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 (through the
r-process and s-process)
if there are sufficient neutrons. SN are
good sites for high neutron fluxes.
Test of the Picture
SN1987A offered a nice test of this picture. It used to be thought that
the light curves for Type I and Type II SN were
grossly
different
in that the luminosity of Type II SN dropped off like bricks after
the star had expanded
greatly and cooled. It turns out that our ideas are changing
(in no small part due to SN 1987A). We now envision that:

SN1987A is not the typical Type II SN, but it is clearly a Type II SN.
However, it went into a
state where it continued to shine much longer than one would have expected if
it exploded (and was formed hot) and then just cooled and marched outward. The
SN 1987A light curve looked like
and, clearly, in many ways resembled a Type I SN light curve.
The implication of this is that there must have been a continued input of energy
into the expanding material to keep it hot.
The first few weeks of SN1987A could be explained as the
simple expansion of a hot gas, but later 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