Editing White dwarf (section)

== Binary stars and novae ==
[[File:White dwarfs circling each other and then colliding.gif|right|thumb|The merger process of two co-orbiting white dwarfs produces [[gravitational wave]]s]]
If a white dwarf is in a binary star system and is accreting matter from its companion, a variety of phenomena may occur, including [[nova]]e and Type Ia supernovae. It may also be a [[super-soft x-ray source]] if it is able to take material from its companion fast enough to sustain fusion on its surface.<ref name=di_stefano_et_al1997>
{{cite book
 |author1=Di Stefano, R.
 |author2=Nelson, L. A.
 |author3=Lee, W.
 |author4=Wood, T. H.
 |author5=Rappaport, S.
 |title=Thermonuclear Supernovae
 |contribution=Luminous Supersoft X-ray Sources as Type Ia Progenitors
 |volume=486
 |pages=148–149 |issue=486
 |series=NATO Science Series C: Mathematical and physical sciences
 |editor=P. Ruiz-Lapuente |editor2=R. Canal |editor3=J. Isern
 |publisher=Springer
 |date=1997
 |isbn=978-0-7923-4359-2
 |bibcode=1997ASIC..486..147D
 |doi=10.1007/978-94-011-5710-0_10
 |url=https://cds.cern.ch/record/305422
}}</ref> On the other hand, phenomena in binary systems such as tidal interaction and star–disc interaction, moderated by magnetic fields or not, act on the rotation of accreting white dwarfs. In fact, the (securely known) fastest-spinning white dwarfs are members of binary systems (the fastest one being the white dwarf in CTCV J2056-3014).<ref>{{cite journal |bibcode=2020ApJ...898L..40L|arxiv= 2007.13932|doi= 10.3847/2041-8213/aba618|title=CTCV J2056-3014: An X-Ray-faint Intermediate Polar Harboring an Extremely Fast-spinning White Dwarf|date=2020|last1=Lopes de Oliveira |first1=R.|last2=Bruch |first2=A.|last3=Rodrigues |first3=C. V.|last4=de Oliveira |first4=A. S.|last5=Mukai |first5=K.|journal=The Astrophysical Journal Letters|volume=898 |issue=2 |pages=L40
|s2cid= 220831174|doi-access= free}}</ref> A close binary system of two white dwarfs can lose angular momentum and radiate energy in the form of [[gravitational wave]]s, causing their mutual orbit to steadily shrink until the stars merge.<ref name=hscfa20101116>
{{cite news
 |author1      = Aguilar, David A.
 |author2      = Pulliam, Christine
 |title        = Astronomers Discover Merging Star Systems that Might Explode
 |date         = 16 November 2010
 |publisher    = Harvard-Smithsonian Center for Astrophysics
 |url          = http://www.cfa.harvard.edu/news/2010/pr201024.html
 |access-date   = 16 February 2011
 |archive-url  = https://web.archive.org/web/20110409095557/http://www.cfa.harvard.edu/news/2010/pr201024.html
 |archive-date = 9 April 2011
 |url-status   = live
}}</ref><ref name=hscfa20110713>
{{cite news
 |author       = Aguilar, David A.
 |author2      = Pulliam, Christine
 |title        = Evolved Stars Locked in Fatalistic Dance
 |date         = 13 July 2011
 |publisher    = Harvard-Smithsonian Center for Astrophysics
 |url          = http://www.cfa.harvard.edu/news/2011/pr201119.html
 |access-date   = 17 July 2011
 |archive-url  = https://web.archive.org/web/20110715224123/http://www.cfa.harvard.edu/news/2011/pr201119.html
 |archive-date = 15 July 2011
 |url-status   = live
}}</ref>

=== Type Ia supernovae ===
{{Main|Type Ia supernova}}
The mass of an isolated, nonrotating white dwarf cannot exceed the Chandrasekhar limit of ~&nbsp;{{solar mass|1.4}}. This limit may increase if the white dwarf is rotating rapidly and nonuniformly.<ref>
{{cite journal
 |bibcode=2004A&A...419..623Y
 |arxiv= astro-ph/0402287
 |doi= 10.1051/0004-6361:20035822
 |title=Presupernova evolution of accreting white dwarfs with rotation
 |date=2004
 |last1=Yoon |first1=S.-C.
 |last2=Langer |first2=N.
