ABSOLUTE DIMENSIONS OF ECLIPSING BINARIES .20. GG LUPI - YOUNG METAL-DEFICIENT B-STARS

The physical properties of the bright, late B-type eclipsing binary GG Lupi (HR 5687; B7V + B9V, P = 1.d85) have been determined from new hi gh-dispersion spectra and uvby light curves. The components have masse s of 4.12 +/- 0.04 and 2.51 +/- 0.02M.) and radii of 2.38 +/- 0.03 and 1.72 +/- 0.02R., respectively. With resulting values of log g of 4.30 and 4.36, the components of GG Lup are some of the least-evolved B st ars with well-determined parameters, The orbit is somewhat eccentric ( e = 0.150 +/- 0.005) and shows apsidal motion with the fairly short pe riod U = 101 +/- 4 yr; measured rotational velocities of the stars cor respond to synchronization at periastron, as expected. GG Lup is locat ed at a distance of about 140 pc. Comparison with stellar evolution mo dels using the most recent input physics again shows GG Lup to be very young; at an age of only approximately 210(7) yr it is essentially un evolved. The density concentration coefficient computed from the apsid al-motion parameters, log k2 = -2.15 +/- 0.03, agrees very well with t hat computed for stellar models that fit the other observed properties of the stars. GG Lup is too young, however, to provide interesting co nstraints on convective overshooting or tidal synchronization and circ ularization theories. With modern evolutionary models, any satisfactor y fit to the gravity of the secondary component of GG Lup requires its metal abundance to be somewhat below solar, Z almost-equal-to 0.015, in agreement with preliminary spectroscopy of the system. Combined wit h results from spectroscopic analyses of nearby B and F stars, this is complementary, supporting evidence that the metal abundance of the '' solar cylinder'' of the thin galactic disk has not been uniformly and monotonically increasing since the formation of the Sun at about half the age of the disk. Large-scale infall of discrete metal-poor gas clo uds to the disk, including an episode preceding the formation of the y oung, local B stars, is a possible interpretation of this basic observ ational result.