EW W Ursae Majoris eclipsing systems

- These are eclipsers with periods shorter than 1 day, consisting of ellipsoidal components almost in contact and having light curves for which it is impossible to specify the exact times of onset and end of eclipses. The depths of the primary and secondary minima are almost equal or differ insignificantly. Light amplitudes are usually < 0.8 mag in V. The components generally belong to spectral types F-G and later. GCVS

Observation Key

Mixed stars gg Small amplitudes f\ Mixed periods <®> CCD or PEP

7.6. ligh' curve variable

The W Ursae Majoris eclipsing binary stars are characterized by continuous light changes due to eclipses and because of their changing aspects resulting from tidal distortions. The minima in the light curves are of almost equal depth, indicating similar surface temperatures of the components and the periods are short, almost exclusively ranging from about seven hours up to one day (Figure 7.6).

The W UMa phenomenon is usually explained by assuming that both stars are in contact and the more massive star is transferring material to the less massive one through a common envelope. The result is probably an equalizing of the surface temperatures.

Period changes are observed in all EW systems and are probably associated with the ongoing mass circulation that transports the material from the primary to the secondary. The long-term evolutionary effects should produce a secular mass loss of the secondary star resulting in a lengthening of the period, if no matter is lost from the system.

EW systems show complex behavior and period changes. Studies indicate that positive and negative period jumps are randomly distributed among the EW binaries but a comprehensive explanation is lacking. The space distributions of W UMa systems indicates that they form within the old disk population and have a typical age of one billion years. It is suggested that they descend from the short-period main sequence RS CVn systems and then evolve into blue stragglers or into FK Comae-type variables.

Discovered in 1888, S Ant was the first W UMa system. The GCVS lists a little over 500 EW systems. Light curves are available for only a small fraction of these stars.

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