 |journal=Astronomy and Astrophysics
 |volume=419 |issue=2 |pages=623–644
 |s2cid= 2963085
}}</ref> White dwarfs in [[binary (astronomy)|binary]] systems can accrete material from a companion star, increasing both their mass and their density. As their mass approaches the Chandrasekhar limit, this could theoretically lead to either the explosive ignition of [[nuclear fusion|fusion]] in the white dwarf or its collapse into a neutron star.<ref name="collapse" />

There are two models that might explain the progenitor systems of [[Type Ia supernova]]e: the ''single-degenerate model'' and the ''double-degenerate model''. In the ''single-degenerate model'', a carbon–oxygen white dwarf accretes mass and compresses its core by pulling mass from a companion non-degenerate star.<ref name="sniamodels">
{{cite journal
 |title=Type IA supernova explosion models
 |last1=Hillebrandt |first1=W.
 |last2=Niemeyer |first2=J. C.
 |date=2000
 |journal=Annual Review of Astronomy and Astrophysics
 |volume=38 |pages=191–230
 |arxiv= astro-ph/0006305
 |bibcode=2000ARA&A..38..191H
 |doi= 10.1146/annurev.astro.38.1.191
}}</ref>{{rp|14}} It is believed that [[compression (physical)|compressional]] heating of the core leads to [[carbon detonation|ignition]] of [[carbon burning process|carbon fusion]] as the mass approaches the Chandrasekhar limit.<ref name="sniamodels" /> Because the white dwarf is supported against gravity by quantum degeneracy pressure instead of by thermal pressure, adding heat to the star's interior increases its temperature but not its pressure, so the white dwarf does not expand and cool in response. Rather, the increased temperature accelerates the rate of the fusion reaction, in a [[thermal runaway|runaway]] process that feeds on itself. The [[thermonuclear]] flame consumes much of the white dwarf in a few seconds, causing a Type Ia supernova explosion that obliterates the star.<ref name="osln" /><ref name="sniamodels" /><ref>
{{cite journal
 |bibcode=2006A&A...453..229B
 |arxiv= astro-ph/0603036
 |doi= 10.1051/0004-6361:20054594
 |title=Theoretical light curves for deflagration models of type Ia supernova
 |date=2006
 |last1=Blinnikov |first1=S. I.
 |last2=Röpke |first2=F. K.
 |last3=Sorokina |first3=E. I.
 |last4=Gieseler |first4=M.
 |last5=Reinecke |first5=M.
 |last6=Travaglio |first6=C.
 |last7=Hillebrandt |first7=W.
 |last8=Stritzinger |first8=M.
 |journal=Astronomy and Astrophysics
 |volume=453 |issue= 1
 |pages=229–240
 |s2cid= 15493284
}}</ref> In another possible mechanism for Type&nbsp;Ia supernovae, the ''double-degenerate model'', two carbon–oxygen white dwarfs in a binary system merge, creating an object with mass greater than the [[Chandrasekhar limit]] in which carbon fusion is then ignited.<ref name="sniamodels" />{{rp|14}} In both cases, the white dwarfs are not expected to survive the Type Ia supernova.<ref name=":6">{{cite journal |last1=Maoz |first1=D. |last2=Mannucci |first2=F. |date=2012-01-18 |title=Type-Ia Supernova Rates and the Progenitor Problem: A Review |url=https://www.cambridge.org/core/journals/publications-of-the-astronomical-society-of-australia/article/typeia-supernova-rates-and-the-progenitor-problem-a-review/7E4AB6A714BF2FBE92DEA13AAFDF1E0D |journal=Publications of the Astronomical Society of Australia |language=en |volume=29 |issue=4 |pages=447–465 |doi=10.1071/AS11052 |issn=1448-6083|arxiv=1111.4492 |bibcode=2012PASA...29..447M }}</ref>

The ''single-degenerate model'' was the favored mechanism for Type Ia supernovae, but now, because of observations, the ''double-degenerate model'' is thought to be the more likely scenario. Predicted rates of white dwarf-white dwarf mergers are comparable to the rate of Type Ia supernovae and would explain the lack of hydrogen in the spectra of Type Ia supernovae.<ref name=":7">{{cite journal |last1=Wang |first1=Bo |last2=Han |first2=Zhanwen |date=2012-06-01 |title=Progenitors of type Ia supernovae |url=https://www.sciencedirect.com/science/article/pii/S138764731200022X |journal=New Astronomy Reviews |volume=56 |issue=4 |pages=122–141 |doi=10.1016/j.newar.2012.04.001 |issn=1387-6473|arxiv=1204.1155 |bibcode=2012NewAR..56..122W }}</ref> However, the main mechanism for Type Ia supernovae remains an open question.