect to the Sun. We concur with previous studies that lithium is fairly abundant in A whereas it is below detection in B: we measured log N(Li)=2.33 in A and place a more stringent upper limit log N(Li)<0.8 for B. We discuss this unusual combination of lithium abundances and the position of the stars in the HR diagram in the light of the complex binary nature of the two components, and of current theories of stellar evolution and of lithium depletion. We suggest that B has kept a high level of activity because its rotation is tidally locked with the orbital motion, and that it has therefore lost about ten times more matter than a single star of the same mass, which explains its strong lithium depletion.