13- Supernovae (mostly core collapse)
Nucleosynthesis in SNe 1a
Nuclear statistic equilibrium, mostly iron peak elements
some intermediate mass elements from partially burned C/O
All other classes
stars >10 Msun will fuse successively heavier elements
each stage requires higher temperature, is less efficient at energy generation, and so lasts for less time
Star develops ogre structure with layers of increasingly heavy elements from H envelope to heavy core
Continues until Si fusion creates an iron core exceeding 1.4 Msun
Stars of ~8-10 M sun Form and O-Ne-Mg core
will also collapse
Threshold for explosion not precisely known, but near 8 Msun
depends on stars heavy element content
Explosions are very aspherical
Collapse of iron core
When fuel is exhausted gravitational collapse occurs, causing heating, igniting next fuel until iron
Mean photon energy becomes such that photo-dissociation of iron can take place
Iron-Helium phase transition
This is endothermic
energy provided at expense of gravitational field, accelerating core collapse
Neutron star formation
Density reaches 10^17 kg/m^3
electrons are forced into protons forming neutrons
creates neutrino burst which carry off 99% of the energy of the collapse, but at this early stage they are trapped
because collapse is faster than neutrino diffusion time
Outer core of star is not collapsing as fast, so it bounces of neutron star
Prompt shock
Creates shock wave which propagates outward
Dissipates nuclei into free protons and neutrons using up 8-9 MeV/nucleon
Neutrino burst emerges at shock breakout
energy loss due to dissociation of nuclei and neutrino cooling will cause shock to stall
Initial shock will not cause explosion
Delayed Neutrino heating mechanism
carry 99% of the supernova energy
neutrino cooling and heating (chem eqns)
if a few % of neutrino energy goes into heating the shock will revive
Explosion!
Explosive Fusion
Pre-existing iron-peak elements are mostly disociated during core collapes
propagating shock will initiate explosive burning of Si,O,Ne and C
Si burning produces iron peak elements, inclusing Ni-56 whos decay powers late time exponential decay of light curve
O burning produces many alpha particles , enhancing production of alpha process isotopes, like O-16, Ne-20, 24Mg (bound states of alphas)
Occurs at tempuratures of several e9 K and high densities
these are conditions for nuclear statistical equilibrium
p and r processes
Early ejecta are proton rich
may be where bypassed nuclei (p) are formed by proton capture (rp)
later ejects may be neutron rich
high neutron number densities give a possible site for r-process
unclear if this happens
for low metallicity SNe r-process may occur further out in He shell of origional star (cold r-process)