<ref>{{cite journal |last1=Maoz |first1=Dan |last2=Mannucci |first2=Filippo |last3=Nelemans |first3=Gijs |date=2014-08-18 |title=Observational Clues to the Progenitors of Type Ia Supernovae |url=https://www.annualreviews.org/doi/10.1146/annurev-astro-082812-141031 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=52 |issue=1 |pages=107–170 |doi=10.1146/annurev-astro-082812-141031 |issn=0066-4146|arxiv=1312.0628 |bibcode=2014ARA&A..52..107M }}</ref> In the single-degenerate scenario, the accretion rate onto the white dwarf needs to be within a narrow range dependent on its mass so that the hydrogen burning on the surface of the white dwarf is stable. If the accretion rate is too low, novae on the surface of the white dwarf will blow away accreted material. If it is too high, the white dwarf will expand and the white dwarf and companion star will be in a common envelope. This stops the growth of the white dwarf thus preventing it from reaching the Chandrasekhar limit and exploding.<ref name=":7" /> For the single-degenerate model its companion is expected to survive, but there is no strong evidence of such a star near Type Ia supernovae sites.<ref name=":6" /> In the double-degenerate scenario, white dwarfs need to be in very close binaries; otherwise their inspiral time is longer than the [[age of the universe]]. It is also likely that instead of a Type Ia supernova, the merger of two white dwarfs will lead to core-collapse. As a white dwarf accretes material quickly, the core can ignite off-center, which leads to gravitational instabilities that could create a [[neutron star]].<ref name=":6" />

The historical bright [[SN 1006]] is thought to have been a Type Ia supernova from a white dwarf, possibly the merger of two white dwarfs.<ref name="hernandez2012">
{{cite journal
 |last1=González Hernández |first1=J.I.
 |last2=Ruiz-Lapuente |first2=P.
 |last3=Tabernero |first3=H. M.
 |last4=Montes |first4 =D.
 |last5=Canal |first5=R.
 |last6=Méndez |first6=J.
 |last7=Bedin |first7=L. R. 
 |title=No surviving evolved companions of the progenitor of SN 1006 
 |year=2012
 |journal=Nature 
 |volume=489 
 |issue=7417 
 |pages=533–536 
 |pmid=23018963
 |arxiv=1210.1948 
 |bibcode=2012Natur.489..533G
 |doi=10.1038/nature11447 |s2cid=4431391
}}</ref> [[Tycho's Supernova]] of 1572 was also a type&nbsp;Ia supernova, and its remnant has been detected.<ref name="Krause2008">{{cite journal |last1=Krause |first1=Oliver |display-authors=etal |date=2008 |title=Tycho Brahe's 1572 supernova as a standard type Ia as revealed by its light-echo spectrum |journal=Nature |volume=456 |issue=7222 |pages=617–619 |doi=10.1038/nature07608 |pmid=19052622 |bibcode=2008Natur.456..617K |arxiv=0810.5106|s2cid=4409995 }}</ref> [[WD 0810–353]], a white dwarf 11 parsecs away from the Sun, is possibly a [[hypervelocity star|hypervelocity runaway]] ejected from a Type Ia supernova, though this has been disputed.<ref name="dlFM2022">
{{cite journal
 | last1=de la Fuente Marcos | first1=Raúl | last2=de la Fuente Marcos | first2=Carlos
 | title=Deep and fast Solar System flybys: The controversial case of WD 0810-353 
 | journal=[[Astronomy & Astrophysics]]
 | url=https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/202245020
 | volume= 668| year=2022 | pages=A14 | issn=0004-6361
 | doi=10.1051/0004-6361/202245020 | bibcode=2022A&A...668A..14D | arxiv=2210.04863| s2cid=252863734
}}</ref><ref>{{cite journal|last1=Landstreet |first1=J. D. |last2=Villaver |first2=E. |last3=Bagnulo |first3=S. |year=2023 |title=Not so fast, not so furious: just magnetic |journal=The Astrophysical Journal |volume=952 |number=2 |page=129 |doi=10.3847/1538-4357/acdac8 |doi-access=free |arxiv=2306.11663|bibcode=2023ApJ...952..129L }}</ref>

=== Post-common envelope binary ===
{{Main|Post common envelope binary}}
