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2.1 Collapse scenario

UpdateJump To The Next Update Information Stars with masses below 8 Mo . end their lives ejecting their envelopes in a possible planetary nebula, leaving behind a white dwarf that gradually cools and fades away. For those white dwarfs in binaries, binary accretion can reheat the white dwarf. If the accreted material ignites degenerately, the resultant nuclear explosion produces a nova and ejects all of the accreted material. But if this material burns non-degenerately, the white dwarf will gain mass. When the mass of the white dwarf exceeds the Chandrasekhar stability limit, it will begin to collapse.

Two possible fates await this collapsing white dwarf. If the collapsing core can achieve high enough temperatures, nuclear burning will begin and the stellar pressure will increase. This nuclear burning can drive nature’s largest nuclear bomb, producing type Ia supernovae. However, neutrino emission from electron capture (e.g., URCA processes) can damp this burning until the core has collapsed too deeply into its gravitational well for nuclear burning to turn around the collapse. The result of this collapse is the formation of a neutron star. Electron capture will dominate if the density at which nuclear ignition occurs exceeds a critical density rcrit. For C-O white dwarfs, rcrit is in the range 9 10 -3 6× 10 - 10 g cm [32Jump To The Next Citation Point]. For O-Ne-Mg white dwarfs, electron capture may be stronger than nuclear burning under most conditions (if a Rayleigh-Taylor instability does not produce a turbulent burn front) [187Jump To The Next Citation Point127Jump To The Next Citation Point]. The collapse of an O-Ne-Mg white dwarf begins when its central density reaches 4 × 109 g cm -3. (For more details about the conditions under which AIC occurs, see [187Jump To The Next Citation Point1273231156Jump To The Next Citation Point].) The dynamics of the collapse itself are somewhat similar to the dynamics of core collapse SNe: the collapse proceeds until the core reaches nuclear densities, the core then bounces and sends out a bounce shock that stalls when it becomes optically thin to neutrinos and loses its thermal energy. However, there is very little envelope around this core to prevent an explosion and the shock can easily be revived to drive a low-mass explosion. Exactly how much matter is ejected depends upon the details of the collapse calculation (compare [114263Jump To The Next Citation Point82Jump To The Next Citation Point]). Less than 10-1Mo. will likely be ejected by the star (due to the bounce itself or due to neutrino absorption/wind mechanisms) [82Jump To The Next Citation Point].


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