Pair-instability supernova

From Wikipedia, the free encyclopedia

This illustration explains the pair-instability supernova process that astronomers think triggered the explosion in SN 2006gy. When a star is very massive, the gamma-rays produced in its core can become so energetic that some of their energy is drained away into production of particle and anti-particle pairs. The resulting drop in pressure causes the star to partially collapse under its own huge gravity. After this violent collapse, runaway thermonuclear reactions (not shown here) ensue and the star explodes, spewing the remains into space.

A pair instability supernova occurs when pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, reduces thermal pressure inside a supermassive star's core. This pressure drop leads to a partial collapse, then greatly accelerated burning in a runaway thermonuclear explosion which blows the star completely apart without leaving a black hole remnant behind.^[1] Pair instability supernovae can only happen in stars with a mass range from around 130 to 250 solar masses and moderate metallicity (low abundance of elements other than hydrogen and helium, a situation common in Population III stars). The recently observed object SN 2006gy is hypothesized to have been a pair instability supernova.

[edit] Physics

[edit] Photon pressure

In very large hot stars, pressure from fusion reaction gamma rays in the stellar core keeps the upper layers of the star supported against gravitational pull from the core. If the stream of gamma rays is reduced, then the outer layers of the star will start to be pulled inwards in a gravitational collapse.

[edit] Pair creation

Pair creation results from gamma-atomic nucleus reactions interacting via the coulomb force (see Pair production by gamma rays). The pair creation cross section for a given material is strongly dependent on the energy of the gamma ray photon - as the gamma rays get more energetic, they are more likely to interact with the atoms they pass through. From Einstein's equation E = mc², gamma rays must have more energy than the mass of the electron-positron pairs, to produce those pairs.

As described in the introduction, the results of pair creation interactions are pairs of electrons and positrons. These particles are released into the star's core and usually recombine (releasing another gamma ray) in very short time periods.

Even though the energy is typically re-released rapidly by the recombination of the electron and positron, the speed at which energy (radiation) transfers through a gas is highly dependent on the average distance between interactions. A photon that is absorbed in pair creation interactions effectively is stopped, and then re-radiated in a random direction.

[edit] Gamma ray production

Gamma rays are produced directly by some of the thermonuclear reactions in stars, and emitted as part of the black body spectrum light emission from the hot gas in the stellar core. Total energy emitted by a material is proportional to the fourth power of the temperature (Stefan-Boltzmann law), and its peak wavelength decreases with temperature as well (Wien's displacement law). The hotter the material, the brighter it is, and the more high energy photons (gamma rays) will be produced.

[edit] Gamma ray absorption

The average distance that gamma rays can travel through matter before they are absorbed (optical thickness) depends on the material (hydrogen has a low cross section; metals much more) and energy of the gamma ray. At low energy, the photoelectric effect and Compton scattering dominate. As gammas get more energetic, the photoelectric and Compton effects are reduced, and the gammas can travel further on the average. Eventually, as gamma energy increases, pair production starts to become significant.

[edit] Pair instability

As described above, the hotter a star's core becomes, the higher energy the gamma rays it produces. Once these reach gamma energies where pair production starts to become the dominant mechanism in gamma ray capture in the gas, then the distance that gamma rays travel in the gas starts to decrease instead of increasing. This decrease in the distance that gamma rays travel is an instability, and causes a feedback loop: as gamma travel distance decreases, the temperature at the core increases, and this increases the gamma energy and further decreases the distance that gammas can travel.

[edit] Stellar susceptibility

Stars which are rotating fast enough, or which have enough metallicity, probably do not collapse in pair-instability supernova due to other effects. Pair instability happens in low metallicity stars, with low to moderate rotation rates.

Very large high metallicity stars are probably unstable due to the Eddington limit, and would tend to shed mass during the formation process.

[edit] Stellar behavior

Several sources describe the stellar behavior for large stars in pair instability conditions. ^{[2] [3]}

[edit] Below 100 solar masses

For lower mass stars (about 100 solar masses and below), the gamma rays are not energetic enough to produce electron-positron pairs, and if these stars become supernovae they do so via other means.

[edit] 100 to 130 solar masses

For stars between 100 and around 130 solar masses, pressure and temperature effects allow larger partial collapses and pressure pulses to occur, initiated by pair production instability in the core, which are too small to fully disrupt the star. These pulses are damped out; they create temporary increased rates of thermonuclear burning, but the star gradually returns to a stable equilibrium. These pulses are expected to lead to ejections of parts of the outer layers of the star, similarly to what happened to the star Eta Carinae in the year 1843, though that may have had a different underlying mechanism. The pulsing mechanism is thought to cause stars in this mass range to shed mass until their remaining core is small enough to collapse in a normal supernova.

[edit] 130 to 250 solar masses

For very high mass stars, with mass at least 130 and up to perhaps roughly 250 solar masses, a true pair-instability supernova can occur. In these stars, the first time that conditions support pair creation instability, the situation runs out of control. The collapse proceeds to efficiently compress the star's core; the overpressure is sufficient to allow runaway nuclear fusion to burn it in a few seconds, creating a thermonuclear explosion.^[3] With more thermal energy released than the stars' gravitational binding energy, it is completely disrupted; no black hole or other remnant is left behind.

In addition to the immediate energy release, a large fraction of the stars' core is transformed to nickel-56, a radioactive isotope which decays with a half-life of 6.1 days into cobalt-56, which has a half-life of 77 days (see Supernova nucleosynthesis). For the hypernova SN 2006gy, studies indicate that perhaps 40 solar masses of the original star were released as Ni-56, almost the entire mass of the star's core regions.^[2] Collision between the exploding star core and gas it ejected earlier, and radioactive decay, release most of the visible light.

[edit] 250 solar masses or more

A different reaction mechanism, photodisintegration, results after collapse starts in stars of at least 250 solar masses. This endothermic reaction (energy-absorbing) causes the star to continue collapse into a black hole rather than exploding due to thermonuclear reactions.

[edit] External links

• SN2006gy
• Pair instability chart
• Fraley, G.S. 1968; "Supernovae Explosions Induced by Pair-Production Instability"

[edit] References