TZ Fornacis : stellar and tidal evolution in a binary with a fully-fledged red giant (absolute dimensions of eclipsing binaries. XVII).

The absolute dimensions for a normal giant star are determined precisely. New radial velocity curves, high-resolution spectroscopy, and complete UVBY light curves for the evolved, but detached, eclipsing binary TZ For are presented. The orbit is circular and the masses of the components are 1.95 and 2.05 solar masses + or - 2 percent. Using spectroscopically determined luminosity ratios at two wavelengths, the light curve solutions yield 3.96 and 8.32 solar radii - both considerably larger than the main-sequence radius for this mass. The effective temperatures are 6350 and 5000 K, and the metal abundance is Fe/H = +0.1 + or - 0.15. Lithium is not detected, implying a depletion by a factor of not less than 300. The rotation of the larger star is synchronized with the orbital motion, while the other component rotates much faster. It is inferred that the main characteristics of TZ For are very similar to those of Capella (alpha Aur).