![Common envelope phase - ejection or merger](https://upload.wikimedia.org/wikipedia/commons/6/61/Common_envelope_phase_-_ejection_or_merger.svg)
In
astronomy
Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
, a common envelope (CE) is gas that contains a
binary star
A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...
system.
The gas does not rotate at the same rate as the embedded binary system. A system with such a configuration is said to be in a common envelope phase or undergoing common envelope evolution.
During a common envelope phase the embedded binary system is subject to drag forces from the envelope which cause the separation of the two stars to decrease. The phase ends either when the envelope is ejected to leave the binary system with much smaller orbital separation, or when the two stars become sufficiently close to merge and form a single star. A common envelope phase is short-lived relative to the lifetime of the stars involved.
Evolution through a common envelope phase with ejection of the envelope can lead to the formation of a binary system composed of a
compact object with a close companion.
Cataclysmic variables
In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since ones with an outburst brightness visible t ...
,
X-ray binaries
X-ray binaries are a class of binary stars that are luminous in X-rays.
The X-rays are produced by matter falling from one component, called the ''donor'' (usually a relatively normal star), to the other component, called the ''accretor'', which ...
and systems of close double
white dwarfs
A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...
or
neutron stars
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...
are examples of systems of this type which can be explained as having undergone common envelope evolution. In all these examples there is a compact remnant (a white dwarf, neutron star or black hole) which must have been the core of a star which was much larger than the current orbital separation. If these systems have undergone common envelope evolution then their present close separation is explained. Short-period systems containing compact objects are sources of
gravitational waves
Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that Wave propagation, propagate as waves outward from their source at the speed of light. They were first proposed by Oliv ...
and
Type Ia supernovae
A Type Ia supernova (read: "type one-A") is a type of supernova that occurs in binary systems (two stars orbiting one another) in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white ...
.
Predictions of the outcome of common envelope evolution are uncertain.
A common envelope is sometimes confused with a
contact binary
In astronomy, a contact binary is a binary star system whose component stars are so close that they touch each other or have merged to share their gaseous envelopes. A binary system whose stars share an envelope may also be called an overconta ...
. In a common envelope binary system the envelope does not generally rotate at the same rate as the embedded binary system; thus it is not constrained by the equipotential surface passing through the L2
Lagrangian point
In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the influence of two massive orbiting bodies. Mathematically, this involves the solution of th ...
.
In a contact binary system the shared envelope rotates with the binary system and fills an equipotential surface.
Formation
![Common envelope evolution](https://upload.wikimedia.org/wikipedia/commons/7/78/Common_envelope_evolution.svg)
A common envelope is formed in a binary star system when the orbital separation decreases rapidly or one of the stars expands rapidly.
The donor star will start mass transfer when it overfills its
Roche lobe
In astronomy, the Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star. It is an approximately teardrop-shaped region bounded by a critical gravitational equipotential, wit ...
and as a consequence the orbit will shrink further causing it to overflow the Roche lobe even more, which accelerates the mass transfer, causing the orbit to shrink even faster and the donor to expand more. This leads to the run-away process of dynamically unstable mass transfer. In some case the receiving star is unable to accept all material, which leads to the formation of a common envelope engulfing the companion star.
[
]
Evolution
The donor's core does not participate in the expansion of the stellar envelope and the formation of the common envelope, and the common envelope will contain two objects: the core of the original donor and the companion star. These two objects (initially) continue their orbital motion inside the common envelope. However, it is thought that because of drag forces inside the gaseous envelope, the two objects lose energy, which brings them in a closer orbit and actually increases their orbital velocities. The loss of orbital energy is assumed to heat up and expand the envelope, and the whole common-envelope phase ends when either the envelope is expelled into space, or the two objects inside the envelope merge and no more energy is available to expand or even expel the envelope.[ This phase of the shrinking of the orbit inside the common envelope is known as a spiral-in.
]
Observational manifestations
Common envelope events (CEEs) are difficult to observe. Their existence has been mainly inferred indirectly from presence in the Galaxy of binary systems that can not be explained by any other mechanism. Observationally CEEs should be brighter than typical nova
A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramati ...
e but fainter than typical supernovae
A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when a ...
. The photosphere of the common envelope should be relatively cool—at about 5,000 K—emitting a red spectrum. However its large size should lead to a large luminosity—on the order of that of a red supergiant
Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) of spectral type K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelgeuse and Anta ...
. A common envelope event should begin with a sharp rise in luminosity followed by a few months long plateau of constant luminosity (much like that of type II-P supernova
A Type II supernova (plural: ''supernovae'' or ''supernovas'') results from the rapid collapse and violent explosion of a massive star. A star must have at least 8 times, but no more than 40 to 50 times, the mass of the Sun () to undergo th ...
) powered by the recombination of hydrogen in the envelope. After that the luminosity should decrease rapidly.
Several events that resemble the description above have been observed in past. These events are called luminous red nova
A luminous red nova (abbr. ''LRN'', pl. ''luminous red novae'', pl.abbr. ''LRNe'') is a stellar explosion thought to be caused by the merging of two stars. They are characterised by a distinct red colour, and a light curve that fades slowly with ...
e (LRNe). They are subset of a broader class of events called intermediate-luminosity red transients (ILRTs). They have relatively slow expansion velocities of 200–1000 km/s and total radiated energies are 1038 to 1040 J.[
The possible CEEs that have been observed so far include:
* ]M85 OT2006-1 M85 or M-85 may refer to:
Military
* M85 machine gun, a machine-gun used in the M60 Patton series of tanks
* Zastava M85, an assault rifle developed and manufactured by Zastava Arms
* Parker-Hale M85 a sniper rifle.
Places
* Messier 85, a lenticul ...
, possible ejection of the whole envelope.
* V1309 Scorpii
V1309 Scorpii (also known as V1309 Sco) is a contact binary that merged into a single star in 2008 in a process known as a luminous red nova. It was the first star to provide conclusive evidence that contact binary systems end their evolution ...
, a possible star merger.
* M31 RV
M31-RV is a possible red cataclysmic variable star located in the Andromeda Galaxy (M31) that experienced an outburst in 1988, which is similar to the outburst V838 Monocerotis experienced in 2002. Such objects have been called luminous r ...
* V838 Monocerotis
V838 Monocerotis (Nova Monocerotis 2002) is a spectroscopic binary star system in the constellation Monoceros (constellation), Monoceros about 19,000 light years (6 parsec, kpc) from the Sun. The previously unremarked star was observed i ...
* Ou 5
Ou 5, also known as IPHASXJ211420.0+434136, is a planetary nebula in the constellation of Cygnus. It was discovered by the IPHAS project in data taken on August 1, 2010, and independently by the French amateur astronomer Nicolas Outters in Sept ...
, a planetary nebula whose progenitor was a common envelope binary
See also
*Binary star
A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...
* Binary system (astronomy)
*Contact binary
In astronomy, a contact binary is a binary star system whose component stars are so close that they touch each other or have merged to share their gaseous envelopes. A binary system whose stars share an envelope may also be called an overconta ...
*Post common envelope binary
A post-common envelope binary (PCEB) or pre-cataclysmic variable is a binary system consisting of a white dwarf or hot subdwarf and a main-sequence star or a brown dwarf. The star or brown dwarf shared a common envelope with the white dwarf prog ...
References
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Stellar astronomy
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