The structure of alpha virginis. II. The apsidal constant

The binary system Spica (a Vir) is analyzed by comparing theoretical models with the masses, radii, and fluxes of the components. The temperature is found from the total measured flux and the radius. It is found that the models computed under the usual physical assumptions are able to fit the present temperature and luminosity, but would need either an increased mass or an increased helium content. These models do not fit the apsidal constant in the sense that the models never become sufficiently centrally condensed, and the observed apsidal constant is too small by a factor of 2. Models with increased opacities have the same need for larger mass or helium content, but for these models the apsidal constant is consistent with the observed value. If the model star is assumed to have arbitrary (nonconvective) mixing to the boundary of the main-sequence convective region, both the photometric and apsidal predictions can be matched with Cox-Stewart opacities. The models show only three acceptable evolutionary stages: the end of core hydrogen burning, the overall contraction phase, or the beginning of the shell-burning stage. Subject headings: apsidal motion - interiors, stellar - stars, individual