Type Ia supernova

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Artist's impression of a type Ia supernova

Type Ia supernovae are a relatively homogeneous group of supernovae . When lit explosively, they show no signs of hydrogen or helium in their spectra . Their characteristic feature are strong absorption lines of the silicon in the time after the maximum. Type Ia supernovae are also known as thermonuclear supernovae because of their presumed explosion mechanism . In contrast to all other supernova types, there is no surviving central star in their supernova remnants . Type Ia supernovae have long been considered the most precisely known standard candles for determining distances in the field of cosmological distances , but the latest findings have cast doubt on this.

observation

Spectrum of the Type Ia supernova SN1998aq one day after the maximum.

The classification of supernovae Type Ia is primarily based on spectroscopic criteria with the complete absence of hydrogen and helium, as well as the detection of strong silicon lines in the spectrum during the rise and the maximum. The spectral properties, the absolute brightness 15 days after the maximum and the shape of the light curve are almost identical in 70 percent of the type Ia supernovae, the normal type Ia supernovae. The optical spectra contain silicon, oxygen, calcium and magnesium at the time of maximum brightness. From this it is concluded that the outer layers of the material ejected in the supernova explosion consist of chemical elements of medium mass. Simply ionized lines of iron dominate the spectrum approximately two weeks after the maximum. About a month later, during the fog phase, forbidden lines of singly and doubly ionized iron and cobalt as well as absorption lines of calcium begin to appear. The strength of the cobalt lines decreases over time, while the strength of the lines of iron increases. The light curve of type Ia supernovae can be modeled after the radioactive decay of 56 Ni through 56 Co and further to 56 Fe . This also fits in with the time course of the strength of the spectral lines.

The early spectra arise from the scattering of a thermal continuum with P-Cygni profiles , the blue end of which reaches up to 25,000 km / s. The maximum rate of expansion decreases rapidly over time. The observed velocities depend on the chemical elements and suggest a layered structure of the products created in the explosion.

Schematic light curve of type Ia supernovae. The luminosity around the maximum is mainly determined by the radioactive decay of nickel, in the later, slower decay of cobalt.

The light curves of the normal Ia supernovae reach a maximum absolute brightness in the blue and visual of the Johnson system of up to −19.3 mag approximately 19 days after the explosion . Within a month the brightness drops by three magnitudes and then further exponentially with one magnitude per month. In the infrared , a second occurs a month after the first maximum. From the light curves it is estimated that between 0.3 and 0.9 solar masses of 56 Ni are synthesized in the supernovae explosions  . In the radio range, type Ia supernovae, in contrast to core collapse supernovae, cannot be detected. Radio radiation occurs only thousands of years later in the supernova remnants through bremsstrahlung , when the ejected material interacts with interstellar matter .

Subgroups

In addition to the normal type Ia supernovae, there are also weaker and brighter supernovae that do not differ, or only slightly, from the normal Ia supernovae in terms of spectroscopy:

  • The super Chandrasekhar supernovae of type Ia with a share of about nine percent of all Ia supernovae. Their absolute brightnesses are brighter by a maximum of one magnitude. Their light curves can be modeled with 1.5 to 1.8 solar masses of 56 Ni. This mass on a synthesized element exceeds the maximum mass of a white dwarf , the Chandrasekhar limit , of approximately 1.44 solar masses.
  • The .Ia type supernovae or also SN-1991bg-like supernovae account for 15 percent of all Ia supernovae. They achieve a lower absolute maximum brightness of no more than −17 mag. The newly synthesized matter at 56 Ni is only 0.1 solar masses and the light curve falls off faster than in normal Ia supernovae. There is no second maximum in the infrared. There is evidence in the spectrum of carbon not consumed in nuclear fusion .
  • The supernovae of the Iax type or also SN-2002cx-like supernovae contribute to five percent of all Ia supernovae. According to their light curves, these underluminous supernovae only produced 0.2 solar masses at 56 Ni. Their maximum brightnesses reach around −18 mag. The rate of expansion is quite slow and their shells do not become transparent even a year after the explosion.
  • The SN-Ia-CSM subgroup shows faint evidence of sharp lines of hydrogen in late spectra a few weeks to months after the maximum. Depending on the author, the SN-Ia-CSM make up between 0.1 and 1 percent of all type Ia supernovae. The hydrogen lines are likely created by the interaction of the material ejected in the supernova explosion with circumstellar matter.

