Ursa Major W type (EW) eclipsing variables are a type of eclipsing binary stars . These stars are so close that their surfaces touch each other, and they constantly exchange material from the outer layers. The substance flows through a narrow neck existing between two stars, which leads to equalization of the masses and temperatures of the components. Both components of systems of this type are located on the main sequence and both fill their Roche cavities . The substance in such systems flows from a more massive star to a less massive one, but this is “on average” at very long time intervals, and at short intervals the process is much more complex in cyclic nature. There is still no complete theory of the evolution of systems of this type. The article [1] provides a fairly detailed discussion of the current state of this problem.
Variables of this type are subdivided into 2 main subclasses: A and W. The eclipsing variables of type A consist of two stars hotter than the Sun and belong to the spectral class A or F, and also have a period of brightness change from 0.4 to 0.8 days . Type W variables belong to the colder spectral classes G or K, and have a shorter period from 0.22 to 0.4 days. The difference between the temperatures on the surface of both stars is less than a few hundred degrees. In 1978, subclass B was identified in which the difference in surface temperatures is greater. In 2004, subclass H was identified in which the mass ratio more ( (mass of the secondary star) (mass of the main star)) and also a larger angular momentum .
The brightness curve also differs from the curve of classical eclipsing binary stars , due to the fact that the stars, being nearby, constantly eclipse one another, and also, due to the powerful gravitational effect, strongly distort each other's shape. The minima on the light curve, in contrast to other eclipsing binary stars, are usually the same, since the stars have the same brightness.
Examples of variables of type W Ursa Major
The prototype of this class of stars is W Ursa Major
| Title | Magnitude | Period (days) | Spectral class |
|---|---|---|---|
| ε of the South Crown | 4.74 - 5 | 0.5914 | F2v |
| S Pump | 6.4 - 6.92 | 0.6483 | A9Vn |
| W Big Dipper | 7.75-8.48 | 0.4387 | G2v |
| Em Cepheus | 7.02 - 7.17 | 0.8062 | B1ve |
| HT Virgin | 7.06 - 7.48 | 0.4077 | F8v |
Notes
- ↑ Lifang Li, Zhanwen Han, Fenghui Zhang. Structure and evolution of low-mass systems of type W UMa (Structure and evolution of low mass W UMa type systems), [1] (2004)