Carbon detonation

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Carbon detonation or carbon deflagration is the violent reignition of thermonuclear fusion in a white dwarf star that was previously slowly cooling. It involves a runaway thermonuclear process which spreads through the white dwarf in a matter of seconds, producing a type Ia supernova which releases an immense amount of energy as the star is blown apart. The carbon detonation/deflagration process leads to a supernova by a different route than the better known type II (core-collapse) supernova (the type II is caused by the cataclysmic explosion of the outer layers of a massive star as its core implodes). [1]

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White dwarf density and mass increase

A white dwarf is the remnant of a small to medium size star (the Sun is an example of these). At the end of its life, the star has burned its hydrogen and helium fuel, and thermonuclear fusion processes cease. The star does not have enough mass to either burn much heavier elements, or to implode into a neutron star or type II supernova as a larger star can, from the force of its own gravity, so it gradually shrinks and becomes very dense as it cools, glowing white and then red, for a period many times longer than the present age of the Universe.

Occasionally, a white dwarf gains mass from another source – for example, a binary star companion that is close enough for the dwarf star to siphon sufficient amounts of matter onto itself; or from a collision with other stars, the siphoned matter having been expelled during the process of the companion's own late stage stellar evolution. If the white dwarf gains enough matter, its internal pressure and temperature will rise enough for carbon to begin fusing in its core. Carbon detonation generally occurs at the point when the accreted matter pushes the white dwarf's mass close to the Chandrasekhar limit of roughly 1.4 solar masses, the mass at which gravity can overcome the electron degeneracy pressure that prevents it from collapsing during its lifetime. This also happens when two white dwarfs merge if the combined mass is over the Chandrasekhar limit, resulting in a type Ia supernova.

A main sequence star supported by thermal pressure would expand and cool which automatically counterbalances an increase in thermal energy. However, degeneracy pressure is independent of temperature; the white dwarf is unable to regulate the fusion process in the manner of normal stars, so it is vulnerable to a runaway fusion reaction.

Fusion and pressure

In the case of a white dwarf, the restarted fusion reactions releases heat, but the outward pressure that exists in the star and supports it against further collapse is initially due almost entirely to degeneracy pressure, not fusion processes or heat. Therefore, even when fusion recommences, the outward pressure that is key to the star's thermal balance does not increase much. One result is that the star does not expand much to balance its fusion and heat processes with gravity and electron pressure, as it did when burning hydrogen (until too late). This increase of heat production without a means of cooling by expansion raises the internal temperature dramatically, and therefore the rate of fusion also increases extremely fast as well, a form of positive feedback known as thermal runaway.

A 2004 analysis of such a process states that:

A deflagration flame burning from the center of the white dwarf star outward leaves hot and light burnt material behind. The fuel in front of it is, however, cold and dense. This results in a density stratification inverse to the gravitational field of the star, which is therefore unstable. Thus, blobs of burning material form and ascend into the fuel. At their interfaces shear flows emerge. These effects lead to strong swirls. The resulting turbulent motions deform the flame and thus enlarge its surface. This increases the net burning rate of the flame and leads to the energetic explosion. [2]

Supercritical event

The flame accelerates dramatically, in part due to the Rayleigh–Taylor instability and interactions with turbulence. The resumption of fusion spreads outward in a series of uneven, expanding "bubbles" in accordance with Rayleigh–Taylor instability. [3] Within the fusion area, the increase in heat with unchanged volume results in an exponentially rapid increase in the rate of fusion – a sort of supercritical event as thermal pressure increases boundlessly. As hydrostatic equilibrium is not possible in this situation, a "thermonuclear flame" is triggered and an explosive eruption through the dwarf star's surface that completely disrupts it, seen as a type Ia supernova.