meaning

Normal supernovae of type Ia are the standard candles for determining distance over cosmological distances. The slightly different light curves can be normalized using the Phillips relationship and then only show a scatter of their absolute brightnesses of 0.1 mag 15 days after the maximum. It was through the application of the Phillips relationship that the accelerated expansion of the universe was discovered, which is currently explained as dark energy . In addition, the supernovae of the interstellar matter add up to 0.7 solar masses of heavy elements, which condense into dust. In addition, they contribute considerable amounts of kinetic energy to the interstellar matter, which can trigger further star formation. The supernova remnants are likely the places where much of cosmic rays are accelerated to near the speed of light.

Home galaxies

SN 2011fe in M101

The frequency for the occurrence of a type Ia supernova is estimated at one to three per hundred years for the Milky Way . Since large parts of the Milky Way cannot be observed due to the extinction by dust of the interstellar matter, supernovae are searched for in nearby galaxies by systematic surveys . Type Ia supernovae occur in all types of galaxies , so unlike core collapse supernovae, they are not associated with massive stars. They are also seen in all types of stellar populations .

In early galaxies ('early' in the Hubble classification) their expansion speeds are systematically lower and the maximum brightness is 0.25 may lower than in late or starburst galaxies . Galaxies with high total masses also show, on average, a lower expansion speed of type Ia supernovae. These relationships also remain valid for high redshifts.

The supernova rate per solar mass is higher for late galaxy types by a factor of 20 than for early galaxies and is inversely linearly dependent on the galaxy mass. In the bulge of galaxies, the supernova rate seems to be lower than in the spiral arms . In the halos the supernovae are fainter than in the spiral arms. Based on these observations, it is assumed that the Type Ia supernovae typically develop from different precursor systems.

Supernova Ia as an exploding white dwarf of the C − O type

In the case of nearby supernovae such as SN 2011fe , conclusions can be drawn about the radius of the exploding star from the time of the shock breakout . This is limited to less than 0.02  solar radii . The synthesized mass of 56 Ni can be calculated from the light curve , which in normal supernovae of type Ia averages 0.5 solar masses. This value is identical to the lower limit of the mass of the precursor star and this combination only occurs with degenerate stars: white dwarfs, neutron stars or the hypothetical quark stars . There is no plausible mechanism by which a neutron star could explode, which is why there is general consensus that the precursor stars of Type Ia supernovae are white dwarfs. White dwarfs can consist primarily of magnesium and neon, or a mixture of carbon and oxygen. Very early spectra of such supernovae show signs of carbon and oxygen. There are thermonuclear reactions known to oxygen mixture carbon to generate all detected in the spectra of the elements of one, while this is not true for magnesium or neon. Hence, it is believed that the precursor stars of type Ia supernovae are CO white dwarfs. A white dwarf can become unstable if a companion in a binary star system transfers matter to the white dwarf and the degenerate star reaches a mass at which the degeneracy pressure can no longer withstand the gravitational pressure. According to the CO white dwarf hypothesis, a signature of the CII can be detected in early spectra about five days before the maximum in 20 percent of the type Ia supernovae. These observations can be interpreted as unburned carbon from the outer layers of the white dwarf or as a result of an asymmetric explosion.

Potential forerunners

So far it has not been possible to unequivocally identify a precursor system of a type Ia supernova in the optical, infrared, UV or X-ray range. This companion will not be in thermal equilibrium for a few thousand years and will have a high space velocity, since its former partner in the binary star system was completely destroyed in the explosion.

