# Common envelope

In astronomy, a common envelope (CE) is gas that contains a binary star system.[1] 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, X-ray binaries and systems of close double white dwarfs or neutron stars 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 and Type Ia supernovae.

Predictions of the outcome of common envelope evolution are uncertain.[2][3][4]

A common envelope is sometimes confused with a contact binary. 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.[1] In a contact binary system the shared envelope rotates with the binary system and fills an equipotential surface.[5]

Key stages in a common envelope phase. Top: A star fills its Roche lobe. Middle: The companion is engulfed; the core and companion spiral towards one another inside a common envelope. Bottom: The envelope is ejected or the two stars merge.

## Formation

Stages in the life of a binary system as a common envelope is formed. The system has mass ratio M1/M2=3. The black line is the Roche equipotential surface. The dashed line is the rotation axis. (a) Both stars lie within their Roche lobes, star 1 on the left (mass M1 in red) and star 2 on the right (mass M2 in orange). (b) Star 1 has grown to nearly fill its Roche lobe. (c) Star 1 has grown to overfill its Roche lobe and transfer mass to star 2: Roche lobe overflow. (d) Transferred too fast to be accreted, matter has built up around star 2. (e) A common envelope, represented schematically by an ellipse, has formed. Adapted from Fig. 1 of Izzard et al. (2012).[6]

A common envelope is formed in a binary star system when the orbital separation decreases rapidly or one of the stars expands rapidly.[2] The donor star will start mass transfer when it overfills its Roche lobe 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.[7]

## 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.[7] 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 novae but fainter than typical supernovae. 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. 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) powered by the recombination of hydrogen in the envelope. After that the luminosity should decrease rapidly.[7]

Several events that resemble the description above have been observed in past. These events are called luminous red novae (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.[7]

The possible CEEs that have been observed so far include:

## References

1. ^ a b Paczyński, B. (1976). "Common Envelope Binaries". In Eggleton, P.; Mitton, S.; Whelan, J. (eds.). Structure and Evolution of Close Binary Systems. IAU Symposium No. 73. Dordrecht: D. Reidel. pp. 75–80. Bibcode:1976IAUS...73...75P.
2. ^ a b Iben, I.; Livio, M. (1993). "Common envelopes in binary star evolution". Publications of the Astronomical Society of the Pacific. 105: 1373–1406. Bibcode:1993PASP..105.1373I. doi:10.1086/133321.
3. ^ Taam, R. E.; Sandquist, E. L. (2000). "Common Envelope Evolution of Massive Binary Stars". Annual Review of Astronomy and Astrophysics. 38: 113–141. Bibcode:2000ARA&A..38..113T. doi:10.1146/annurev.astro.38.1.113.
4. ^ Ivanova, N.; Justham, S.; Chen, X.; De Marco, O.; Fryer, C. L.; Gaburov, E.; Ge, H.; Glebbeek, E.; Han, Z.; Li, X. D.; Lu, G.; Podsiadlowski, P.; Potter, A.; Soker, N.; Taam, R.; Tauris, T. M.; van den Heuvel, E. P. J.; Webbink, R. F. (2013). "Common envelope evolution: where we stand and how we can move forward". The Astronomy and Astrophysics Review. 21: 59. arXiv:1209.4302. Bibcode:2013A&ARv..21...59I. doi:10.1007/s00159-013-0059-2.
5. ^ Eggleton, P. (2006). Evolutionary Processes in Binary and Multiple Stars. Cambridge: Cambridge University Press. ISBN 978-0521855570.
6. ^ Izzard, R. G.; Hall, P. D.; Tauris, T. M.; Tout, C. A. (2012). "Common envelope evolution". Proceedings of the International Astronomical Union. 7: 95. doi:10.1017/S1743921312010769.
7. Ivanova, N.; Justham, S.; Nandez, J. L. A.; Lombardi, J. C. (2013). "Identification of the Long-Sought Common-Envelope Events". Science. 339 (6118): 433–435. arXiv:1301.5897. Bibcode:2013Sci...339..433I. doi:10.1126/science.1225540. PMID 23349287.
8. ^ "Mystery of Strange Star Outbursts May Be Solved". Retrieved 2015-08-30.
AM Canum Venaticorum star

An AM Canum Venaticorum star (AM CVn star), is a rare type of cataclysmic variable star named after their type star, AM Canum Venaticorum. In these hot blue binary variables, a white dwarf accretes hydrogen-poor matter from a compact companion star.

