Type Ia supernova

A Type Ia supernova is a sub-category of cataclysmic variable stars that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion. However, white dwarfs of the common carbon-oxygen variety are capable of further fusion reactions that release a great deal of energy if their temperatures rise high enough.

Physically, white dwarfs with a low rate of rotationcite journal
author=Yoon, S.-C.; Langer, L.
title=Presupernova Evolution of Accreting White Dwarfs with Rotation
journal=Astronomy and Astrophysics | year=2004
volume=419 | issue=2 | pages=623
url=http://www.citebase.org/fulltext?format=application%2Fpdf&identifier=oai%3AarXiv.org%3Aastro-ph%2F0402287
accessdate=2007-05-30
doi=10.1051/0004-6361:20035822
] are limited to masses that are below the Chandrasekhar limit of about 1.38cite journal
author=Mazzali, P. A.; K. Röpke, F. K.; Benetti, S.; Hillebrandt, W.
title=A Common Explosion Mechanism for Type Ia Supernovae
journal=Science | year=2007 | volume=315
issue=5813 | pages=825–828
doi=10.1126/science.1136259
accessdate=2007-05-24
] solar masses. This is the maximum mass that can be supported by electron degeneracy pressure. Beyond this limit the white dwarf would begin to collapse. If a white dwarf gradually accretes mass from a binary companion, its core is believed to reach the ignition temperature for carbon fusion as it approaches the limit. If the white dwarf merges with another star (a very rare event), it will momentarily exceed the limit and begin to collapse, again raising its temperature past the nuclear fusion ignition point. Within a few seconds of initiation of nuclear fusion, a substantial fraction of the matter in the white dwarf undergoes a runaway reaction, releasing enough energy (1-2 × 1044 joules)cite journal
author=Khokhlov, A.; Mueller, E.; Hoeflich, P.
title=Light curves of Type IA supernova models with different explosion mechanisms
journal=Astronomy and Astrophysics
year=1993
volume=270
issue=1-2
pages=223–248
url=http://adsabs.harvard.edu/cgi-bin/bib_query?1993A%26A...270..223K
accessdate=2007-05-22
] to unbind the star in a supernova explosion. [cite web
author=Staff
date=September 7, 2006
url=http://heasarc.gsfc.nasa.gov/docs/objects/snrs/snrstext.html
title=Introduction to Supernova Remnants
publisher=NASA Goddard/SAO
accessdate=2007-05-01
]

This category of supernovae produces consistent peak luminosity because of the uniform mass of white dwarfs that explode via the accretion mechanism. The stability of this value allows these explosions to be used as standard candles to measure the distance to their host galaxies because the visual magnitude of the supernovae depends primarily on the distance.

Consensus model

Citation
last =Matheson
first =Thomas
last2 =Kirshner
first2 =Robert
last3 =Challis
first3 =Pete
last4 =Jha
first4 =Saurabh
year =2008
title =Optical Spectroscopy of Type Ia Supernovae
periodical =Astronomical Journal
url =http://arxiv.org/abs/0803.1705
accessdate = 2008-05-19
]
There are several means by which a supernova of this type can form, but they share a common underlying mechanism. When a slowly-rotating, carbon-oxygen white dwarf accretes matter from a companion, it cannot exceed the Chandrasekhar limit of about 1.38 solar masses, beyond which it would no longer be able to support its weight through electron degeneracy pressurecite journal | author=E. H. Lieb, H.-T. Yau
title=A rigorous examination of the Chandrasekhar theory of stellar collapse
journal=Astrophysical Journal
year=1987 | volume=323 | issue=1 | pages=140–144
url=http://adsabs.harvard.edu/abs/1987ApJ...323..140L
accessdate = 2007-02-01
doi=10.1086/165813
] and begin to collapse. In the absence of a countervailing process, the white dwarf would collapse to form a neutron star, [cite journal | author=R. Canal, J. Gutiérrez
title=The possible white dwarf-neutron star connection
journal=Astrophysics and Space Science Library
year=1997 | volume=214 | pages=49
url=http://adsabs.harvard.edu/abs/1997astro.ph..1225C
accessdate = 2007-02-01
] as normally occurs in the case of a white dwarf that is primarily composed of magnesium, neon and oxygen. [cite web
author=Fryer, C. L.; New, K. C. B.
date =January 24, 2006
url =http://www.livingreviews.org/Articles/Volume6/2003-2new
title =2.1 Collapse scenario
work=Gravitational Waves from Gravitational Collapse
publisher =Max-Planck-Gesellschaft | accessdate = 2007-06-07
]

