Common Envelope

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The red giant (left in the picture) fills its dashed Roche boundary and transfers matter to the star on the right in the picture. Since the red giant expands faster than it loses mass due to this mass transfer, it is an unstable process with the result of a common atmosphere enveloping both stars.

The common envelope ( German Joint cover, abbreviated CE ) is a relatively short period with unstable mass transfer in an interacting binary star system with a period of months to several years. During the common envelope, the companion star is 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 over-contact systems, a common shell can exist for several million years and ensure energy transfer between the components of the binary star system . The energy and mass transfer during a common envelope enables the formation of stars and planets with properties that cannot develop from a single star. In the event of a common envelope event, enough energy is released to accelerate part of the envelope up to escape speed. The expanding gas masses resulting from a common shell are likely to be one of the primary sources of dust in the interstellar medium alongside AGB stars and supernova remnants .

Common envelope in a red giant

Medium-mass stars expand in the course of their evolution due to their increasing luminosity. This applies in particular to the phase of a red giant or AGB star , in which a degenerate core develops. In a close binary star system, the expansion of the outer atmosphere can lead to a crossing of the Roche interface and, as a result, matter flows onto the companion star.

This mass transfer has the consequence, on the one hand, that the red giant tries to restore its thermodynamic equilibrium through further expansion, and on the other hand, it causes a loss of orbital torque . As a result, the distance between the two stars decreases, the mass transfer rate continues to increase, the companion star cannot accrete the mass for a short time and a common shell forms, with orbital torque being transferred to the common shell and large parts of the accelerated common shell into the interstellar space are lost (Common Envelope Ejection). Reducing the distance can lead to the two stars merging.

A common envelope phase in a binary star system with a red giant has not yet been observed because of its short duration. The modeling depends heavily on parameters such as viscosity.

At the end of the common envelope phase there can be different forms of binary star systems:

  • Cataclysmic Mutable . Here a white dwarf is orbited in a close binary star system by a main sequence star or subgiant . In the case of single stars, the star develops into a red giant that sheds its atmosphere and the core remains as a white dwarf. In the case of cataclysmic variables, the companion star would orbit within the atmosphere of the former red giant and therefore the binary star system has probably gone through a common envelope phase.
  • The formation of X-ray binary stars of small mass, in which a neutron star or black hole is orbited by a main sequence star at a short orbital distance.
  • In close binary star systems , the proportion of white dwarfs with strong magnetic fields is orders of magnitude higher than that of individual white dwarfs. This magnetic field is interpreted as a consequence of the movement of the companion star through the atmosphere of the red giant.
  • The common shell is a development path to the emergence of blue stragglers . These stars are too massive for their age and therefore have accreted matter from a companion or have merged with the companion.
  • The bipolar structure of many planetary nebulae could be the result of a common shell.
  • A potential formation channel for blue subdwarfs could be the phase of a common shell, in which matter falls back on the white dwarf and this then appears as a hot star with a hydrogen-rich atmosphere.
  • Type IIn supernovae show signs of expansion of the supernova ejecta through a dense circumstellar envelope that may have been created by a common envelope.
  • Type Ia supernovae arise when a white dwarf crosses its Chandrasekhar limit and the degenerative pressure can no longer compensate for gravity. The accretion of matter required for this from a companion is likely to occur predominantly in binary star systems that were brought into a small orbital distance by a common shell phase.
  • A superluminous supernova of type Ia can occur if a white dwarf penetrates the shell of a AGB star during a second common envelope phase and the core of the AGB star is then destroyed and accreted by the white dwarf. In this scenario, the mass of the exploding merger can significantly exceed the Chandrasekhar limit. These supernovae are considerably more luminous than normal Type Ia supernovae and should also show signs of a strong interaction with a dense stellar envelope from the common envelope phase. Supernova PTF 11kx is an example of such a core degenerate scenario .
  • With two white dwarfs in a close binary star system such as B. With the AM Canum Venaticorum stars a common envelope is passed through twice. This is the only way to get two remains of red giants into an orbit with an orbit that takes less than an hour.
  • R-Coronae-Borealis stars are hydrogen-poor and carbon-rich giants whose atmospheres consist of around 98% helium. Their visual brightness drops at irregular intervals by up to 8  mag and rises again to rest brightness over months or years. This is interpreted as a consequence of a darkening of the line of sight by clouds of soot that the star is ejecting. R-Coronae-Borealis stars show a chemical composition that is very different from that of other stars. The most likely development scenario is a merger of a helium and a CO white dwarf. The small distance between these burned-out former cores of red giants, which leads to a merger, is a consequence of going through a common envelope phase twice.
  • If a common shell forms during the migration of a star from the main sequence to the red giant branch, then the core of the star has only a small mass at this point and consists almost exclusively of helium. If the shell of the star is lost in the common envelope, an ELM helium white dwarf is created in a double star. These are white dwarfs with extremely low masses of less than 0.2 solar masses .
  • In addition to double stars with a white dwarf, neutron stars can also go through a common envelope phase. Some of the X-ray binary stars , in which a neutron star accretes matter from a companion, is destroyed when the neutron star immerses in the atmosphere of its companion. The process of merging only takes around 1000 years.
  • The J-type carbon stars differ from normal carbon stars in that they are enriched with nitrogen, have a low 12 C / 13 C isotope ratio and above-average luminosity and are rich in lithium in their stellar atmospheres . It is very unusual in stellar astrophysics for all of these stars to be single stars. Since over 50% of all stars are components of binary star systems, it is assumed that the J-type carbon stars emerged from mergers of two stars. Their chemical composition can be simulated when a helium-rich white dwarf and a red giant go through a common envelope phase, with the white dwarf sinking into the core of the red giant and merging with it.
  • A formation channel for gamma ray bursts could be a common envelope phase in a very massive binary star system. A star loses its outer hydrogen- and helium-rich layers via the shared shell and explodes as a type Ic supernova . Since the two stars rotate bound in the narrow binary star system , a long gamma ray burst with a soft gamma spectrum can arise according to the Collapsar model .
  • EL Canum Venaticorum stars are eclipse-changing binary star systems and consist of an AF dwarf and a precursor of a white dwarf with an extremely low mass of less than 0.35 solar masses. A white dwarf with such a small mass can only arise in an interacting binary star system, since at the current age of the universe red dwarfs have not yet finished the hydrogen burning phase.

