Paper detail

The Short Period End of the Distribution of Contact Binary Stars

Contact binary systems (also known as W UMa systems) consist of a pair of hydrogen-burning dwarf stars orbiting each other so closely that they share a common envelope. Although they are relatively common, there is as yet no established consensus on the principle evolutionary questions surrounding them: how do they form, how do they evolve over time, what do they become? One observational clue to their evolutionary history has been the abrupt termination of the orbital period distribution around 5.2 hours. We have undertaken an observational study of this by 1) discovery of fast W UMa systems in our Calvin-Rehoboth Observatory data archive, 2) follow-up with the Calvin-Rehoboth Observatory of candidate fast systems from the Catalina Sky Survey, and 3) follow-up of other reports of potentially fast systems in other recently published surveys. We find the follow-up to have been particularly important as many surveys taken for other purposes lead to ambiguous or incorrect claims for periods less than five hours. Our results to date may be characterized as showing two distinct components: the steeply decaying tail associated with the previously known cutoff along with a low-amplitude, but apparently uniform distribution that extends down to 3.6 hours. The confirmation at greater sensitivity of the abruptness of the cutoff seems to imply that the dominant mechanism for system formation (or the mechanism that determines system lifetime) does have a strong period dependence. At the same time, there appears to be a second mechanism at work as well which leads to the formation of the ultrafast component of the histogram.