The current view (among astronomers who model Type Ia supernova explosions) is that this limit is never actually attained, however, so that collapse is never initiated. Instead, the increase in pressure and density due to the increasing weight raises the temperature of the core, and as the white dwarf approaches to within about 1% of the limit [cite book
last = [http://www.as.utexas.edu/~wheel/ J. Craig Wheeler]
first =
title = Cosmic Catastrophes: Supernovae, Gamma-Ray Bursts, and Adventures in Hyperspace
publisher = Cambridge University Press
date = 2000-01-15
location = Cambridge, UK
pages = p. 96
url = http://www.cambridge.org/catalogue/catalogue.asp?isbn=9780521857147
isbn = 0521651956
] , a period of convection ensues, lasting approximately 1,000 years.cite journal
author=W. Hillebrandt, J. C. Niemeyer
title=Type IA Supernova Explosion Models
journal=Annual Review of Astronomy and Astrophysics
year=2000 | volume=38 | pages=191–230
url=http://adsabs.harvard.edu/abs/2000ARA&A..38..191H
accessdate = 2007-02-01
doi=10.1146/annurev.astro.38.1.191
] At some point in this simmering phase, a deflagration flame front is born, powered by carbon fusion. (The details of the ignition are still unknown, including the location and number of points where the flame begins.)cite web | year = 2001
url = http://flash.uchicago.edu/website/research/home.py?submit=science.txt
title = Science Summary
publisher = ASC / Alliances Center for Astrophysical Thermonuclear Flashes
accessdate = 2006-11-27
)] Oxygen fusion is initiated shortly thereafter, but this fuel is not consumed as completely as carbon.cite journal | author = F. K. Röpke, W. Hillebrandt
title = The case against the progenitor's carbon-to-oxygen ratio as a source of peak luminosity variations in Type Ia supernovae
journal = Astronomy and Astrophysics
volume = 420 | pages = L1–L4 | date = 2004
]

Once fusion has begun, the temperature of the white dwarf starts to rise. A main sequence star supported by thermal pressure would expand and cool in order to counter-balance an increase in thermal energy. However, degeneracy pressure is independent of temperature; the white dwarf is unable to regulate the burning process in the manner of normal stars, and is vulnerable to a runaway fusion reaction. The flame accelerates dramatically, in part due to the Rayleigh–Taylor instability and interactions with turbulence. It is still a matter of considerable debate whether this flame transforms into a supersonic detonation from a subsonic deflagration . [cite journal | author = V. N. Gamezo, A. M. Khokhlov, E. S. Oran, A. Y. Chtchelkanova, R. O. Rosenberg
title=Thermonuclear Supernovae: Simulations of the Deflagration Stage and Their Implications
journal=Science
date=January 3, 2003
volume=299 | issue=5603 | pages=77–81
url=http://www.sciencemag.org/cgi/content/full/299/5603/77
accessdate=2006-11-28
doi=10.1126/science.1078129
]

Regardless of the exact details of nuclear burning, it is generally accepted that a substantial fraction of the carbon and oxygen in the white dwarf is burned into heavier elements within a period of only a few seconds, raising the internal temperature to billions of degrees. This energy release from thermonuclear burning (1-2 × 1044 joules) is more than enough to unbind the star; that is, the individual particles making up the white dwarf gain enough kinetic energy that they are all able to fly apart from each other. The star explodes violently and releases a shock wave in which matter is typically ejected at speeds on the order of 5–20,000 km/s, or roughly 3% of the speed of light. The energy released in the explosion also causes an extreme increase in luminosity. The typical visual absolute magnitude of Type Ia supernovae is Mv = −19.3 (≈ 5 billion times brighter than the Sun), with little variation. Whether or not the supernova remnant remains bound to its companion depends on the amount of mass ejected. The theory of this type of supernovae is similar to that of novae, in which a white dwarf accretes matter more slowly and does not approach the Chandrasekhar limit. In the case of a nova, the infalling matter causes a hydrogen fusion surface explosion that does not disrupt the star. This type of supernova differs from a core-collapse supernova, which is caused by the cataclysmic explosion of the outer layers of a massive star as its core implodes.cite journal
last = Gilmore
first = Gerry
title=The Short Spectacular Life of a Superstar
journal=Science
year=2004
volume=304
issue=5697
pages=1915–1916
url=http://www.sciencemag.org/cgi/content/full/sci;304/5679/1915
accessdate=2007-05-01
doi=10.1126/science.1100370
]