These binaries have extremely short orbital periods (shorter than about one hour) and have unusual spectra dominated by helium with hydrogen absent or extremely weak. They are predicted to be strong sources of gravitational radiation, strong enough to be detected with the Laser Interferometer Space Antenna.

Antlia

Antlia (; from Ancient Greek ἀντλία) is a constellation in the Southern Celestial Hemisphere. Its name means "pump" in Latin; it represents an air pump. Originally Antlia Pneumatica, the constellation was established by Nicolas-Louis de Lacaille in the 18th century, though its name was later abbreviated by John Herschel. Located close to the stars forming the old constellation of the ship Argo Navis, Antlia is completely visible from latitudes south of 49 degrees north.

Antlia is a faint constellation; its brightest star is Alpha Antliae, an orange giant that is a suspected variable star, ranging between apparent magnitudes 4.22 and 4.29. S Antliae is an eclipsing binary star system, changing in brightness as one star passes in front of the other. Sharing a common envelope, the stars are so close they will one day merge to form a single star. Two star systems with known exoplanets, HD 93083 and WASP-66, lie within Antlia, as do NGC 2997, a spiral galaxy, and the Antlia Dwarf Galaxy.

Bright giant

The luminosity class II in the Yerkes spectral classification is given to bright giants. These are stars which straddle the boundary between ordinary giants and supergiants, based on the appearance of their spectra.

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 overcontact binary. Almost all known contact binary systems are eclipsing binaries; eclipsing contact binaries are known as W Ursae Majoris variables, after their type star, W Ursae Majoris.Contact binaries are not to be confused with common envelopes. Whereas the configuration of two touching stars in a contact binary has a typical lifetime of millions to billions of years, the common envelope is a dynamically unstable phase in binary evolution that either expels the stellar envelope or merges the binary in a timescale of months to years.

DY Centauri

DY Centauri is a variable star in the constellation Centaurus. From its brightness, it is estimated to be 7000 parsecs (23000 light-years) away from Earth.DY Centauri is classified as a R Coronae Borealis variable (RCB), a rare class of supergiant stars which show rapid and irregular decreases in brightness due to the formation of dust clouds on the stellar surface. However, DY Centauri is not an active RCB star anymore, and the last registered obscuration event was in 1934. This seems to be related to evolutionary changes in the star, represented by a very fast horizontal movement across the top of the HR diagram. Spectroscopic and photometric evidence show DY Centuari has increased its effective temperature from 5800 K in 1906 to 24800 K in 2010, while maintaining constant luminosity. As consequence, its visual apparent magnitude has faded from about 11.75 in the beginning of the 20th century to 13.2 in 2010 (due to changes in the bolometric correction), while its radius is calculated to have decreased from 100 R☉ to 8 R☉. There are only three other known stars with this behavior, called hot RCB stars.Periodic changes in the radial velocity of DY Centauri have been detected, indicating that the star in a single-lined spectroscopic binary in an eccentric orbit (e = 0.44) with a period of 39.67 days. The companion star has an estimated minimum mass of 0.2 M☉, so it can be a low mass white dwarf or main sequence star. With an estimated separation of only 10 R☉ at periastron, the system must have interacted in the past when the primary had larger dimensions, forming a common envelope.DY Centauri has a peculiar chemical composition and is poor in hydrogen and rich in helium and carbon, being identified as an extreme helium star (EHe). In comparison to other RCB and EHe stars, however, its hydrogen content is relatively high. Stars of this type are believed to be the product of the merger of two white dwarfs, therefore being single stars, which is inconsistent with the identification of DY Centauri as a close binary. Thus, the origin and evolutionary state of the DY Centauri system remain uncertain. In the future, it is likely that the primary will evolve to a B subdwarf, a class of stars frequently found in binary systems.The spectrum of DY Centauri indicates the presence of a low density expanding nebula around it, formed by gas ionized by ultraviolet radiation from the star. The nebula has an estimated dimension of 1.2 arcseconds and, from its expansion velocity, was probably created about a thousand years ago.