Regardless of the exact details of this nuclear fusion, it is generally accepted that a substantial fraction of the carbon and oxygen in the white dwarf is converted into heavier elements within a period of only a few seconds, [4] raising the internal temperature to billions of degrees. This energy release from thermonuclear fusion (1–2×1044  J [5] ) is more than enough to unbind the star; that is, the individual particles making up the white dwarf gain enough kinetic energy to fly apart from each other. The star explodes violently and releases a shock wave in which matter is typically ejected at speeds on the order of 5,000–20000 km/s, roughly 6% of the speed of light. The energy released in the explosion also causes an extreme increase in luminosity. The typical visual absolute magnitude of type Ia supernovae is Mv = −19.3 (about 5 billion times brighter than the Sun), with little variation. [6] This process, of a volume supported by electron degeneracy pressure instead of thermal pressure gradually reaching conditions capable of igniting runaway fusion, is also found in a less dramatic form in a helium flash in the core of a sufficiently massive red giant star.

See also

Related Research Articles

The Chandrasekhar limit is the maximum mass of a stable white dwarf star. The currently accepted value of the Chandrasekhar limit is about 1.4 M☉ (2.765×1030 kg).

<span class="mw-page-title-main">Nova</span> Nuclear explosion in a white dwarf star

A nova is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star that slowly fades over weeks or months. Causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars. All observed novae involve white dwarfs in close binary systems. The main sub-classes of novae are classical novae, recurrent novae (RNe), and dwarf novae. They are all considered to be cataclysmic variable stars.

<span class="mw-page-title-main">Supernova</span> Explosion of a star at its end of life

A supernova is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed to form a diffuse nebula. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months.

<span class="mw-page-title-main">Stellar evolution</span> Changes to stars over their lifespans

Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the current age of the universe. The table shows the lifetimes of stars as a function of their masses. All stars are formed from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.

<span class="mw-page-title-main">White dwarf</span> Type of stellar remnant composed mostly of electron-degenerate matter

A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to Earth's. A white dwarf's low luminosity comes from the emission of residual thermal energy; no fusion takes place in a white dwarf. The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star. There are currently thought to be eight white dwarfs among the hundred star systems nearest the Sun. The unusual faintness of white dwarfs was first recognized in 1910. The name white dwarf was coined by Willem Luyten in 1922.

<span class="mw-page-title-main">Black dwarf</span> Theoretical stellar remnant

A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently to no longer emit significant heat or light. Because the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe, no black dwarfs are expected to exist in the universe at the present time. The temperature of the coolest white dwarfs is one observational limit on the universe's age.

Degenerate matter occurs when the Pauli exclusion principle significantly alters a state of matter at low temperature. The term is used in astrophysics to refer to dense stellar objects such as white dwarfs and neutron stars, where thermal pressure alone is not enough to avoid gravitational collapse. The term also applies to metals in the Fermi gas approximation.

In astronomy, the term compact object refers collectively to white dwarfs, neutron stars, and black holes. It could also include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects have a high mass relative to their radius, giving them a very high density, compared to ordinary atomic matter.

<span class="mw-page-title-main">Helium flash</span> Brief thermal runaway nuclear fusion in the core of low mass stars

A helium flash is a very brief thermal runaway nuclear fusion of large quantities of helium into carbon through the triple-alpha process in the core of low mass stars during their red giant phase. The Sun is predicted to experience a flash 1.2 billion years after it leaves the main sequence. A much rarer runaway helium fusion process can also occur on the surface of accreting white dwarf stars.

<span class="mw-page-title-main">Gravitational collapse</span> Contraction of an astronomical object due to the influence of its gravity

Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the center of gravity. Gravitational collapse is a fundamental mechanism for structure formation in the universe. Over time an initial, relatively smooth distribution of matter, after sufficient accretion, may collapse to form pockets of higher density, such as stars or black holes.

<span class="mw-page-title-main">Thermal runaway</span> Situation where an increase in temperature causes a further increase in temperature

Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback.

Supernova nucleosynthesis is the nucleosynthesis of chemical elements in supernova explosions.

<span class="mw-page-title-main">Type Ia supernova</span> Type of supernova in binary systems

A Type Ia supernova is a type of supernova that occurs in binary systems in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white dwarf.

<span class="mw-page-title-main">IK Pegasi</span> Star in the constellation Pegasus

IK Pegasi is a binary star system in the constellation Pegasus. It is just luminous enough to be seen with the unaided eye, at a distance of about 154 light years from the Solar System.