Other mechanisms

A number of hypothetical models have been developed that can lead to the destruction of a CO white dwarf by thermonuclear reactions:

  • According to the simply degenerate scenario , a white dwarf in a binary star system receives matter from a hydrogen- or helium-burning companion because the companion exceeds its Roche limit volume . At a certain accretion rate there is a steady hydrogen burning near the surface of the white dwarf with symbiotic stars and the super soft X-ray sources , whereby the mass of the white dwarf increases. Above a certain mass, mostly near the Chandrasekhar limit , an explosive carbon burn begins in the core of the degenerate star .
  • In the double degenerate scenario or double detonation model, the companion of the white dwarf is another white dwarf, which also accretes matter from it . The accreted helium condenses on the surface of the more massive white dwarf and helium flames ignite. This causes a shock wave to run into the core of the white dwarf and ignite the carbon burning there. This model is popular for sub-Chandrasekhar supernovae of the Iax and Ia types. However, it is difficult to explain the homogeneity of the normal SN-Ia, since in the double detonation model the ignition can take place in a wide mass range of the white dwarf.
  • In the double degenerate merger scenario , there is a stable mass transfer from a white dwarf to a heavy companion who is also a white dwarf. If the distance between the two stars is small, the lighter white dwarf breaks in the gravitational field of its companion due to tidal effects . As a result, an accretion disk forms around the surviving white dwarf and the degenerate star gains mass until an explosive carbon burn is triggered inside by the increasing pressure. Computational simulations of this scenario tend to lead to the white dwarf consisting of carbon and oxygen transforming into a neon-magnesium-white dwarf. As with electron capture supernovae , electrons are captured in the nucleus near the Chandrasekhar boundary . The result of this process is not an explosion, but an Accretion Induced Collapse, in which the white dwarf transforms into a neutron star.
  • In the violent merger scenario , type Ia supernovae can also arise if z. B. two white dwarfs collide in a globular cluster . However, this process takes place far too rarely to make a significant contribution to the rate of these supernovae
  • In the core-degenerate scenario , a white dwarf plunges into the extended atmosphere of a AGB star and is slowed down by friction in the common shell . The orbital axis decreases until the white dwarf merges with the core of the AGB star. What remains is a rapidly rotating white dwarf near the Chandrasekhar boundary, whose rate of rotation slowly decreases due to magnetic interaction. The centrifugal force stabilizes the collapse and the white dwarf explodes as a type Ia supernova.

Time Delay

In astrophysics, the time delay describes the distance between the star formation and the explosion as a supernova. From the distribution of the observed time delays, conclusions can be drawn about the population of stars or double stars that end in a Type Ia supernova; it thus serves to discriminate between the models listed in the previous section. This works particularly well in galaxies that have only produced one generation of stars (e.g. some dwarf galaxies ), or in former starburst galaxies , where most of the stars were formed in a short period of time. The result of these investigations suggests two populations of precursor systems:

  • a rapid population that ends as a Type Ia supernova in less than 500 million years;
  • a slow population that exploded within a period between 400 million years and the Hubble period .

The observed distribution of the time delay cannot be represented by just one of the preceding systems described above.

Simulation of the explosion process

In contrast to the quasi-static equilibrium in other phases of the life of stars, a supernova explosion is a highly dynamic process. Therefore, the influence z. B. the turbulence can no longer be described by an average mixture length theory, but the turbulence must be calculated over all scale lengths from micro to macro turbulence. This is not possible with the computing power available today , which is why the physical models must be greatly simplified. So far it has not been possible to simulate the normal type Ia supernovae explosions satisfactorily. This can be the result of excessive simplifications in the modeling, or because the correct predecessor systems or explosion mechanisms have not yet been found.

