Element Production in SN

Cosmic Abundances

The Universe is primarily hydrogen and helium: cosmic abundances. The Big Bang produced mainly hydrogen and helium with trace amounts of some lighter elements with no appreciable production of carbon, nitrogen, oxygen, iron, magnesium, silicon, ... , and other elements heavier than iron. So, where did these elements come from?

Nucleosynthesis

The pre-supernova star has an onion-skin structure -- an iron core surrounded by various layers of material. The prediction is that:

the core collapse generates a shock wave which moves out through the outer layers of the star.
Initially, the shock travels at high energy and therefore heats the inner layers of the star to high temperatures. These high temperatures lead to nuclear re-processing of the inner outer layers of material into elements ranging from magnesium to iron.
As the shock moves outward (losing energy all of the time), eventually a point is reached where the temperatures generated by the shock cannot ignite nuclear reactions.
The shock then just pushes the outer layers away and so these layers reflect the normal evolution of the star. The transition occurs around the neon-oxygen layer.

What about elements heavier than iron? Well, the SN outburst is a strong source of neutrons. This is a key point since there is no electrical barrier for the addition of neutrons to nuclei. This means that one can build up very massive elements if there are sufficient neutrons. SN are good sites for high neutron fluxes.

n + (A,Z) ===> (A+1,Z) ===> (A+1,Z+1) + e + neutrino
n + (A+1,Z+1) ===> (A+2,Z+1) ===> (A+2,Z+2) + e + neutrino
and so on ...

Can also follow other routes, but the idea is the same. Add neutrons, let them beta-decay into protons (transmuting the elements). Can have rapid addition of neutrons (r-process) or slow addition of neutrons (s-process).

Test of the Picture

SN1987A offered a nice test of this picture. It used to be thought that the light curves Type I and Type II SN were grossly different in that the luminosity of Type II SN would drop off like bricks after the star had expanded greatly and cooled. It turns out that this is not true for SN1987A.

SN1987A looks a Type II SN initially, but then goes into this state where it continues to shine much longer than one would have thought if it only exploded (and was formed hot) and then just marched outward. The implication is that there must be a continued input of energy into the expanding material to keep it hot. That is, the first few weeks of SN1987A can be explained as the simple expansion of a hot gas, but after the first few weeks a new source of is energy is needed to explain the light curve.

A natural source for this energy is nuclear power. In particular, tapping the energy contained in the decay of radioactive elements. This picture works very nicely. We have that

Nickel(56) ===> Cobalt(56) with a half-life of 6.1 days
Cobalt(56) ===> Iron(56) with a half-life of 77.1 days
at longer times, the analysis gets messier but there are indications that other radioactive decays like Cobalt(57) ===> Iron(57) may be important.
An interesting by-product of this exercise is that it makes firm predictions for how much nickel (an iron peak element) and eventually iron is produced in the explosion. This calibrates the model and detailed predictions on the elemental yield of the explosion can be made! The results of SN1987A corrborate many assumptions we made about nucleosynthesis and also helped steer us into other promising directions.