A post-common envelope binary (PCEB) is a binary consisting of a white dwarf or [[hot subdwarf]] and a closely tidally-locked red dwarf (in other cases this might be a [[brown dwarf]] instead of a red dwarf).<ref>{{cite journal|title=A Catalog of Potential Post–Common Envelope Binaries |first1=Matthias U. |last1=Kruckow |first2=Patrick G. |last2=Neunteufel |first3=Rosanne |last3=Di Stefano |first4=Yan |last4=Gao |first5=Chiaki |last5=Kobayashi |journal=The Astrophysical Journal |date=2021 |arxiv=2107.05221 |volume=920 |number=2 |page=86 |doi=10.3847/1538-4357/ac13ac|doi-access=free |bibcode=2021ApJ...920...86K }}</ref> These binaries form when the red dwarf is engulfed in the [[red giant]] phase. As the red dwarf orbits inside the [[common envelope]], it is slowed down in the denser environment. This slowed orbital speed is compensated with a decrease of the orbital distance between the red dwarf and the core of the red giant. The red dwarf spirals inwards towards the core and might merge with the core. If this does not happen and instead the common envelope is ejected, then the binary ends up in a close orbit, consisting of a white dwarf and a red dwarf. This type of binary is called a post-common envelope binary. The evolution of the PCEB continues as the two dwarf stars orbit closer and closer due to [[magnetic braking (astronomy)|magnetic braking]] and by releasing gravitational waves. The binary might then evolve into one of several dramatic outcomes: a high-field magnetic white dwarf, a white dwarf pulsar, a double-degenerate binary, or even a Type Ia supernova.<ref>{{cite journal|first1=J. |last1=Nordhaus |first2=S. |last2=Wellons |first3=D. S. |last3=Spiegel |first4=B. D. |last4=Metzger |first5=E. G. |last5=Blackman |title=Formation of high-field magnetic white dwarfs from common envelopes |journal=PNAS |volume=108 |number=8 |pages=3135–3140 |doi=10.1073/pnas.1015005108 |year=2011|doi-access=free |pmid=21300910 |pmc=3044383 |arxiv=1010.1529 |bibcode=2011PNAS..108.3135N }}</ref><ref>{{cite journal|first1=A. |last1=Rebassa-Mansergas |first2=E. |last2=Solano |first3=F. M. |last3=Jiménez-Esteban |first4=S. |last4=Torres |first5=C. |last5=Rodrigo |first6=A. |last6=Ferrer-Burjachs |first7=L. M. |last7=Calcaferro |first8=L. G. |last8=Althaus |first9=A. H. |last9=Córsico |title=White dwarf–main-sequence binaries from Gaia EDR3: the unresolved 100 pc volume-limited sample |journal=Monthly Notices of the Royal Astronomical Society |volume=506 |number=4 |date=October 2021 |pages=5201–5211 |doi=10.1093/mnras/stab2039|doi-access=free |arxiv=2107.06303 }}</ref> Because a PCEB may evolve at some point into a [[cataclysmic variable]], some of them are also called pre-cataclysmic variables.<ref>{{cite journal|first1=M. R. |last1=Schreiber |first2=B. T. |last2=Gänsicke |title=The age, life expectancy, and space density of Post Common Envelope Binaries |journal=Astronomy and Astrophysics |volume=406 |pages=305–321 |year=2003 |doi=10.1051/0004-6361:20030801|arxiv=astro-ph/0305531 |bibcode=2003A&A...406..305S }}</ref><ref name="GaiaDR3"/>

=== Cataclysmic variables ===
{{Main|Cataclysmic variable star}}
Before accretion of material pushes a white dwarf close to the Chandrasekhar limit, accreted hydrogen-rich material on the surface may ignite in a less destructive type of thermonuclear explosion powered by [[Nuclear fusion|hydrogen fusion]]. These surface explosions can be repeated as long as the white dwarf's core remains intact. This weaker kind of repetitive cataclysmic phenomenon is called a (classical) nova. Astronomers have also observed [[dwarf nova]]e, which have smaller, more frequent luminosity peaks than the classical novae. These are thought to be caused by the release of [[gravitational potential energy]] when part of the [[accretion disc]] collapses onto the star, rather than through a release of energy due to fusion. In general, binary systems with a white dwarf accreting matter from a stellar companion are called [[cataclysmic variable]]s. As well as novae and dwarf novae, several other classes of these variables are known, including [[Polar (star)|polars]] and [[intermediate polar]]s, both of which feature highly magnetic white dwarfs.