A W Ursae Majoris variable, also known as a low mass contact binary, is a type of eclipsing binary variable star. These stars are close binaries of spectral types F, G, or K that share a common envelope of material and are thus in contact with one another. They are termed contact binaries because the two stars touch and transfer mass and energy through the connecting neck, although astronomer Robert E. Wilson argues that the term "overcontact" is more appropriate.
The class is divided into two subclasses: A-type and W-type. A-type W UMa binaries are composed of two stars both hotter than the Sun, having spectral types A or F, and periods of 0.4 to 0.8 day. The W-types have cooler spectral types of G or K and shorter periods of 0.22 to 0.4 day. The difference between the surface temperatures of the components is less than several hundred kelvins. A new subclass was introduced in 1978: B-type. The B-types have larger surface temperature difference. In 2004 the H (high mass ratio) systems were discovered by Szilárd Csizmadia and Peter Klagyivik. The H-types have a higher mass ratio than ( = (secondary's mass)/(primary's mass)) and they have extra angular momentum.
These stars were first shown to follow a period-color relation (shorter period systems are redder) by Olin J. Eggen. In 2012, Terrell, Gross and Cooney published a color-survey of 606 W UMa systems in the Johnson-Cousins photometric system.
Their light curves differ from those of classical eclipsing binaries, undergoing a constant ellipsoidal variation rather than discrete eclipses. This is because the stars are gravitationally distorted by one another, and thus the projected area of the stars is constantly changing. The depths of the brightness minima are usually equal because both stars have nearly equal surface temperatures.
W Ursae Majoris is the prototype of this class.
Designation (name) Constellation DiscoveryApparent magnitude (Maximum)Apparent magnitude (Minimum)Range of magnitude Period Spectral types(eclipsing components)Comment AB And Andromeda 10.40 11.27 d G5 G5V S Ant Antlia H.M.Paul, 1891 6.27 6.83 dA9VA9V 44 (or i) Boo B Boötes 5.8 6.4 d G2V G2V Triple system, A being non-variableTU BooBoötes 11.812.5 dG3 VW Cep Cepheus 7.23 7.68 dG5K0Ve WZ Cep Cepheus 11.4 12.0 dF5 Possible triple system ε CrA Corona Australis 4.74 5.0 d SX Crv Corvus 8.99 9.25 dF7V? V1191 Cyg Cygnus 10.82 11.15 dF6VG5VBW DraDraco 8.619.080.470.2921671 dG3VG0V V571 Dra Draco Barquin, 2018 14.43 14.77 d XY Leo Leo 9.459.93 dK0V K0CE LeoLeo 11.812.6 d TV Pic Pictor Verschuren, 1987 7.37 7.53 dA2VA9-F0V Y Sex Sextans 9.81 10.23 d F3/5V or F5/6V Possibly two substellar companions W UMa Ursa Major 7.75 8.48 dF8Vp F8Vp Prototype, possible triple system
Notes
References
External links OGLE Atlas of Variable Star Light Curves - W Ursae Majoris type binaries