HR 5171

HR 5171, also known as V766 Centauri, is a triple star system in the constellation Centaurus, around 12,000 light years from Earth. It is either a red supergiant or recent post-red supergiant yellow hypergiant, and one of the largest known stars. Its diameter is uncertain but likely to be around either 1300 or 1500 times that of the Sun. It is a contact binary, sharing a common envelope of material with a smaller yellow star, the two orbiting each other every 1,304 ± 6 days.

HW Virginis

HW Virginis, abbreviated HW Vir, is an eclipsing binary system (of the Algol type) approximately 590 light-years away (based on the stellar properties and magnitudes: the Hipparcos trigonometric parallax measurement has too high an error value to be useful) in the constellation of Virgo. The system comprises an eclipsing B-type subdwarf star and red dwarf star. The two stars orbit each other every 0.116795 days.

IPTF14hls

iPTF14hls is an unusual supernova star that has erupted continuously for about 1,000 days before becoming a remnant nebula. It had previously erupted in 1954. None of the theories nor proposed hypotheses fully explain all the aspects of the object.

K 1-2

K1-2 is a planetary nebula in the constellation Pyxis. It was discovered by Czech astronomer Luboš Kohoutek in 1961. The central star of the nebula—VW Pyxidis—is a post-common-envelope binary composed of a hot degenerate star and a cooler companion in a close orbit. A best-fit calculation from its orbit and spectra yields a white dwarf-like star with around 50% of the Sun's mass and a main sequence lie star around 70% as massive as the Sun. Jets of matter are emanating from the system. One study yielded a surface temperature of 85,000 K for the hotter star.

Lead star

A lead star is a low-metallicity star with an overabundance of lead and bismuth as compared to other products of the S-process.

MY Camelopardalis

MY Camelopardalis (MY Cam) is a binary star system located in the Alicante 1 open cluster, some 13 kly (4.0 kpc) away in the constellation Camelopardalis. It is one of the most massive known binary star systems and a leading candidate for a massive star merger. MY Cam is the brightest star in Alicante 1.The system consists of two hot blue O-type stars with one component having a mass of 32 solar masses and the other 38 solar masses. MY Cam is a contact binary and eclipsing binary, with an orbital period of 1.2 days, and an orbital velocity of 1,000,000 km/h (620,000 mph).

NGC 545

NGC 545 is a lenticular galaxy located in the constellation Cetus. It is located at a distance of circa 250 million light years from Earth, which, given its apparent dimensions, means that NGC 545 is about 180,000 light years across. It was discovered by William Herschel on October 1, 1785. It is a member of the Abell 194 galaxy cluster and is included along with NGC 547 in the Atlas of Peculiar Galaxies.

A weak radio source with radio jets has been associated with NGC 545. The short jet crosses the much more prominent jet of NGC 547. Observations of the centre of the galaxy by Hubble Space Telescope didn't reveal the presence of dust or disk features. In the centre of the galaxy is believed to exist a supermassive black hole whose mass is estimated to be about 600 million (108.79) M☉ based on the stellar tidal disruption rate.NGC 545 forms a pair with the equally bright NGC 547, which lies 0.5 arcminutes away. They share a common envelope, however, despite their close position, no tidal features like tails or bridges have been observed. A stellar bridge has been detected between the galaxy pair and NGC 541, which lies 4.5 arcminutes to the southwest (projected distance circa 100 kpc).Observations of the galaxy by the Chandra X-Ray Observatory revealed sharp surface brightness edges on the northeastern part of the galaxy and an extended tail in the soft band. It has been presumed that these are the result of motion of NGC 545 towards the centre of the cluster that has been identified as the location of NGC 547.

