AType Ia supernova (read: "type one-A") is asupernova that occurs inbinary systems (twostars orbiting one another) in which one of the stars is awhite dwarf. The other star can be anything from agiant star to an even smaller white dwarf.[1]
Physically, carbon–oxygen white dwarfs with a low rate of rotation are limited to below 1.44 solar masses (M☉).[2][3] Beyond this "critical mass", they reignite and in some cases trigger a supernova explosion; this critical mass is often referred to as the Chandrasekhar mass, but is marginally different from the absoluteChandrasekhar limit, whereelectron degeneracy pressure is unable to prevent catastrophic collapse. If a white dwarf gradually accretes mass from a binary companion, or merges with a second white dwarf, the general hypothesis is that a white dwarf's core will reach the ignition temperature forcarbon fusion as it approaches the Chandrasekhar mass. Within a few seconds of initiation of nuclear fusion, a substantial fraction of the matter in the white dwarf undergoes arunaway reaction, releasing enough energy (1×1044 J)[4] to unbind the star in a supernova explosion.[5]
The Type Ia category of supernova produces a fairly consistent peak luminosity because of the fixed critical mass at which a white dwarf will explode. Their consistent peak luminosity allows these explosions to be used asstandard candles to measure the distance to their host galaxies: thevisual magnitude of a Type Ia supernova, as observed from Earth, indicates its distance from Earth.
The Type Iasupernova is a subcategory in the Minkowski–Zwicky supernova classification scheme, which was devised by German-American astronomerRudolph Minkowski and Swiss astronomerFritz Zwicky.[7] There are several means by which a supernova of this type can form, but they share a common underlying mechanism. Theoretical astronomers long believed theprogenitor star for this type of supernova is awhite dwarf, and empirical evidence for this was found in 2014 whenSN 2014J was observed in thegalaxyMessier 82.[8] When a slowly-rotating[2]carbon–oxygen white dwarfaccretes matter from a companion, it can exceed the Chandrasekhar limit of about 1.44 M☉, beyond which electron degeneracy pressure will no longer support its mass against gravitational collapse.[9] In the absence of a countervailing process, the white dwarf would collapse to form aneutron star, in an accretion-induced non-ejective process,[10] as normally occurs in the case of a white dwarf that is primarily composed ofmagnesium,neon, and oxygen.[11]
The current view among astronomers who model Type Ia supernova explosions, however, is that this limit is never actually attained and collapse is never initiated. Instead, the increase in pressure and density due to the increasing mass raises the temperature of the core,[3] and as the white dwarf approaches about 99% of the limit,[12] a period ofconvection ensues, lasting approximately 1,000 years.[13] At some point in this simmering phase, adeflagration flame front is born, powered bycarbon fusion. The details of the ignition are still unknown, including the location and number of points where the flame begins.[14]Oxygen fusion is initiated shortly thereafter, but this fuel is not consumed as completely as carbon.[15]
Once fusion begins, the temperature of the white dwarf increases. Amain sequence star supported bythermal pressure can expand and cool which automatically regulates the increase in thermal energy. However,degeneracy pressure is independent of temperature; white dwarfs are unable to regulate temperature in the manner of normal stars, so they are vulnerable torunaway fusion reactions. The flare accelerates dramatically, in part due to theRayleigh–Taylor instability and interactions withturbulence. It is still a matter of considerable debate whether this flare transforms into asupersonicdetonation from asubsonic deflagration.[13][16]
Regardless of the exact details of how the supernova ignites, it is generally accepted that a substantial fraction of the carbon and oxygen in the white dwarf fuses into heavier elements within a period of only a few seconds,[15] with the accompanying release of energy increasing the internal temperature to billions of degrees. The energy released (1–2×1044 J)[17] is more than sufficient tounbind the star; that is, the individual particles making up the white dwarf gain enoughkinetic energy to fly apart from each other. The star explodes violently and releases ashock wave in which matter is typically ejected at speeds on the order of5,000–20,000 km/s, roughly 6% of thespeed of light. The energy released in the explosion also causes an extreme increase in luminosity. The typical visualabsolute magnitude of Type Ia supernovae isMv = −19.3 (about 5 billion times brighter than the Sun), with little variation.[13] The Type Ia supernova leaves no compact remnant, but the whole mass of the former white dwarf dissipates through space.