<span class="mw-page-title-main">Type II supernova</span> Explosion of a star 8 to 45 times the mass of the Sun

A Type II supernova or SNII results from the rapid collapse and violent explosion of a massive star. A star must have at least eight times, but no more than 40 to 50 times, the mass of the Sun (M☉) to undergo this type of explosion. Type II supernovae are distinguished from other types of supernovae by the presence of hydrogen in their spectra. They are usually observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies; those are generally composed of older, low-mass stars, with few of the young, very massive stars necessary to cause a supernova.

<span class="mw-page-title-main">Pair-instability supernova</span> Type of high-energy supernova in very large stars

A pair-instability supernova is a type of supernova predicted to occur when pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, temporarily reduces the internal radiation pressure supporting a supermassive star's core against gravitational collapse. This pressure drop leads to a partial collapse, which in turn causes greatly accelerated burning in a runaway thermonuclear explosion, resulting in the star being blown completely apart without leaving a stellar remnant behind.

Deflagration to detonation transition (DDT) refers to a phenomenon in ignitable mixtures of a flammable gas and air when a sudden transition takes place from a deflagration type of combustion to a detonation type of explosion.

p-nuclei (p stands for proton-rich) are certain proton-rich, naturally occurring isotopes of some elements between selenium and mercury inclusive which cannot be produced in either the s- or the r-process.

<span class="mw-page-title-main">KPD 1930+2752</span>

KPD 1930+2752 is a binary star system including a subdwarf B star and a probable white dwarf with relatively high mass. Due to the nature of this astronomical system, it seems like a likely candidate for a potential type Ia supernova, a type of supernova which occurs when a white dwarf star takes on enough matter to approach the Chandrasekhar limit, the point at which electron degeneracy pressure would not be enough to support its mass. However, carbon fusion would occur before this limit was reached, releasing enough energy to overcome the force of gravity holding the star together and resulting in a supernova.

Pycnonuclear fusion is a type of nuclear fusion reaction which occurs due to zero-point oscillations of nuclei around their equilibrium point bound in their crystal lattice. In quantum physics, the phenomenon can be interpreted as overlap of the wave functions of neighboring ions, and is proportional to the overlapping amplitude. Under the conditions of above-threshold ionization, the reactions of neutronization and pycnonuclear fusion can lead to the creation of absolutely stable environments in superdense substances.

References

  1. ↑ Gilmore, Gerry (2004). "The Short Spectacular Life of a Superstar". Science. 304 (5697): 1915–1916. doi:10.1126/science.1100370. PMID   15218132. S2CID   116987470.
  2. ↑ Röpke, Friedrich; Hillebrandt, Wolfgang (October 2004). "Current Research Highlight: Three-dimensional simulations of Type Ia supernova explosions". Max-Planck-Institut für Astrophysik. Archived from the original on 2021-09-10. Retrieved 2022-01-25.
  3. ↑ "Archived copy" (PDF). www.jinaweb.org. Archived from the original (PDF) on 2016-03-04.{{cite web}}: CS1 maint: archived copy as title (link)
  4. ↑ Röpke, F. K.; Hillebrandt, W. (2004). "The case against the progenitor's carbon-to-oxygen ratio as a source of peak luminosity variations in Type Ia supernovae". Astronomy and Astrophysics . 420 (1): L1–L4. arXiv: astro-ph/0403509 . Bibcode:2004A&A...420L...1R. doi:10.1051/0004-6361:20040135. S2CID   2849060.
  5. ↑ Khokhlov, A.; Müller, E.; Höflich, P. (1993). "Light curves of Type IA supernova models with different explosion mechanisms". Astronomy and Astrophysics. 270 (1–2): 223–248. Bibcode:1993A&A...270..223K.
  6. ↑ Hillebrandt, W.; Niemeyer, J. C. (2000). "Type IA Supernova Explosion Models". Annual Review of Astronomy and Astrophysics. 38 (1): 191–230. arXiv: astro-ph/0006305 . Bibcode:2000ARA&A..38..191H. doi:10.1146/annurev.astro.38.1.191. S2CID   10210550.
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