NGC 547

NGC 547 is an elliptical galaxy and radio galaxy (identified as 3C 40) located in the constellation Cetus. It is located at a distance of circa 220 million light years from Earth, which, given its apparent dimensions, means that NGC 547 is about 120,000 light years across. It was discovered by William Herschel on October 1, 1785. It is a member of the Abell 194 galaxy cluster and is included along with NGC 547 in the Atlas of Peculiar Galaxies.

NGC 547 is a prominent radio galaxy, with two large radio jets of Fanaroff-Riley class I with wide-angle tails. The galaxy is identified as 3C 40B (3C 40A is less prominent and is associated with the nearby galaxy NGC 541), and the source extends for 10 arcminutes in the south-north direction. A small, smooth, dark feature has been observed running across the nucleus in images by the Hubble Space Telescope. Its projected size is 0.3 kpc and its shape suggests it is the near side of a small dust disk.NGC 547 forms a pair with the equally bright NGC 545, which lies 0.5 arcminutes away. They share a common envelope, however, despite their close position, no tidal features like tails or bridges have been observed. A stellar bridge has been detected between the galaxy pair and NGC 541, which lies 4.5 arcminutes to the southwest (projected distance circa 100 kpc).Observations of the galaxy by the Chandra X-Ray Observatory revealed a large very luminous X-ray corona around the galaxy. The gas distribution appears symmetric, without evidence of tails, indicating its relatively low velocity, and thus it has been identified as the centre of the cluster, with NGC 541 and NGC 545 moving towards it.

NN Serpentis

NN Serpentis (abbreviated NN Ser) is an eclipsing post-common envelope binary system approximately 1670 light-years away. The system comprises an eclipsing white dwarf and red dwarf. The two stars orbit each other every 0.13 days.

Q star

A Q-Star, also known as a grey hole, is a hypothetical type of a compact, heavy neutron star with an exotic state of matter. The Q stands for a conserved particle number. A Q-Star may be mistaken for a stellar black hole.

RR Caeli

RR Caeli is a double star in the constellation Caelum. It is approximately 66 light years from Earth. It was first noted to be a high-proper motion star in 1955 by Jacob Luyten, and given the name LFT 349. Discovered to be an eclipsing binary in 1979, it has a baseline magnitude of 14.36, dimming markedly every 7.2 hours for an interval of around 10 minutes, due to the total eclipse of the brighter star by the fainter one. Its variability in brightness led to its being given the variable star designation RR Caeli in 1984. This star system consists of a red dwarf of spectral type M6 and a white dwarf that orbit each other every seven hours; the former is 18% as massive as the Sun, while the latter has 44% of the Sun's mass. The red dwarf is tidally locked with the white dwarf, meaning it displays the same side to the heavier star. The system is also a post-common-envelope binary, and the red dwarf star is transferring material onto the white dwarf. In approximately 9–20 billion years, RR Caeli will likely become a cataclysmic variable star due to the period's gradual shortening, leading to increasing rates of transfer of hydrogen to the surface of the white dwarf.

Stellar envelope

Stellar envelope may mean:

The region of a star that transports energy from the stellar core to the stellar atmosphere

Common envelope in a binary system

V838 Monocerotis

V838 Monocerotis (V838 Mon) is a red star in the constellation Monoceros about 20,000 light years (6 kpc) from the Sun. The previously unknown star was observed in early 2002 experiencing a major outburst, and was possibly one of the largest known stars for a short period following the outburst. Originally believed to be a typical nova eruption, it was then identified as something completely different. The reason for the outburst is still uncertain, but several conjectures have been put forward, including an eruption related to stellar death processes and a merger of a binary star or planets.

The remnant is evolving rapidly. By 2009 its temperature had increased a bit (since 2005) to 3,270 K and its luminosity was 15,000 times solar, but its radius had decreased to 380 times that of the Sun although the ejecta continues to expand. The opaque ejected dust cloud has completely engulfed a B-type companion.

W Ursae Majoris variable

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 R.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 Sz. Csizmadia and P. Klagyivik. The H-types have a higher mass ratio than ${\displaystyle q=0.72}$ (${\displaystyle q}$ = (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.

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