  • In the Chandrasekhar-mass delayed detonations , the white dwarf should explode when the total mass of the star approaches the Chandrasekhar mass. In order to achieve the chemical composition of the supernova ejection observed in the calculations, the explosion would first have to propagate as a deflagration and later run through the star as a detonation . The transition from a propagation speed with less than the speed of sound to the supersonic speed must be timed extremely precisely so that a deflagration does not take place in the white dwarf, which is not observed in this way. There is also no physical cause for the change in the propagation speed of the thermonuclear reactions. Furthermore, the simulations do not produce more than 0.3 solar masses of nickel, which is too little for normal Type Ia supernovae.
  • In the case of the Sub-Chandrasekhar-mass double detonations , an explosive helium burn should first start near the surface of the white dwarf and a shock wave run into the core of the star, which leads to the ignition of the carbon burn. While the second part is undisputed, the ignition of the helium near the surface in the simulations does not succeed without ad hoc assumptions. According to this, a massive helium layer with a mass of 0.2 solar masses would have to accumulate and ignite. In the process, however, elements of the iron group would be formed that cannot be detected in the observed spectra.
  • The violent mergers and the core-degenerate scenario were not examined very intensively because they should not play a major role in the observed supernovarate. In the case of the Violent Merger, a thermonuclear reaction does appear to be igniting, but it is not clear why they should produce supernovae with such less heterogeneity .

Criticism of the standard model

The standard model for type Ia supernovae has been worked out intensively. But even 40 years after the proposal to understand these supernovae as the result of the destruction of a white dwarf, there are still unsolved problems:

  • The predecessor system cannot simply be degenerate because there are far too few Super Soft X-ray Sources for it . The doubly degenerate scenario, the merging of two white dwarfs, cannot be reconciled with the low polarization of these eruptions.
  • The supernovar rate of 1 to 2 per century for the Milky Way exceeds the birth rate for white dwarfs near the Chandrasekhar mass limit by several orders of magnitude. An increase in mass is also unlikely, since more mass is shed in cataclysmic variables during nova eruptions than was previously accreted . In order to bring the supernovar rate in line with the twofold degenerate scenario, the accretion rate should not exceed 10 −12 solar masses per year, which does not agree with observations in the X-ray range.
  • The evenness of the supernova Ia, so important for the cosmological determination of distance, is an insoluble problem for the twofold degenerate scenario, since the different masses of two white dwarfs merge here.
  • In the simply degenerate scenario, there is the partner problem. The companion star must have already lost its hydrogen shell to prevent a nova outbreak and the transfer rate must be precisely set to avoid constant nuclear reactions on the white dwarf and mixing of the white dwarf. There is no known star model for a companion that meets these requirements.
  • The ejecta mass, derived from the supernova remnants , scatters considerably. If the Chandrasekhar limit mass is a universal constant, this would only be expected in the case of an incomplete deflagration, which is again not compatible with the polarization measurements.

literature

  • Pilar Ruiz-Lapuente: New approaches to SNe Ia progenitors . In: Astrophysics. Solar and Stellar Astrophysics . 2014, arxiv : 1403.4087v1 .
  • Dan Maoz, Filippo Mannucci, Gijs Nelemans: Observational clues to the progenitors of Type-Ia supernovae . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1312.0628v2 .
  • Laura Chomiuk: SN 2011fe: A Laboratory for Testing Models of Type Ia Supernovae . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1307.2721v1 .
  • W. Hillebrandt, M. Kromer, FK Röpke, AJ Ruiter: Towards an understanding of Type Ia supernovae from a synthesis of theory and observations . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1302.6420v1 .
  • Bo Wang, Zhanwen Han: Progenitors of type Ia supernovae . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1204.1155v2 .
  • Dan Maoz, Filippo Mannucci: Type-Ia supernova rates and the progenitor problem, a review . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1111.4492v2 .

Web links

Individual evidence

  1. ↑ Yijung Kang et al .: Early-type host Galaxies of Type Ia supernovae. II. Evidence for Luminosity Evolution in Supernova Cosmology. January 18, 2020, accessed on January 24, 2020 .
  2. ^ Matheson, Thomas et al .: Optical Spectroscopy of Type Ia Supernovae . In: Astronomical Journal . 135, No. 4, 2008, pp. 1598-1615. arxiv : 0803.1705 . bibcode : 2008AJ .... 135.1598M . doi : 10.1088 / 0004-6256 / 135/4/1598 .
  3. ^ L. Clavelli: Six indications of radical new physics in supernovae Ia . In: Astrophysics. Solar and Stellar Astrophysics . 2017, arxiv : 1706.03393v1 .