On the A-type W Ursae Majoris systems

Abstract

It is shown that eight out of eight A-type W Ursae Majoris systems, selected for having accurately known parameters, have larger than zero-age main-sequence radii. This result is in accord with earlier findings that zero-age structural models cannot be produced for these binaries except by incorporating one or more implausible features into the models. The A-type binaries can probably be understood as systems which are essentially in equilibrium, but not zero age. The few published models for evolved W Ursae Majoris stars exhibit the proper main features of the observed systems, except for having much less extreme mass ratios. It is shown that the use of mean component ('equal volume') radii rather than 'side' radii, as in previous work, can be expected to help considerably in bringing theoretical and observational mass ratios into agreement.