Common Envelope

The Common- Envelope ( German Joint Cover, abbreviated as CE) is a relatively short phase with unstable mass transfer in an interacting binary system with a period of months to several years. While the common- envelope is the companion star in the atmosphere of the primary star with the result of a loss of torque and the ejection of part of the atmosphere of the primary star. In case of contact systems, a common envelope also exist several million years and provide an energy transfer between the components of the binary system. The energy and mass transfer during a common- envelope allows the formation of stars and planets with properties that can not develop from a single star. In a common - envelope event sufficient energy is released by a part of the shell to accelerate to escape velocity. The expanding gas masses resulting from a common sheath likely one of the primary sources of dust in the interstellar medium in addition to AGB stars and supernova remnants to be.

Common envelope with a red giant

Intermediate-mass stars expand as they evolve due to their increasing luminosity. This is especially true for the phase of a Red Riesens or AGB star, in which crystallizes into a degenerate core. In a close binary system, the expansion of the outer atmosphere lead to exceeding the Roche interface and subsequently matter flows on the companion star.

This mass transfer has two consequences that the red giant is trying to restore its balance by a further expansion, on the other hand to a loss of train torque. This increases the distance between the two stars from the mass transfer rate continues to rise, the companion star, the mass in the short term not accrete and leads to the formation of a common envelope, said train torque is transmitted to the common envelope and a large part of the so- accelerated common sheath in the interstellar space are lost (Common Envelope Ejection ). The reduction of the distance can lead to a merger of the two stars.

A common- envelope phase in a binary system with a red giant has not yet been observed due to their short duration. Modeling strongly depends on parameters such as the viscosity.

At the end of the common- envelope phase, different forms of binary systems can be:

• A cataclysmic variables. Here a white dwarf in a close binary system is unkreist of a main sequence star or subgiant. For single stars, the star evolves into a red giant, which throws off its atmosphere and the core remains as a white dwarf. In cataclysmic variables of the companion star would circulate within the atmosphere of the former red giant, and therefore the binary star system is probably gone through a common hull phase
• The emergence of low-mass X-ray binaries in which a neutron star or black hole of a main sequence star is orbited a small stepover.
• In close binary systems, the proportion of white dwarfs with strong magnetic fields is orders of magnitude higher than that of single white dwarfs. This magnetic field is interpreted as a consequence of the motion of the companion star by the atmosphere of the red giant
• The common envelope is a development path to the creation of Blue stragglers. These stars are too massive for their age and have therefore accrete matter from a companion or are merged with the companion
• The bipolar structure of many planetary nebulae could be the result of common envelope
• A potential formation channel for blue subdwarfs could be the phase of a common envelope falls back to the matter on the white dwarf and this then appears as a hot star with a hydrogen-rich atmosphere
• Supernovae of type IIn show signs of expansion of the Supernovaejekta by a dense circumstellar shell that might have been generated by a common envelope
• Supernovae of type Ia occur when a white dwarf exceeds its Chandrasekhar limit and the degeneracy pressure of gravity can no longer resist. The need for accretion of matter from a companion should mainly be done in binary systems, which were brought by a common sheath phase in a low stepover
• At a higher luminous supernova of type Ia can occur when a white dwarf penetrates during a second common- envelope phase in the shell of an AGB star and then the core of the AGB star is destroyed and accreted by the white dwarf. In this scenario, the mass of the exploding Mergers exceed the Chandrasekhar limit significantly. These supernovae are significantly more luminous than normal TypIa supernovae and should also show evidence of a strong interaction with a dense stellar envelope of the common- envelope phase. Supernova PTF 11kx is considered as an example of such a such a core Degenerate scenario
• When two white dwarfs in a close binary system such as a common- envelope is twice through in the AM Canum - Venaticorum stars. Only thus can pass with an orbital period of less than an hour remains of two red giants in a web.
• R Coronae Borealis stars are hydrogen- poor carbon-rich giants, their atmospheres consist of approximately 98 % helium. Their visual brightness falls at irregular intervals to 8 mag and rises over months to years to rest again brightness. This is interpreted as a consequence of a darkening of the line of sight through soot clouds which ejects the star. R Coronae Borealis star show a distinctly different from other stars chemical composition. The most likely development scenario is a merger of a helium white dwarf and a CO. The close proximity of these burned- former cores of red giants, which leads to a merger is a result of a two-time passing through a common- envelope phase.
• A common envelope forms already in the migration of a star from the main sequence to the red giant branch as the core of the star has at this time only a small mass and consists almost entirely of helium. Go into the common envelope, the envelope of the star is lost there arises in a binary ELM -helium white dwarf. These are white dwarf with extremely low weight of less than 0.2 solar masses.
• In addition to binaries with a white dwarf and neutron stars may undergo a common- enevelope phase. Part of the X-ray binaries in which a neutron star accretes matter from a companion is destroyed by the immersion of the neutron star in the atmosphere of his companion. The merger transaction will only take about 1000 years.
• A certain class of carbon stars, the J -type carbon stars, differ from normal carbon stars by enrichment of nitrogen, a low 12C/13C isotope ratio, above average luminosity and lithium - rich in their stellar atmospheres. It is very unusual in stellar astrophysics that all these stars are single stars. Since over 50 % of all stars are components of binary systems is believed that the J- type carbon stars are the result of mergers of two stars. Their chemical composition can be simulated when a helium- rich white dwarf and a red giant through a common- envelope phase, in which the white dwarf decreases in the core of the red giant's and fuses with him.
• A channel formation for gamma -ray bursts could be in a very massive binary star system, a common- envelope phase. About the common envelope loses a star its outer hydrogen-and helium-rich layers and explodes as a supernova of type Ic. Since in the close binary system, the two stars can rotate bound by the Collapsar model a long gamma -ray burst with a soft gamma spectrum arise