<ref name="osln" /><ref name="sniamodels" /><ref name="nasa1">{{cite web |url=http://imagine.gsfc.nasa.gov/docs/science/know_l2/cataclysmic_variables.html |series=Imagine the Universe! |title=Cataclysmic Variables |archive-url=https://web.archive.org/web/20070709185919/http://imagine.gsfc.nasa.gov/docs/science/know_l2/cataclysmic_variables.html |archive-date=9 July 2007 |department=fact sheet |publisher=NASA Goddard |access-date=4 May 2007}}</ref><ref name="nasa2">{{cite web |url=http://heasarc.gsfc.nasa.gov/docs/objects/cvs/cvstext.html |title=Introduction to Cataclysmic Variables (CVs) |archive-url=https://web.archive.org/web/20120206213752/http://heasarc.gsfc.nasa.gov/docs/objects/cvs/cvstext.html |archive-date=6 February 2012 |url-status=live |department=fact sheet |publisher=NASA Goddard |access-date=4 May 2007}}</ref> Both fusion- and accretion-powered cataclysmic variables have been observed to be X-ray sources.<ref name="nasa2" />

=== Other multiple-star systems ===
Other binaries include those that consist of a [[Main-sequence star|main sequence star]] (or giant) and a white dwarf. The binary Sirius AB is an example pair of this type.<ref>{{cite book|first1=Andrew |last1=Fraknoi |display-authors=etal |title=Astronomy 2e |publisher=OpenStax |isbn=978-1-951693-50-3 |year=2024 |chapter=18.4 The H–R Diagram |chapter-url=https://openstax.org/books/astronomy-2e/pages/18-4-the-h-r-diagram |page=618}}</ref> White dwarfs can also exist as binaries or multiple star systems that only consist of white dwarfs. An example of a resolved triple white dwarf system is [[WD J1953−1019]], discovered with [[Gaia DR2]] data.<ref>{{cite journal|first1=M. |last1=Perpinyà-Vallès |first2=A. |last2=Rebassa-Mansergas |first3=B. T. |last3=Gänsicke |first4=S. |last4=Toonen |first5=J. J. |last5=Hermes |first6=N. P. |last6=Gentile Fusillo |first7=P.-E. |last7=Tremblay |title=Discovery of the first resolved triple white dwarf |journal=Monthly Notices of the Royal Astronomical Society |volume=483 |number=1 |date=February 2019 |pages=901–907 |doi=10.1093/mnras/sty3149|doi-access=free |arxiv=1811.07752 }}</ref> One interesting field is the study of [[#Debris disks and planets|remnant planetary systems]] around white dwarfs. It is expected that planets orbiting several [[Astronomical unit|AU]] from a star will survive the star's post-main-sequence transformation into a white dwarf. Moreover, white dwarfs, being much smaller and correspondingly less luminous than their progenitors, are less likely to outshine any bodies in orbit around them. This makes white dwarfs advantageous targets for direct-imaging searches for [[exoplanet]]s and [[brown dwarf]]s. The first brown dwarf to be detected by direct imaging was the companion to the white dwarf [[GD 165|GD 165 A]], discovered in 1988.<ref>{{cite journal|first1=Wolfgang |last1=Brandner |first2=Hans |last2=Zinnecker |first3=Taisiya |last3=Kopytova |title=Search for giant planets around seven white dwarfs in the Hyades cluster with the Hubble Space Telescope |journal=Monthly Notices of the Royal Astronomical Society |year=2021 |volume=500 |issue=3 |pages=3920–3925 |doi=10.1093/mnras/staa3422 |doi-access=free |arxiv=2011.03562}}</ref> More recently, the white dwarf [[WD 0806−661]] was found to have a cold companion body of substellar mass, variously described as a brown dwarf<ref>{{cite journal | last1=Luhman |first1=K. L. |last2=Burgasser |first2=A. J. |last3=Bochanski |first3=J. J. | year=2011 | title=Discovery of a Candidate for the Coolest Known Brown Dwarf | journal=The Astrophysical Journal Letters |volume=730 |number=1 |pages=L9 |arxiv=1102.5411 |doi=10.1088/2041-8205/730/1/L9 |bibcode=2011ApJ...730L...9L }}</ref><ref>{{cite journal |last1=Rodriguez |first1=David R. |last2=Zuckerman |first2=B. |last3=Melis |first3=Carl |last4=Song |first4=Inseok |date=May 2011 |title=The Ultra Cool Brown Dwarf Companion of WD 0806-661B: Age, Mass, and Formation Mechanism |journal=The Astrophysical Journal Letters |volume=732 |number=2 |pages=L29 |doi=10.1088/2041-8205/732/2/L29 |arxiv=1103.3544 |bibcode=2011ApJ...732L..29R }}</ref> or an exoplanet.<ref>{{cite journal|first1=Dimitri |last1=Veras |first2=N. Wyn |last2=Evans |title=Exoplanets beyond the Solar neighbourhood: Galactic tidal perturbations |journal=Monthly Notices of the Royal Astronomical Society |volume=430 |number=1 |date=21 March 2013 |pages=403–415 |doi=10.1093/mnras/sts647 |doi-access=free |arxiv=1212.4150}}</ref>