Formation

One model for the formation of this category of supernova is a close binary star system. The progenitor binary system consists of main sequence stars, with the primary possessing more mass than the secondary. Being greater in mass, the primary is the first of the pair to evolve onto the asymptotic giant branch, where the star's envelope expands considerably. If the two stars share a common envelope then the system can lose significant amounts of mass, reducing the angular momentum, orbital radius and period. After the primary has degenerated into a white dwarf, the secondary star later evolves in a red giant and the stage is set for mass accretion onto the primary. During this final shared-envelope phase, the two stars spiral in closer together as angular momentum is lost. The resulting orbit can have a period as brief as a few hours. [cite conference | first = B. | last = Paczynski
title = Common Envelope Binaries
booktitle = Structure and Evolution of Close Binary Systems
pages = 75–80
publisher = Dordrecht, D. Reidel Publishing Co.
date = July 28-August 1, 1975
location = Cambridge, England
url = http://adsabs.harvard.edu/abs/1976IAUS...73...75P
accessdate = 2007-01-08
] [cite web | author=K. A. Postnov, L. R. Yungelson
year = 2006
url = http://relativity.livingreviews.org/open?pubNo=lrr-2006-6&page=articlesu8.html
title = The Evolution of Compact Binary Star Systems
publisher = Living Reviews in Relativity
accessdate = 2007-01-08
] If the accretion continues long enough, the white dwarf may eventually approach the Chandrasekhar limit.

A second possible, but much less likely, mechanism for triggering a Type Ia supernova is the merger of two white dwarfs, with the combined mass exceeding the Chandrasekhar limit (which is called a super-Chandrasekhar mass white dwarf). [cite web | author=Staff
url =http://cosmos.swin.edu.au/entries/typeiasupernovaprogenitors/typeiasupernovaprogenitors.html?e=1
title =Type Ia Supernova Progenitors
publisher =Swinburne University | accessdate = 2007-05-20
] [cite news | title=Brightest supernova discovery hints at stellar collision
publisher=New Scientist
date=January 3, 2007
url=http://space.newscientist.com/article/dn10883-brightest-supernova-discovery-hints-at-stellar-collision.html
accessdate=2007-01-06
] In such a case, the total mass would not be constrained by the Chandrasekhar limit. This is one of several explanations proposed for the anomalously massive (2 solar mass) progenitor of the SN 2003fg. [cite news | title=The Weirdest Type Ia Supernova Yet
publisher=Lawrence Berkeley National Laboratory
date=September 20, 2006
url=http://www.lbl.gov/Science-Articles/Archive/Phys-weird-supernova.html
accessdate=2006-11-02
] [cite news | title=Bizarre Supernova Breaks All The Rules
publisher=New Scientist
date=September 20, 2006
url=http://space.newscientist.com/article/dn10114
accessdate=2007-01-08
]

Collisions of solitary stars within our galaxy are thought to occur only once every 107–1013 years; far less frequently than the appearance of novae. [cite journal | last=Whipple | first=Fred L.
title=Supernovae and Stellar Collisions
journal=Proceedings of the National Academy of Sciences of the United States of America
year=1939 | volume=25 | issue=3 | pages=118–125
url=http://adsabs.harvard.edu/abs/1939PNAS...25..118W
accessdate = 2007-02-01
doi=10.1073/pnas.25.3.118
] However, collisions occur with greater frequency in the dense core regions of globular clusters. [cite journal | author=V. C. Rubin, W. K. J. Ford
title=A Thousand Blazing Suns: The Inner Life of Globular Clusters
journal=Mercury
year=1999 | volume=28 | pages=26
url=http://www.astrosociety.org/pubs/mercury/9904/murphy.html
accessdate=2006-06-02
] ("C.f." blue stragglers.) A likely scenario is a collision with a binary star system, or between two binary systems containing white dwarfs. This collision can leave behind a close binary system of two white dwarfs. Their orbit decays and they merge together through their shared envelope. [cite journal | last=Middleditch | first=J.
title=A White Dwarf Merger Paradigm for Supernovae and Gamma-Ray Bursts
journal=The Astrophysical Journal
year=2004 | volume=601 | issue=2 | pages=L167–L170
url=http://adsabs.harvard.edu/abs/2003astro.ph.11484M
accessdate = 2007-02-01
doi=10.1086/382074
]

The white dwarf companion could also accrete matter from other types of companions, including a subgiant or (if the orbit is sufficiently close) even a main sequence star. The actual evolutionary process during this accretion stage remainsuncertain, as it can depend both on the rate ofaccretion and the transfer of angular momentum to the whitedwarf companion. [cite conference
author=Langer, N.; Yoon, S.-C.; Wellstein, S.; Scheithauer, S.
editors=Gänsicke, B. T.; Beuermann, K.; Rein, K.
title =On the evolution of interacting binaries which contain a white dwarf
booktitle =The Physics of Cataclysmic Variables and Related Objects, ASP Conference Proceedings
pages =252 | publisher = Astronomical Society of the Pacific
year=2002 | location =San Francisco, California
url =http://adsabs.harvard.edu/abs/2002ASPC..261..252L
accessdate = 2007-05-25
]