Common envelope with the merger of two main-sequence stars

A binary star system can fuse before one of the star has left the main sequence. Cause loss of torque may be the emission of gravitational waves or magnetic torque loss. In the latter matter is frozen in the stellar wind in the magnetic field lines and the star must drag along this ionized matter during its rotation. Both effects, the radius of the orbit is reduced in a double star system, and at a short distance to the friction leads to a rapid fusion. Such a merger has been observed in the Beta Lyrae star V1309 Sco and resulted in a luminescent Strengthening the Red Nova. In addition to Beta Lyrae stars, the contact systems of the type W Ursae Majoris regarded as a precursor to a merger bursts in which the orbital energy is converted into an expansion of the common envelope with a temporary increase in luminosity. The merger initially goes a rapidly rotating giant FK Comae - type Berenices, the long-term evolves into a blue straggler.

Common envelope in case of contact systems

The W Ursae Majoris stars are eclipsing About Contact systems that take advantage of a common envelope energy. Although the masses of the components of close binary systems can differ by up to a factor of 10, the two companions almost the same surface temperature. The W Ursae Majoris stars are formed as separate binary star systems and come with a loss of torque due to magnetic activity in contact. The W UMa - phase lasts a few to several 100 million years, and during the whole time of the double star is embedded in a common enevelope. Also the W Ursae Majoris stars should merge with another loss of torque and a blue stragglers form.

Common envelope in eruptive variables

The eruption on variable stars a shell is ejected and dropped from the star. If this example happens in novae and supernovae in a binary system of companion for a time runs within a common envelope around the common center of gravity. The density of the shell is to have a significant impact on low usually on the guide, but the attendant transmits kinetic energy to the shell and thus forming the structure of the mist. Bipolar form some Nova remains is contacted with the common envelope phase in combination, for example, with slow novae.

Planets in tight orbits around white dwarfs and blue subdwarfs

If there is a common envelope phase as the kinetic energy of the immersed in the atmosphere is transmitted to guide them and in many cases leads to an ejection of the envelope. This falls at least partially along the orbital plane back and forms a disk around the binary star system or the company resulting from the merger single star. Within this disk planets can form in very narrow alleys, and seems to be a possible explanation for the observation of planets on short orbits around white dwarfs and subdwarfs to Blue to be. In their present orbits the planet would not have survived the red giant stage.

Besides the formation of a planet from an accretion disk also formerly massive planets can survive the CE phase. As shown by simulations, they lose especially by ram pressure during immersion in the atmosphere of the red giant's part of their mass. It may consist of a gas planet with the properties of a Jupiter a terrestrial planet arise, which consists only of the former core of the gas planets.

However, the existence zirkumbinärer planets around post- common- envelope systems is questioned by other authors. All alleged proofs are based on the light-time effect in eclipsing binary systems, where the planet leads to a slight shift of the minimum brightness times due to a change of the common center of gravity. If this planetary would exist at the time of the cover could be predicted more accurately, but this is not the case. Also reported are often the paths of exoplanets allegedly found not stable. Furthermore, the emergence of these zirkumbinärer planet is not without problems. From the age of some cooling of white dwarfs in post- common- envelope systems with exoplanets reported an age of less than a million years been closed. This is too little for a planet formation from a protoplanetary disk after the end of the common- envelope phase. Gas planets are, however, not observed around a binary star system consists of two main-sequence stars, the forerunners of the post- common- envelope systems. An alternative hypothesis for the irregular Bedeckungsminima is suspected in a change in the shape of the red dwarf due to magnetic activity.

Post Common Envelope Binaries

Post Common Envelope Binaries ( PCEB ) are binary stars, which consist of a main sequence star and a white dwarf. They are the most common result of a common- envelope evolution and the observation of these stars allows the parameters of the common- envelope phase to examine indirectly as the viscosity. The systems with the shortest circulation times have to also show the highest probability of a light cover change. They often consist of a hot white dwarf and a faint red dwarf. These stars will evolve into a cataclysmic binary star system when the mass transfer from the Red used to the white dwarf.