A CASE-STUDY OF THE QUADRUPLE SYSTEM XI-URSAE-MAJORIS - ITS ACTIVITY AND LITHIUM DEPLETION

The quadruple system xi UMa is an interesting test case for the numero us problems encountered when studying the evolutionary status, chromos pheric activity and lithium depletion of solar-type stars. We have car ried out a detailed analysis of the major components A and B of xi UMa from high resolution, high signal-to-noise CFHT spectra. We determine accurate temperatures for each of the principal components, neither d is turbed by their invisible, much less massive companion, and find th at the two differ in temperature by 300 K, one being somewhat hotter t han the Sun, the other cooler. The metal abundances derived from iron and a few other elements are identical for the two stars, and the syst em is moderately metal-deficient, by a factor of two with respect to t he Sun. We concur with previous studies that lithium is fairly abundan t in A whereas it is below detection in B: we measured log N(Li) = 2.3 3 in A and place a more stringent upper limit log N(Li) < 0.8 for B. W e discuss this unusual combination of lithium abundances and the posit ion 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 evolu tion and of lithium depletion. We suggest that B has kept a high level of activity because its rotation is tidally locked with the orbital m otion, and that it has therefore lost about ten times more matter than a single star of the same mass, which explains its strong lithium dep letion.