Supernova nucleosynthesis

Supernova nucleosynthesis is the production of new chemical elements inside supernovae. It occurs primarily due to explosive nucleosynthesis during explosive oxygen burning and silicon burning . Those fusion reactions create the elements silicon, sulfur, chlorine, argon, sodium, potassium, calcium, scandium, titanium and iron peak elements: vanadium, chromium, manganese, iron, cobalt, nickel. As a result of their ejection from individual supernovae, their abundances grow increasingly larger within the interstellar medium. Heavy elements (heavier than nickel) are created primarily by a neutron capture process known as the r process. However, there are other processes thought to be responsible for some of the element nucleosynthesis, notably a proton capture process known as the rp process and a photodisintegration process known as the gamma (or p) process. The latter synthesizes the lightest, most neutron-poor, isotopes of the heavy elements.

Supernovae

A supernova is a massive explosion of a star that occurs under two principal scenarios. The first is that a white dwarf star undergoes a nuclear based explosion after it reaches its Chandrasekhar limit from absorbing mass from a neighboring star (usually a red giant). The second, and more common, cause is when a massive star, usually a red giant, reaches iron in its nuclear fusion (or burning) processes. Iron 56 has the highest binding energies of all of the elements and is the last element that can be produced by nuclear fusion, exothermically. All nuclear fusion reactions from here on are endothermic and so the star loses energy. The star's gravity then pulls its outer layers rapidly inward. The star collapses very quickly, and then explodes.

Elements fused

Due to the large amounts of energy released in a supernova explosion, much higher temperatures are reached than stellar temperatures. Higher temperatures allow for an environment where elements up to the atomic mass of 254 are formed, californium being the heaviest known, though it is seen only as a synthetic element on Earth. In nuclear fusion processes in stellar nucleosynthesis, the maximum weight for an element fused is that of nickel, reaching an isotope with an atomic mass of 56. Fusion of elements between silicon and nickel occurs only in the largest of stars, which end as supernova explosions (see Silicon burning process). A neutron capture process known as the s process which also occurs during stellar nucleosynthesis can create elements up to bismuth with an atomic mass of approximately 209. However, the s process occurs primarily in low-mass stars that evolve more slowly.

The R-Process

During supernova nucleosynthesis, the r process (r for rapid) creates very neutron-rich heavy isotopes, which decay after the event to the first stable isotope, thereby creating the neutron-rich stable isotopes of all heavy elements. This neutron capture process occurs in high neutron density with high temperature conditions. In the r process, any heavy nuclei are bombarded with a large neutron flux to form highly unstable neutron rich nuclei which very rapidly undergo beta decay to form more stable nuclei with higher atomic number and the same atomic weight. The neutron flux is astonishingly high, about 10²² neutrons per square centimeter per second. First calculation of a dynamic r process, showing the evolution of calculated results with time , also suggested that the r process abundances are a superposition of differing neutron fluences. Small fluence produces the first r process abundance peak near atomic weight A=130 but no actinides, whereas large fluence produces the actinides uranium and thorium but no longer contains the A=130 abundance peak. These processes occur in a fraction of a second to a few seconds, depending on details. Hundreds of subsequent papers published have utilized this time-dependent approach. Interestingly, the only modern nearby supernova, 1987A, has not revealed r process enrichments. Modern thinking is that the r process yield may be ejected from some supernovae but swallowed up in others as part of the residual neutron star or black hole.

References

E. M. Burbidge, G. R. Burbidge, W. A. Fowler, F. Hoyle, Synthesis of the Elements in Stars, Rev. Mod. Phys. 29 (1957) 547 (article at the Physical Review Online Archive (subscription required)).
D. D. Clayton, "Handbook of Isotopes in the Cosmos", Cambridge University Press, 2003, ISBN 0 521 823811.