Absolute dimensions of eclipsing binaries. XX. 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^d.85) have been determined from new high-dispersion spectra and uvby light curves. The components have masses of 4.12±0.04 and 2.51±lO.O2 M_sun and radii of 2.38±0.03 and 1.72±O.O2 R_sun, 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 stars with well- determined parameters. The orbit is somewhat eccentric (e = 0.150±0.005) and shows apsidal motion with the fairly short period U = 101±4 yr; measured rotational velocities of the stars correspond to synchronization at periastron as expected. GG Lup is located at a distance of about 140 pc.

Comparison with stellar evolution models using the most recent input physics again shows GG Lup to be very young; at an age of only ∼2 1O⁷ yr it is essentially unevolved. The density concentration coefficient computed from the apsidal-motion parameters, log k₂ = -2.15±0.03, agrees very well with that computed for stellar models that fit the other observed properties of the stars. GG Lup is too young, however, to provide interesting constraints on convective overshooting or tidal synchronization and circularization theories.

With modern evolutionary models, any satisfactory fit to the gravity of the secondary component of GG Lup requires its metal abundance to be somewhat below solar, Z ≈ 0.015, in agreement with preliminary spectroscopy of the system. Combined with 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 clouds to the disk, including an episode preceding the formation of the young, local B stars, is a possible interpretation of this basic observational result.