Convective overturn
Encyclopedia
The convective overturn model of supernova
e was proposed by Bethe and Wilson in 1985, and received a dramatic test with SN 1987A
, and the detection of neutrinos from the explosion
. The model is for type II supernovae, which take place in star
s more massive than 8 solar masses.
When the iron core of a super massive star becomes heavier than electron degeneracy pressure
can support, the core of the star collapses, and the iron core is compressed by gravity until nuclear
densities are reached when a strong rebound sends a shock wave throughout the rest of the star and tears it apart in a large supernova explosion. The remains of this core will eventually become a neutron star
. The collapse produces two reactions: one breaks apart iron
nuclei into 13 helium
atoms and 4 neutron
s, absorbing energy; and the second produces a wave of neutrinos that form a shock wave
. While all models agree that there is a convective shock, there is disagreement as to how important that shock is to the supernova explosion.
In the convective overturn model, the core collapses faster and faster, exceeding the speed of sound
inside the star, and producing a supersonic
shock wave
. This shock wave explodes outward until it stalls when it reaches the neutrinosphere, where the pressure
of the star collapsing inward exceeds the pressure of the neutrinos radiating outwards. This point produces heavier elements
as the neutrinos are absorbed.
The stalling of the shock wave represents the supernova problem, because once stalled, the shock wave should not be "reenergized". The prompt convection model states that the shock wave will increase the luminosity
of the neutrinos produced by the core collapse, and this increase in energy will start the shock wave going again. The neutron fingers model has instability near the core expel another wave of energized neutrinos which reenergizes the shock wave. The entropy convection model has matter falling inward from above the shock layer down to the gain radius, which would not increase neutrino luminosity, but would allow the shock wave to continue outwards.
All of these models exhibit convective overturn in that they rely on a convection
mechanism to re-energize the stalled shock wave and complete the supernova explosion.
There are still open issues in both the convective models and in the more general core collapse model, which include not taking into account flavor mixing and mass of neutrinos, and the inability to model large explosions. Current models indicate that the collapse may occur more slowly than thought before, which would mean the shock wave would penetrate farther into the upper layers of the star. The proto-neutron star boosts neutrino luminosities, and the additional neutrinos emitted help re-energize the shock wave. These changes remove some, but not all, of the supernova problem, and strengthen the idea of convection being an important factor in supernova explosions.
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
e was proposed by Bethe and Wilson in 1985, and received a dramatic test with SN 1987A
SN 1987A
SN 1987A was a supernova in the outskirts of the Tarantula Nebula in the Large Magellanic Cloud, a nearby dwarf galaxy. It occurred approximately 51.4 kiloparsecs from Earth, approximately 168,000 light-years, close enough that it was visible to the naked eye. It could be seen from the Southern...
, and the detection of neutrinos from the explosion
Explosion
An explosion is a rapid increase in volume and release of energy in an extreme manner, usually with the generation of high temperatures and the release of gases. An explosion creates a shock wave. If the shock wave is a supersonic detonation, then the source of the blast is called a "high explosive"...
. The model is for type II supernovae, which take place in star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s more massive than 8 solar masses.
When the iron core of a super massive star becomes heavier than electron degeneracy pressure
Electron degeneracy pressure
Electron degeneracy pressure is a particular manifestation of the more general phenomenon of quantum degeneracy pressure. The Pauli Exclusion Principle disallows two half integer spin particles from occupying the same quantum state at a given time. The resulting emergent repulsive force is...
can support, the core of the star collapses, and the iron core is compressed by gravity until nuclear
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
densities are reached when a strong rebound sends a shock wave throughout the rest of the star and tears it apart in a large supernova explosion. The remains of this core will eventually become a neutron star
Neutron star
A neutron star is a type of stellar remnant that can result from the gravitational collapse of a massive star during a Type II, Type Ib or Type Ic supernova event. Such stars are composed almost entirely of neutrons, which are subatomic particles without electrical charge and with a slightly larger...
. The collapse produces two reactions: one breaks apart iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
nuclei into 13 helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
atoms and 4 neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s, absorbing energy; and the second produces a wave of neutrinos that form a shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
. While all models agree that there is a convective shock, there is disagreement as to how important that shock is to the supernova explosion.
In the convective overturn model, the core collapses faster and faster, exceeding the speed of sound
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
inside the star, and producing a supersonic
Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
. This shock wave explodes outward until it stalls when it reaches the neutrinosphere, where the pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
of the star collapsing inward exceeds the pressure of the neutrinos radiating outwards. This point produces heavier elements
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
as the neutrinos are absorbed.
The stalling of the shock wave represents the supernova problem, because once stalled, the shock wave should not be "reenergized". The prompt convection model states that the shock wave will increase the luminosity
Luminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
of the neutrinos produced by the core collapse, and this increase in energy will start the shock wave going again. The neutron fingers model has instability near the core expel another wave of energized neutrinos which reenergizes the shock wave. The entropy convection model has matter falling inward from above the shock layer down to the gain radius, which would not increase neutrino luminosity, but would allow the shock wave to continue outwards.
All of these models exhibit convective overturn in that they rely on a convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
mechanism to re-energize the stalled shock wave and complete the supernova explosion.
There are still open issues in both the convective models and in the more general core collapse model, which include not taking into account flavor mixing and mass of neutrinos, and the inability to model large explosions. Current models indicate that the collapse may occur more slowly than thought before, which would mean the shock wave would penetrate farther into the upper layers of the star. The proto-neutron star boosts neutrino luminosities, and the additional neutrinos emitted help re-energize the shock wave. These changes remove some, but not all, of the supernova problem, and strengthen the idea of convection being an important factor in supernova explosions.