Hypernova

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Eta Carinae (with Homunculus Nebula), a star that could become a hypernova

A hypernova is a supernova with an electromagnetically radiated energy of more than 10 45 joules assuming a spatially isotropic emission . A hypernova represents the upper end of the super luminous or super bright supernovae .

properties

Hypernovae are divided into three classes according to their light curves and spectral properties:

  • Type I does not show any traces of hydrogen in its spectra.
  • With type II, on the other hand, hydrogen can be detected in the spectra during the explosion.
  • For type R, the tail of the light curve can be described by the radioactive decay by an unusually large amount of 56 Ni . The amount required is in the order of five solar masses .

Compared to core collapse supernovae, hypernovae occur very rarely, with 1,000 to 10,000 core collapse supernovae for each hypernova . They are observed almost exclusively in small galaxies with high star formation rates similar to Magellanic Clouds .

Pair instability supernova

The term hypernova was first used by Woosley & Weaver to describe what is now known as the pair instability supernova . Very massive stars with masses of more than 100 or - depending on the source - even 150 solar masses , reach a temperature of more than 10 10 Kelvin in their core . After the central carbon burn, a process of pair instability begins when extremely high-energy photons are converted into electron - positron pairs and gravitational instability occurs as a result. The cause of this instability is that the mass and gravity do not change when the photons are converted into electron-positron pairs, but the radiation pressure as a counteraction to gravity does not apply . Depending on the mass, the star is either completely torn apart or becomes a black hole . This can produce up to 50 solar masses of 56 Ni, the radioactive decay of which is the main source of energy for the energy shown in the light curve and radiated by the hypernova. Amounts of energy of up to 10 46 joules can be released.

Light curve from the SN 2006gy identified as a pair instability supernova (upper curve) compared to the light curves from core collapse and thermonuclear supernovae

The pair instability supernovae were particularly common in Population  III. These are the first stars that were formed from the three elements (hydrogen, helium and lithium) of primordial nucleosynthesis or from the first subsequent generation. In contrast to today's population I, the vanishingly low metallicity did not limit the intensity of the stellar winds caused by the radiation pressure and thus the upper mass limit of the blue giants to around 150 solar masses. Therefore, hypernovae in the form of pair instability supernovae were much more common in the early universe. Today such massive stars are mainly formed by the merging of two stars in a close binary system.

CSM model

A normal core collapse supernova can release additional energy if the precursor star was a supergiant or a luminous blue variable . Before their gravitational collapse, these stars lost considerable amounts of matter via stellar winds and the matter accelerated during the supernova explosion collided with the circumstellar matter. This type of hypernova shows a broader light curve, as the additional energy through the conversion of kinetic energy into electromagnetic radiation only occurs after the explosion process. It also shows the spectral properties of type IIn supernovae.

The collapsar model

The Kollapsar model describes core collapse supernovae that create a black hole. During the supernova explosion, a protoneutron star and expanding matter are initially created . However, the kinetic energy released in the process is insufficient to break out of the star's surface, and the matter falls back through an accretion disk onto the neutron star, which then exceeds its stable mass limit and collapses into a black hole. If the precursor star rotates fast enough, relativistic jets can form along the axis of rotation and emerge from the star. If the jets are directed towards the earth, they appear as gamma-ray bursts . Even more energy can be released if the protoneutron star has a magnetic field with a magnetic flux density of more than 10 11  Tesla (10 15  Gauss ). The decay of the magnetic field can release energies of up to a few 10 46  joules (10 53  ergs ). The Kollapsar models also include a variant according to which a massive star collapses directly into a black hole and the additional energy of the supernova is generated from the rapid accretion of matter into the black hole. In this scenario, the forerunner of the hypernova is a blue supergiant whose gravitational potential prevents the supernova's shock wave from accelerating most of the atmosphere beyond its escape speed.

Core collapse models

The observed luminosity of hypernovae can also be simulated with traditional gravitational collapse models. The luminosity would arise if the stripped-envelope supernovae succeeds in generating more than 3.5 solar masses at 56 Ni and there is an asymmetrical supernova explosion in the direction of the observer. According to computational simulations, stars with an original mass of more than 100 solar masses and a metallicity that is just sufficient to avoid a pair instability event can produce this amount of radioactive nuclides . However, this is heavily dependent on the little-known rate of mass loss shortly before the explosion as a supernova.

literature

Web links

Individual evidence

  1. ↑ Taichi Kato et al. a .: Massive Stars and their Supernovae . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1008.2144 .
  2. ↑ EPJ van den Heuvel, SF Portegies Zwart: Are Super-Luminous supernovae and Long GRBs produced exclusively in young dense star clusters? In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1303.6961v1 .
  3. ^ SE Woosley, TA Weaver: Theoretical Models for Supernovae . In: MJ Rees, RJ Stoneham (eds.): NATO ASIC Proc. 90: Supernovae: A Survey of Current Research . 1982.
  4. ^ Hans-Thomas Janka: Explosion Mechanisms of Core-Collapse Supernovae . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.2503 .
  5. ↑ L. Muijres, Jorick S. Vink, A. de Koter, R. Hirschi, N. Langer, S.-C. Yoon: Mass-loss predictions for evolved very metal-poor massive stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1209.5934 .
  6. ↑ Sambaran Banerjee, Pavel Kroupa, Seungkyung Oh: The emergence of super-canonical stars in R136-type star-burst clusters . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1208.0826 .
  7. ↑ N. Smith, R. Chornock, W. Li, M. Ganeshalingam, JM Silverman, RJ Foley, AV Filippenko, AJ Barth: SN 2006tf: Precursor Eruptions amd the optically thick regime of extremely Luminous Type IIn supernovae . In: The Astrophysical Journal . tape 686 , 2008, p. 467-484 , doi : 10.1086 / 591021 .
  8. ^ A. Gal-Yam, DC Leonard: A massive hypergiant star as the progenitor of the supernova SN 2005gl . In: Nature . tape 458 , 2009, p. 865–867 , doi : 10.1038 / nature07934 .
  9. ^ SI Fujimoto, N. Nishimura, MA Hashimoto: Nucleosynthesis in Magnetically Driven Jets from Collapsars . In: The Astrophysical Journal . tape 680 , 2008, p. 1350-1358 , doi : 10.1086 / 529416 .
  10. ^ N. Bucciantini: Magnetars and Gamma Ray Bursts . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1204.2658 .
  11. Jump up ↑ D. Vanbeveren, N. Mennekens, W. Van Rensbergen, C. De Loore: Blue supergiant progenitor models of Type II supernovae . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1212.4285 .
  12. ↑ Takashi Yoshida, Shinpei Okita, Hideyuki Umeda: Type Ic core-collapse supernova explosions evolved from very massive stars . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1312.7043v1 .