Editing White dwarf (section)

=== 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>