W Ursae Majoris variable

W Ursae Majoris stars, and W Ursae Majoris variables ( GCVS nomenclature abbreviation: EW) are eclipsing stars, the binary star pair is in surface contact and show a continuous change of light. They are surrounded by a common sheath that has formed between the inner and outer Roche limit. They are named after the prototype W Ursae Majoris.

Survey

W Ursae Majoris stars usually have a spectral type from F to K, where its components are about the same brightness at a different mass. The orbital period is usually less than a day, with almost all periods from 0.22 to 0.8 days lie. The amplitude of the visual light is usually less than 0.8 magnitudes, the two minima differ only slightly. The total mass of a W Ursae Majoris binary star system is less than 2.5 solar masses.

The light curve differs from the classical discrete blackout eclipsing binary by a continuous change in brightness. This is a result of the elliptical shape of the stars because of the proximity, wherein these sets by gravitational bias, and the centrifugal force. The light curve is repeated often not strict, since due to the short orbital period and the convective energy transport in the stellar envelope activity occurs. Star spots and flares are frequently observed. Characteristic of W Ursae Majoris stars is still the constant color index over the entire light change, which is also used to distinguish between pulsating variables such as Delta Scuti stars and RR Lyrae stars. From the constant color index follows a nearly identical surface temperature for two stars with different masses. This is a violation of the Vogt- Russell theorem, according to which the mass and chemical composition clearly defines both the radius and the luminosity of the star. Today, it is assumed that a W- Ursae Majoris star is embedded in a common sheath, and this leads to the identical surface temperature.

W Ursae Majoris variables occur at a high frequency among the variable stars. In the galactic area include about one percent of main sequence stars with the spectral types F to K to the W Ursae Majoris stars.

The primary star is in its evolution on the main sequence, while the companion with a lower mass over a larger radius has up to seven times as a single star with an identical mass and chemical composition. The enlarged diameter could be the result of convective energy transfer from the primary star to the companion.

Classification

The W Ursae Majoris stars are divided into the following subclasses:

• Type A: The more massive star of the two has the larger radius and a higher effective temperature, both stars have a higher surface temperature than the Sun with a spectral type A or F at a web orbital period of 0.4 to 0.8 days
• Type W: The more massive star has a larger radius and a lower effective temperature than its partner. Both stars have a spectral type G or K with an orbital period from .22 to 0.4 days
• Type H: This W Ursae Majoris stars have a mass ratio q = M1 / M2 is more than 0.72. In these double stars, the energy transfer between the components is very inefficient.

O'Connell effect

In many contact systems and especially in W Ursae Majoris variable O'Connell effect can be observed, in which the maxima in the light curve show a different amount of up to 0.1 mag. The asymmetry in the light of change increases, the more the stars are distorted elliptical and the greater is the ratio of the radii of the stars. The O'Connell effect is either as the result of a hot spot between the two stars due to mass exchange, star spots explains the components of the binary system and by circumstellar matter in a ring around the eclipsing binaries. This is accompanied by the so-called W - phenomenon. After that, the deeper minimum of the eclipsing change at most W UMa stars occurs when the secondary star is covered by the more massive primary star. This is associated with a collection of star spots on the primary star in compound whereby the average temperature of its photosphere is lower than that of his companion.

Development

W Ursae Majoris variables and other contact systems occur either in star-forming regions even in young open clusters. They are, however, common in the older open clusters with an age of more than a billion years, and to find the approximately 12 billion year old globular clusters. Contact systems result in a temporal process that is referred to as magnetic torque loss. Since narrow, initially separated binary systems the rotation of the star is bound, the period of rotation of these stars can only be identical to the orbital period in a double star system of a few days. Because convection dominates the energy transport to the surface of the late star, global magnetic fields are formed. The opinion given in the stellar wind matter is ionized, therefore frozen in the magnetic field and the rotation of the star must follow. This reduces the entrainment present in the binary star system torque and consequently reduces the distance between the two components, to form a common envelope. In more massive W Ursae Majoris variable dominates the nuclear development. After the exhaustion of hydrogen inventories by thermonuclear processes expands the star to remain in hydrostatic equilibrium and thus gets in touch with his companion. This path of development is characteristic of the W UMa - type stars A. In both development paths the binary star system is only for 10 percent of its characteristic lifetime of up to about 8 billion years in the contact and exchanges of matter. The mass ratio is not more extreme than a tenth.

The continuous exchange of matter and energy between the two stars in a common sheath, the total torque of the binary system is further reduced. Therefore, the distance between the two components is decreased until the two star merge. In the process of merger of a close binary system is a large amount of energy is released and this is observed as Luminous Red Nova. In the case of V1309 Sco even the Bedeckungsveränderlichkeit has been documented before the outbreak. As a result, the light Strengthening Red Nova is a rapidly rotating single star forms consisting of the mass of the two components of the binary system. As the successor to the merger, FK Comae Berenices - star and the blue stragglers are considered.

Period distribution

The distribution of orbital periods of these systems contact has a maximum at 0.37 days. For smaller periods towards the frequency drops rapidly and below 0.21 days is not a W Ursae Majoris star known. This distribution is explained as a consequence of an unstable mass transfer. The primary star in such a tight contact system has the property that its radius faster in a loss of mass increases as the Roche limit in a double star system. The result is an exponential increase in the mass transfer rate, if this period lower limit is reached. This leads to a fast melting of the binary system, and the result is a rotating high speed single star.

A search for contact systems at the lower end of the period distribution using the data from the SuperWASP experiment has shown that only 3 out of 53 systems show a marked shortening of the orbital period. This period changes can be caused either by magnetic interaction nor by emission of gravitational waves. However, the small number of binary systems with possibly unstable mass transfer problem for the present hypotheses and also not statistically significant, as there are a comparable number of contact systems with strong period extensions.

A period lower limit of 0.21 days does not seem to exist for main-sequence stars consisting of two M- dwarfs. It even separate systems were found below the period boundary and the shortest known orbital period in a contact system of two M- dwarfs is 0.112 days. So far it has been assumed that two M- dwarfs may have lost in the Hubble time not enough torque to reach such short periods. Whether M dwarfs are coupled by stellar activity in a position to quickly convert torque, or whether they have already come out as a very close double star system of star formation, is the subject of current research.

The circulation periods of contact systems, measured as the distance between two minima, varies with an amplitude of up to 0.01 days with a quasi-period of a few hundred days. This is caused by star marks on the surface of the star. Star spots are areas with lower surface temperature, which can move the point at minimum brightness by their location on the hemisphere. The quasi-periods, in turn are the result of a differential rotation of the star.