Unlike the other types of supernovae, Type Ia supernovae generally occur in all types of galaxies, including ellipticals. They show no preference for regions of current stellar formation. [cite journal | last = van Dyk | first = Schuyler D.
title=Association of supernovae with recent star formation regions in late type galaxies
journal=Astronomical Journal
year=1992 | volume=103 | issue=6 | pages=1788–1803
url=http://adsabs.harvard.edu/abs/1992AJ....103.1788V
accessdate = 2007-02-01
doi=10.1086/116195
] As white dwarf stars form at the end of a star's main sequence evolutionary period, such a long-lived star system may have wandered far from the region where it originally formed. Thereafter a close binary system may spend another million years in the mass transfer stage (possibly forming persistent nova outbursts) before the conditions are ripe for a Type Ia supernova to occur. [cite journal | author=N. Langer, A. Deutschmann, S. Wellstein, P. Höflich
title=The evolution of main sequence star + white dwarf binary systems towards Type Ia supernovae
journal=Astronomy and Astrophysics
year=1999 | volume=362 | pages=1046–1064
url=http://adsabs.harvard.edu/abs/2000astro.ph..8444L
accessdate = 2007-02-01
]

Light curve

Type Ia supernovae have a characteristic light curve, their graph of luminosity as a function of time after the explosion. Near the time of maximum luminosity, the spectrum contains lines of intermediate-mass elements from oxygen to calcium; these are the main constituents of the outer layers of the star. Months after the explosion, when the outer layers have expanded to the point of transparency, the spectrum is dominated by light emitted by material near the core of the star, heavy elements synthesized during the explosion; most prominently isotopes close to the mass of iron (or iron peak elements). The radioactive decay of nickel-56 through cobalt-56 to iron-56 produces high-energy photons which dominate the energy output of the ejecta at intermediate to late times.

The similarity in the absolute luminosity profiles of nearly all known Type Ia supernovae has led to their use as a secondary [cite journal | author=L.M. Macri, K.Z. Stanek, D. Bersier, L. J. Greenhill, M. J. Reid
title=A New Cepheid Distance to the Maser-Host Galaxy NGC 4258 and Its Implications for the Hubble Constant
journal=Astrophysical Journal
year=2006 | volume= 652 | issue=2 | pages= 1133–1149
url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006ApJ...652.1133M
accessdate = 2007-02-01
doi=10.1086/508530
] standard candle in extragalactic astronomy. [cite journal | author=S. A. Colgate
title=Supernovae as a standard candle for cosmology
journal=Astrophysical Journal
year=1979 | volume=232 | issue=1 | pages=404–408
url=http://adsabs.harvard.edu/abs/1979ApJ...232..404C
accessdate = 2007-02-01
doi=10.1086/157300
] The cause of this uniformity in the luminosity curve is still an open question. In 1998, observations of distant Type Ia supernovae indicated the unexpected result that the universe seems to undergo an accelerating expansion.cite journal | author=B. Leibundgut, J. Sollerman
title=A cosmological surprise: the universe accelerates
journal=Europhysics News
year=2001 | volume=32 | issue=4
url=http://www.eso.org/~bleibund/papers/EPN/epn.html
accessdate = 2007-02-01
] cite news | title=Confirmation of the accelerated expansion of the Universe
publisher=Centre National de la Recherche Scientifique
date=September 19, 2003
url=http://www2.cnrs.fr/en/45.htm?&debut=160xt/
accessdate=2006-11-03
]

ee also

* Carbon detonation
* History of supernova observation
* Supernova remnant
* Extragalactic Distance Scale

References

External links

* cite web
last = Falck | first = Bridget | year=2006
url =http://www.pha.jhu.edu/~bfalck/SeminarPres.html
title =Type Ia Supernova Cosmology with ADEPT
publisher =Johns Hopkins University | accessdate = 2007-05-20

* cite web
author=Staff | date =February 27, 2007
url =http://www.sdss.org/supernova/aboutsprnova.html
title =Sloan Supernova Survey
publisher =Sloan Digital Sky Survey
accessdate = 2007-05-25

* cite web
url =http://www.peripatus.gen.nz/Astronomy/Novae.html
title =Novae and Supernovae
publisher =peripatus.gen.nz | accessdate = 2007-05-25

* cite web
date =August 6, 2003
url =http://www.spaceflightnow.com/news/n0308/06supernova/
title =Source for major type of supernova
publisher =Pole Star Publications Ltd
accessdate = 2007-11-25
(A Type Ia progenitor found)
* cite web
url =http://www.peripatus.gen.nz/Astronomy/Novae.html
title =Novae and Supernovae explosions found
publisher =peripatus.gen.nz | accessdate = 2007-05-25


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