A post-common envelope binary (PCEB) or pre-cataclysmic variable is a

binary system A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that t ...

consisting of a

white dwarf 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 f ...

or hot subdwarf and a

main-sequence star In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hert ...

or a

brown dwarf Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen ( 1H) into helium in their cores, unlike a main-sequence star. Instead, they have a mass between the most m ...

. The star or brown dwarf shared a

common envelope In astronomy, a common envelope (CE) is gas that contains a binary star 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 commo ...

with the white dwarf progenitor in the

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

phase. In this scenario the star or brown dwarf loses

angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syste ...

as it orbits within the envelope, eventually leaving a main-sequence star and white dwarf in a short-period orbit. A PCEB will continue to lose angular momentum via magnetic braking and

gravitational wave Gravitational waves are waves of the intensity of gravity generated by the accelerated masses of an orbital binary system that propagate as waves outward from their source at the speed of light. They were first proposed by Oliver Heaviside in 1 ...

s and will eventually begin mass-transfer, resulting in a

cataclysmic variable 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 to ...

. While there are thousands of PCEBs known, there are only a few eclipsing PCEBs, also called ePCEBs. Even more rare are PCEBs with a brown dwarf as the secondary. A brown dwarf with a mass lower than 20 might

evaporate Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidi ...

during the common envelope phase and therefore the secondary is supposed to have a mass higher than 20 . The material ejected from the common envelope forms a

planetary nebula A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelated ...

. One in five planetary nebulae are ejected from common envelopes, but this might be an underestimate. A planetary nebula formed by a common envelope system usually shows a bipolar structure. The suspected PCEB HD 101584 is surrounded by a complex nebula. During the common envelope phase the red giant phase of the primary was terminated prematurely, avoiding a stellar merger. The remaining hydrogen envelope of HD 101584 was ejected during the interaction between the red giant and the companion and it now forms the circumstellar medium around the binary. Many eclipsing post-common envelope binaries show variations in the timing of eclipses, the cause of which is uncertain. While orbiting

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

s are often proposed as the cause of these variations, planetary models often fail to predict subsequent changes in eclipse timing. Other proposed causes, such as the

Applegate mechanism The Applegate mechanism (Applegate's mechanism or Applegate effect) explains long term orbital period variations seen in certain eclipsing binaries. As a main sequence star goes through an activity cycle, the outer layers of the star are subject t ...

, often cannot fully explain the observed eclipse timing variations either.

List of post-common envelope binaries

Sorted by increasing

orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets, ...

References

{{Reflist, refs= {{Cite journal, last1=Olofsson, first1=H., last2=Khouri, first2=T., last3=Maercker, first3=M., last4=Bergman, first4=P., last5=Doan, first5=L., last6=Tafoya, first6=D., last7=Vlemmings, first7=W. H. T., last8=Humphreys, first8=E. M. L., last9=Lindqvist, first9=M., last10=Nyman, first10=L., last11=Ramstedt, first11=S., date=March 2019, title=HD 101584: circumstellar characteristics and evolutionary status, journal= Astronomy & Astrophysics, language=en, volume=623, pages=A153, doi=10.1051/0004-6361/201834897, arxiv=1902.02153, bibcode=2019A&A...623A.153O, s2cid=102480818, issn=0004-6361 {{cite journal , last1=Pulley , first1=D. , last2=Sharp , first2=I. D. , last3=Mallett , first3=J. , last4=von Harrach , first4=S. , date=August 2022 , title=Eclipse timing variations in post-common envelope binaries: Are they a reliable indicator of circumbinary companions? , journal=

Monthly Notices of the Royal Astronomical Society ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal covering research in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes letters and papers reporting origina ...

, volume=514 , issue=4 , pages=5725-5738 , doi=10.1093/mnras/stac1676 , arxiv=2206.06919 , bibcode=2022MNRAS.514.5725P Binary stars White dwarfs Stellar phenomena

List of post-common envelope binaries

See also

References