The theory of this type of supernova is similar to that ofnovae, 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.[13]
Type Ia supernovae differ fromType II supernovae, which are caused by the cataclysmic explosion of the outer layers of a massive star as its core collapses, powered by release ofgravitational potential energy vianeutrino emission.[18]
One model for the formation of this category of supernova is a closebinary 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 theasymptotic 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 theangular momentum, orbital radius andperiod. After the primary has degenerated into a white dwarf, the secondary star later evolves into 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.[19][20] If the accretion continues long enough, the white dwarf may eventually approach theChandrasekhar limit.
The white dwarf companion could also accrete matter from other types of companions, including asubgiant or (if the orbit is sufficiently close) even a main sequence star. The actual evolutionary process during this accretion stage remains uncertain, as it can depend both on the rate of accretion and the transfer of angular momentum to the white dwarf companion.[21]
It has been estimated that single degenerate progenitors account for no more than 20% of all Type Ia supernovae.[22]
A second possible mechanism for triggering a Type Ia supernova is the merger of two white dwarfs whose combined mass exceeds theChandrasekhar limit. The resulting merger is called a super-Chandrasekhar mass white dwarf.[23][24] In such a case, the total mass would not be constrained by the Chandrasekhar limit.
Collisions of solitary stars within the Milky Way occur only once every107 to1013 years; far less frequently than the appearance of novae.[25] Collisions occur with greater frequency in the dense core regions ofglobular clusters[26] (cf.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. Theirorbit decays and they merge through their shared envelope.[27] A study based onSDSS spectra found 15 double systems of the 4,000 white dwarfs tested, implying a double white dwarf merger every 100 years in the Milky Way: this rate matches the number of Type Ia supernovae detected in our neighborhood.[28]
A double degenerate scenario is one of several explanations proposed for the anomalously massive (2 M☉) progenitor ofSN 2003fg.[29][30] It is the only possible explanation forSNR 0509-67.5, as all possible models with only one white dwarf have been ruled out.[31] It has also been strongly suggested forSN 1006, given that no companion star remnant has been found there.[22] Observations made withNASA'sSwift space telescope ruled out existing supergiant or giant companion stars of every Type Ia supernova studied. The supergiant companion's blown out outer shell should emitX-rays, but this glow was not detected by Swift's XRT (X-ray telescope) in the 53 closest supernova remnants. For 12 Type Ia supernovae observed within 10 days of the explosion, the satellite's UVOT (ultraviolet/optical telescope) showed no ultraviolet radiation originating from the heated companion star's surface hit by the supernova shock wave, meaning there were no red giants or larger stars orbiting those supernova progenitors. In the case ofSN 2011fe, the companion star must have been smaller than theSun, if it existed.[32] TheChandra X-ray Observatory revealed that the X-ray radiation of fiveelliptical galaxies and the bulge of theAndromeda Galaxy is 30–50 times fainter than expected. X-ray radiation should be emitted by the accretion discs of Type Ia supernova progenitors. The missing radiation indicates that few white dwarfs possessaccretion discs, ruling out the common, accretion-based model of Ia supernovae.[33] Inward spiraling white dwarf pairs are strongly-inferred candidate sources ofgravitational waves, although they have not been directly observed.
It has been proposed that a group of sub-luminous supernovae should be classified asType Iax.[34][35] This type of supernova may not always completely destroy the white dwarf progenitor, but instead leave behind azombie star.[36] Known examples of Type Iax supernovae include: the historical supernovaSN 1181,SN 1991bg,SN 2002cx, andSN 2012Z.
