The evolution of W Ursae Majoris systems

In an attempt to understand the structure of W UMa binaries, the evolution of close binary models in and near contact has been investigated. Calculations have been performed for a total system mass of 1.8 solar masses, and for various orbital angular momenta and degrees of nuclear evolution. Cyclic behaviour on a thermal time-scale, with alternating contact and semi-detached phases, is generally found although in some circumstances a stable equal-mass system is obtained. Both nuclear evolution and lower angular momentum cause the cycles to be centred on more extreme mass ratios, and the fraction of time spent in contact appears to increase considerably. One evolved model settles in an apparently stable state of shallow contact with q approximately 0.4. It is suggested that this may represent a fairly typical A-type W UMa system. The drawback of the cyclic models is that during the semi-detached phase the systems would be expected to exhibit EB-type light curves, and it must be admitted that (so far) not short-period (P < approximately 0,four days) EB binaries have been observed. Reasons are given for believing that this difficulty is not insurmountable. (author)