Supernova thermonuclear

Stars of mass greater than 1.4 solar masses have thermonuclear reactions that generate heavier elements (see Table 4.3) and ultimately stars of approximately 20 solar masses are capable of generating the most stable nucleus by fusion processes, Fe. The formation of Fe terminates all fusion processes within the star. Heavier elements must be formed in other processes, usually by neutron capture. The ejection of neutrons during a supernova allows neutron capture events to increase the number of neutrons in an atomic nucleus. Two variations on this process result in the production of all elements above Fe. A summary of nucleosynthesis processes is summarised in Table 4.4. Slow neutron capture - the s-process - occurs during the collapse of the Fe core of heavy stars and produces some higher mass elements, however fast or rapid neutron capture - the r-process - occurs during the supernova event and is responsible for the production of the majority of heavy nuclei. 

In 1960 Fred Hoyle and William Fowler discovered that thermonuclear combustion in the dense core of a degenerate star (the word degenerate is used in the sense of quantum theory and will be made exphcit later) could trigger the explosion and volatilisation of the star. If we add the idea that post mortem light emissions are fuelled by the gradual disintegration of an unstable radioactive isotope, nickel-56, a subject to be discussed in great detail later, we obtain the universal explanation of what are now known as type la supernovas. 

Thermonuclear supernovas are close in operation to bombs of the same name. Their brutal and complex physics involves matter exchange between two coupled stars, one of which is a white dwarf (Fig. 7.7). The trade-off is controlled by gravity in rather opaque conditions that render the whole process difficult to discern. 

However, the entombment of iron is only perpetrated by gravitational-collapse supernovas. Their thermonuclear counterparts are more liberal and, one might say, more final, for they leave behind no corpse, no bones, and no scrap iron. They owe this propensity for total destraction to the rigidity and fragility of the exploding body, the white dwarf, a porcelain ornament that is sure to break when it falls. But thermonuclear supernovas, though lavish providers of iron, are rare. Very special conditions must be fulfilled for these explosions to occur. 

Astronomers use a variety of methods to determine the distance to objects in the universe. One of the most effective is the standard candle provided by Type la supemovae. These supemovae originate in a binary star system when a white dwarf star accretes matter from its companion. When the white dwarf reaches the Chandrasekhar limit of 1.4 solar masses, a thermonuclear runaway occurs that completely disrupts the star in a cataclysmic explosion that makes the supernova as bright as an entire galaxy. Because Type la supemovae occur in stars with similar masses and because the nuclear burning affects the entire star, they all have essentially the same intrinsic brightness and their apparent brightness observed from Earth can be used to derive the distance to the supernova. 

For massive supernovae the absolute luminosity after about 120 days, together with the age of the supernova, gives a relatively accurate measure of the amount of 56Co synthesised in the explosion Hamuy et al. 2003B Elmhamdi et al. 2003. This measurement is now available for many core-collapse supernovae and is typically a factor 10 less than assumed in thermonuclear supernovae but spans almost a factor of 100 Pastorello et al. 2004. 

See 5 Fe (to which 5 Ni decays) for discussion of the nucleosynthesis of 5 Ni by equilibrium and quasiequilibrium processes during supernova nuclear burning. Roughly 0.5 to 0.8 solar masses of 56Ni is created by the thermonuclear explosions of white dwarfs (Type la supernovae). Only about l/loth as much is created by each core-collapse supernova (Type II), but they are about 4 to 5 times more frequent than Type la explosions. This means that about two-thirds of the galactic total of 5 Ni is synthesized by la explosions and about one-third by core-collapse Type II. 

A related question is this Which types of stars or supernova explosions produce the extinct radioactive nuclei thatare found in the solar system Here the reader is referred to the entry for each specific isotope. But this much must be appreciated first. Some radioactivities appear to be made primarily by the thermonuclear explosions of white-dwarf stars, called Type la supernovae. Others are created primarily in massive stars whose cores collapse to become neutron stars to initiate an explosive ejection (Type II supernovae). Type II supernovae occur three to five times more frequendy than do Type la supernovae. Some radioactive nuclei are made within differing portions of each event, some prior to the ejection, but some during the heat of the ejection process. And still other radioactive nuclei are created within evolved stars that do not become supernovae (red giants). This diversity of origin renders uncertain the identity of those extinct radioactivities that are to be attributed specifically to that supernova thatis supposed to have triggered the formation of the solar system. In recent scientific... 

Keywords Supernovae, core collapse, thermonuclear explosions, gamma-ray bursts... 

Here we focus on some recent highlights in both core collapse and thermonuclear supernova studies, which became possible mainly due to increasingly accurate radiation hydrodynamic calculations with a detailed treatment of neutrino processes. We also briefly describe recent success of asymmetric SN simulations (2D magneto-rotational collapse). Next we focus on recently established link between GRB explosions and energetic type Ibc supemovae (hypernovae) and discuss recent ideas on the GRB progenitors. We hypothesize that different core collapse outcomes may lead to the formation of different classes of GRBs. 

An impressive progress has been done in multidimensional calculations of thermonuclear explosions of degenerate dwarfs for type la supernovae. It is still however unclear whether pure deflagration or delayed detonation is at work in SN la. The important problem is to more precisely determine the initial ignition conditions. Detailed radiation hydrodynamic modeling revealed that SN la light curves proved to be very sensitive to the explosion models and thus can be used to check the models. 

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