Formation and evolution of W Ursa Majoris stars: fallacies and corrections

Abstract

Period histograms of eclipsing binaries generated with data from the All-Sky Automated Survey cannot be interpreted only using orbital evolution. Eclipse probabilities, selection effects and space distributions in the solar neighbourhood should be considered before any interpretations are made. Depending upon the physical dimensions (the total mass and the period) of the progenitor stars and the efficiency of the angular momentum loss mechanism, a newly formed W Ursa Majoris type binary can be at any age up to several Gyr. Also, the evolution in the contact stage is controlled not only by angular momentum and mass loss but also by mass transfer between the component stars. Thus, the mean life of the contact stages should be about 1.6 Gyr. A different time-scale would cause inconsistencies.