Common envelope when two main sequence stars merge

A binary star system can merge before either star has left the main sequence . The cause of the loss of angular momentum can be the radiation of gravitational waves or loss of magnetic angular momentum. In the latter, matter is frozen in the stellar wind in the magnetic field lines and the star has to drag this ionized matter with it as it rotates. Both effects reduce the radius of the orbit in the binary star system and if the distance is too small, the friction leads to a rapid merging. Such a merger has been observed in the Beta Lyrae star V1309 Sco and resulted in a bright red nova . In addition to Beta-Lyrae stars, the W-Ursae-Majoris type contact systems are considered the forerunners of a merger burst, in which the orbital energy is converted into an expansion of the common shell with a temporary increase in luminosity. From the merger, a rapidly rotating giant of the FK-Comae-Berenices type emerges, which in the long term develops into a blue straggler .

Common envelope in overcontact systems

The W-Ursae-Majoris stars are eclipsed overcontact systems that exchange energy through a common shell. Although the masses of the components of these close binary star systems can differ by up to a factor of 10, the two companions have almost the same surface temperature. The W-Ursae-Majoris stars arise as separate binary star systems and come into contact through a loss of angular momentum due to magnetic activity. The W-UMa phase lasts for a few to a few hundred million years, and during the entire time the double star remains embedded in a common envelope. The W-Ursae-Majoris stars should also merge through further loss of angular momentum and form a blue straggler.

Common envelope for eruptive variables

When erupting on variable stars , a shell is ejected and thrown off the star. If this z. B. happens in novae or supernovae in a binary star system, the companion walks for a certain time within a common shell around the common center of gravity. The density of the envelope is usually too low to have a significant impact on the companion, but the companion transfers kinetic energy to the envelope and thereby forms the structure of the nebula . The bipolar form of some nova remnants has been associated with the common envelope phase, e.g. B. with slow novae.

Planets in tight orbits around white dwarfs and blue sub-dwarfs

If there is a common envelope phase, the kinetic energy of the companion immersed in the atmosphere is transferred to the latter and in many cases leads to an ejection of the envelope. This at least partially falls back along the plane of the orbit and forms a disc around the double star system or the single star resulting from the merger. Planets can form in very narrow orbits in this disk, and this seems to be a possible explanation for the observation of planets in short orbits around white dwarfs and blue sub- dwarfs . The planets would not have survived the red giant stage on their current orbits.

In addition to the formation of a planet from an accretion disk, planets that were previously massive can also survive a common envelope phase. As simulation calculations show, they lose part of their mass, particularly due to dynamic pressure when they are immersed in the atmosphere of the red giant. A gas planet with the properties of Jupiter can be turned into an earth-like planet that only consists of the former core of the gas planet.

However, the existence of circumbinary planets around post-common-envelope systems is questioned by other authors. All alleged evidence is based on the light transit time effect in eclipsed binary star systems , whereby the planet leads to a slight shift in the times of minimum brightness due to a change in the common center of mass. If these planets existed, the timing of the occultation could be more accurately predicted, but it is not. Also, the reported orbits of the supposedly found exoplanets are often not dynamically stable. Furthermore, the formation of these circumbinary planets is not without problems. From the cooling age of some white dwarfs in post-common-envelope systems with reported exoplanets, an age of less than a million years has been inferred. This is not enough for a planet to emerge from a protoplanetary disk after the end of the common envelope phase. Gas planets, on the other hand, have not been observed around binary star systems made up of two main sequence stars, the forerunners of the post-common envelope systems. An alternative hypothesis for the irregular cover minima is assumed to be 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 that consist of a main sequence star and a white dwarf. They are the most common result of a common envelope development and the observation of these stars enables the parameters of the common envelope phase such as viscosity to be investigated indirectly. The systems with the shortest cycle times also have the highest probability of showing a cover light change. They often consist of a hot white dwarf and a faint red dwarf . These stars will continue to evolve into a cataclysmic binary star system when the mass transfer from the red to the white dwarf begins.

Individual evidence

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