The supernovaSN 1181 is believed to be associated with the supernova remnant Pa 30 and its central starIRAS 00500+6713, which is the result of a merger of a CO white dwarf and an ONe white dwarf. This makes Pa 30 and IRAS 00500+6713 the only SN Iax remnant in theMilky Way.[37]
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.[39] 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.[40]
A long-standing problem in astronomy has been the identification of supernova progenitors. Direct observation of a progenitor would provide useful constraints on supernova models. As of 2006, the search for such a progenitor had been ongoing for longer than a century.[41] Observation of the supernova SN 2011fe has provided useful constraints. Previous observations with the Hubble Space Telescope did not show a star at the position of the event, thereby excluding ared giant as the source. The expandingplasma from the explosion was found to contain carbon and oxygen, making it likely the progenitor was a white dwarf primarily composed of these elements.[42]Similarly, observations of the nearby SN PTF 11kx,[43] discovered January 16, 2011 (UT) by thePalomar Transient Factory (PTF), lead to the conclusion that this explosion arises from single-degenerate progenitor, with a red giant companion, thus suggesting there is no single progenitor path to SN Ia. Direct observations of the progenitor of PTF 11kx were reported in the August 24 edition of Science and support this conclusion, and also show that the progenitor star experienced periodic nova eruptions before the supernova – another surprising discovery.[43][44]However, later analysis revealed that thecircumstellar material is too massive for the single-degenerate scenario, and fits better the core-degenerate scenario.[45]
In May 2015, NASA reported that theKepler space observatory observed KSN 2011b, a Type Ia supernova in the process of exploding. Details of the pre-nova moments may help scientists better judge the quality of Type Ia supernovae as standard candles, which is an important link in the argument fordark energy.[46]
In July 2019, theHubble Space Telescope took three images of a Type Ia supernova through agravitational lens. This supernova appeared at three different times in the evolution of its brightness due to the differing path length of the light in the three images; at −24, 92, and 107 days from peak luminosity. A fourth image will appear in 2037 allowing observation of the entire luminosity cycle of the supernova.[47]
Type Ia supernovae have a characteristiclight curve, their graph of luminosity as a function of time after the explosion. Near the time of maximal luminosity, the spectrum contains lines of intermediate-mass elements from oxygen tocalcium; 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 (iron-peak elements). Theradioactive decay ofnickel-56 throughcobalt-56 toiron-56 produces high-energyphotons, which dominate the energy output of the ejecta at intermediate to late times.[13]
The use of Type Ia supernovae to measure precise distances was pioneered by a collaboration of Chilean and US astronomers, theCalán/Tololo Supernova Survey.[48] In a series of papers in the 1990s the survey showed that while Type Ia supernovae do not all reach the same peak luminosity, a single parameter measured from the light curve can be used to correct unreddened Type Ia supernovae to standard candle values. The original correction to standard candle value is known as thePhillips relationship[49] and was shown by this group to be able to measure relative distances to 7% accuracy.[50] The cause of this uniformity in peak brightness is related to the amount of nickel-56 produced in white dwarfs presumably exploding near the Chandrasekhar limit.[51]
The similarity in the absolute luminosity profiles of nearly all known Type Ia supernovae has led to their use as a secondary standard candle in extragalactic astronomy.[52]Improved calibrations of theCepheid variable distance scale[53] and direct geometric distance measurements toNGC 4258 from the dynamics ofmaser emission[54] when combined with theHubble diagram of the Type Ia supernova distances have led to an improved value of theHubble constant.
In 1998, observations of distant Type Ia supernovae indicated the unexpected result that theuniverse seems to undergo anaccelerating expansion.[55][56]Three members from two teams were subsequently awarded Nobel Prizes for this discovery.[57]
There is significant diversity within the class of Type Ia supernovae. Reflecting this, a plethora of sub-classes have been identified. Two prominent and well-studied examples include 1991T-likes, an overluminous subclass that exhibits particularly strong ironabsorption lines and abnormally small silicon features,[59] and 1991bg-likes, an exceptionally dim subclass characterized by strong early titanium absorption features and rapid photometric and spectral evolution.[60] Despite their abnormalluminosities, members of both peculiar groups can be standardized by use of thePhillips relation, defined at blue wavelengths, to determinedistance.[61]
