The apsidal constant and structure of alpha Virginis.

The binary system Spica (a Vir) is analyzed by means of theoretical models. Masses, radii, and orbital elements are taken from the work of Herbison-Evans et al. The primary is assumed to be evolving from the zero-age main sequence, the secondary on or near it. Comparison of the models is made with the observed apsidal constant and with the flux ratio F(A2905)/F(A5470), which has been observed by Bless. It is not possible to fit both of these quantities, even by adjusting the observed quantities within reasonable limits; the worst discrepancy is that the observed apsidal constant is only about one-half that of the models. The A2905/ V flux ratio cannot be fitted with the model atmospheres of Kurucz, but can with those of Van Citters and Morton. A model with a helium-rich atmosphere, as Stothers and Simon have suggested for p Ma stars, is found to have an even greater difficulty in fitting the apsidal constant than the homogeneous ones. It is suggested that mixing of the inner regions from a mechanism other than convection (e.g., rotation or convective overshoot) would resolve the difficulty. Naturally this mixing, if present, would affect subsequent evolution. It is unlikely but not impossible that an unseen third component of the system could be the cause. Subject headings: apsidal motion - binaries - interiors, stellar - stars, individual - fl Cephei stars