In the case of Type Ia supernovae, a white dwarf in a multiple system has accreted enough material to exceed critical mass and initiate nuclear reactions as it gravitationally collapses. The result is a spectacular explosion with complete disappearance of the white dwarf. However, in some cases, it is theoretically possible for a stellar remnant to remain as a zombie star.
Type Ia supernovae — also called thermonuclear supernovae — occur only in multiple systems, involving a white dwarf and a companion star (usually a giant that has left the main sequence). Carbon-oxygen white dwarfs have a critical mass of about 1.44 solar masses.
When the companion star overflows its Roche lobe, it begins to pour gas on the white dwarf, thus causing an accretion phenomenon gradually increasing the mass of the latter. The thermal pressure becomes sufficient to initiate the nuclear fusion reaction of carbon and then that of oxygen, drastically increasing the temperature of the star.
Since the degeneracy pressure at the heart of the white dwarf is independent of temperature, the object is extremely sensitive to runaway nuclear reactions. This runaway is accentuated by phenomena such as Rayleigh–Taylor instability and turbulence.
Within seconds, the internal temperature reaches several billion degrees, releasing an energy of approximately 1–2×10 44 J; enough energy to break up the star.
A shock wave is then generated, blowing the peripheral layers of the white dwarf and disintegrating the interior layers. The gas is ejected at a speed of about 6% the speed of light.
The state of degeneracy cancels out, the pressure again becomes a direct function of the temperature, leading to a spectacular explosion in which the star is completely disintegrated. The energy released is responsible for the peak of luminosity equivalent to 5 billion Suns.
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However, theoretical models predict that in 5 to 30% of cases, the event does not completely destroy the white dwarf. There would then remain a kind of star corpse called stellar remnant. This category of supernova is called type Iax supernova, and has a lower ejection velocity and luminosity compared to an ordinary type Ia supernova. Although zombie stars are hypothetical, some candidates have been proposed.
This is the case of SN 2012Z, located in the galaxy NGC 1309, which astrophysicists suspect to be an Iax-type supernova. It was observed in January 2012 using the Katzman Automatic Imaging Telescope (KAIT).
In this binary system, the white dwarf would have accreted enough mass from its companion star to exceed the critical limit and explode into a supernova. Images of the area were taken before the supernova, allowing scientists to make a before/after comparison.
New luminosity measurements must still be carried out to confirm the presence of the stellar remnant. According to the authors of the discovery, there would be a 99% chance that it was indeed a zombie star. The other hypothesis explaining the observations would be the second explosion of a massive star of about 30-40 solar masses.
