Type Ia Supernova - Consensus Model

Consensus Model

The Type Ia supernova is a sub-category in the Minkowski-Zwicky supernova classification scheme, which was devised by American astronomers Rudolph Minkowski and Fritz Zwicky. There are several means by which a supernova of this type can form, but they share a common underlying mechanism. When a slowly-rotating, carbon-oxygen white dwarf accretes matter from a companion, it cannot exceed the Chandrasekhar limit of about 1.38 solar masses, beyond which it would no longer be able to support its weight through electron degeneracy pressure and begin to collapse. In the absence of a countervailing process, the white dwarf would collapse to form a neutron star, as normally occurs in the case of a white dwarf that is primarily composed of magnesium, neon and oxygen.

The current view among astronomers who model Type Ia supernova explosions, however, is that this limit is never actually attained, so that collapse is never initiated. Instead, the increase in pressure and density due to the increasing weight raises the temperature of the core, and as the white dwarf approaches to within about 1% of the limit, a period of convection ensues, lasting approximately 1,000 years. At some point in this simmering phase, a deflagration flame front is born, powered by carbon fusion. The details of the ignition are still unknown, including the location and number of points where the flame begins. Oxygen fusion is initiated shortly thereafter, but this fuel is not consumed as completely as carbon.

Once fusion has begun, the temperature of the white dwarf starts to rise. A main sequence star supported by thermal pressure would expand and cool in order to counterbalance an increase in thermal energy. However, degeneracy pressure is independent of temperature; the white dwarf is unable to regulate the burning process in the manner of normal stars, and is vulnerable to a runaway fusion reaction. The flame accelerates dramatically, in part due to the Rayleigh–Taylor instability and interactions with turbulence. It is still a matter of considerable debate whether this flame transforms into a supersonic detonation from a subsonic deflagration.

Regardless of the exact details of nuclear burning, it is generally accepted that a substantial fraction of the carbon and oxygen in the white dwarf is burned into heavier elements within a period of only a few seconds, raising the internal temperature to billions of degrees. This energy release from thermonuclear burning (1–2×1044 J) is more than enough to unbind the star; that is, the individual particles making up the white dwarf gain enough kinetic energy that they are all able 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 5,000–20,000 km/s, or roughly up to 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 M_v = −19.3 (about 5 billion times brighter than the Sun), with little variation.

The theory of this type of supernovae is similar to that of novae, in which a white dwarf accretes matter more slowly and does not approach the Chandrasekhar limit. In the case of a nova, the infalling matter causes a hydrogen fusion surface explosion that does not disrupt the star. This type of supernova differs from a core-collapse supernova, which is caused by the cataclysmic explosion of the outer layers of a massive star as its core implodes.