Type II supernovae explosions

Question

I'm quite confused about the explosion of a Type II supernova.

As far as I understand, when the Fe-56 core has been created, and the star has all the other layers around it, it starts to collapse, because, yeah, Fe-56 cannot give energy by fusion. This in turn gives a huge pressure on the core, and it exceeds the Chandrasekhar limit, and collapses further.

Now, during this collapse, the star becomes so hot and dense, that photodisintegration sets in, and produces a lot of neutrons, and electrons and proton fuse into neutrons and neutrinos as well. So now we a core with neutron degeneracy.

When this neutron degeneracy happens, the core sort of stops collapsing, and the layers outside the core, which is still collapsing, hits the hard surface of the neutron star surface, and bounces outwards like a tennis ball on a basket ball - i.e. a huge shock wave.

Now, my problem is, that I've also read, that during the collapse into a neutron star, the star gets so hot, that if much hotter, the neutrons will "boil" away. So in order to cancel this, it produces a lot of neutrinos, which can't escape the neutron star surface that well, and we then get a huge energy burst in the outer layers of the neutron star, which at last burst through everything in a major explosion and we get our supernova, leaving the core of the neutron star alone.

So basically: What causes the supernova explosion ? Is it the in-ward collapse from the outer layers, giving rise to the out-ward bounce when it hits the surface of the star, or is it actually the neutrinos that are the main force ? Or does the in-ward collapse only raise the temperature of the star to achieve the high temperature in order to produce the extra neutrinos, or something else I've missed ? :)

Thanks in advance.

Answer 1

In the final phases of a pre-supernova, massive star, most of the energy loss is through neutrino emission from the core. The losses are strongly temperature dependent, and combined with the decreasing yield from advanced nuclear burning phases, this means the final phase of silicon burning has a timescale of only about a day.

The iron core continues to contract and is close to completely degenerate. The standard Chandrasekhar mass for such a configuration is of order $1.25M_{\odot}$, but in any case when the core mass gets close to this, collapse is triggered either by electron capture or photo-disintegration in the core, both of which reduce the adiabatic index and cause instability. The collapse occurs on almost a freefall timescale.

The proximity of neutrons to each other halts the core-collapse - as you accurately summarise - because the equation of state suddenly hardens both due to non-relativistic neutron degeneracy pressure and the repulsion felt by nucleons in asymmetric n-rich nucleonic matter (i.e. strong force interactions).

The ultimate energy for the supernova explosion comes from the gravitational potential energy released during the core collapse. Only a small fraction (1%) of this energy is required to blow the envelope to infinity, but the difficulty is transferring it.

One idea is that the core bounce generates an outwardly propagating shockwave and that this shock has sufficient kinetic energy to drive the explosion. However, I think it is now thought that a lot of this kinetic energy is dissipated by (i) disintegrating the remains of the still-infalling outer parts of the core; (ii) consumed in neutrino production caused by electron captures onto free protons behind the shock. Because of this, it is commonly thought that the shock would stall.

The neutrinos that are produced during the collapse are mainly produced in electron capture reactions and they have energies of order a fraction of the Fermi energy of the degenerate electrons. However, in the final phases of core collapse, neutrinos at these energies become effectively trapped by scattering reactions. That is, the "optical depth" of the core to neutrinos becomes large. So an alternative view of what happens to drive the explosion is that after a short delay (a few seconds, because the core neutrinos become degenerate and this slows down neutronisation and prevents complete neutronisation of the core), the shock can become revived using the energy stored in the trapped neutrinos. The trick is to get them to deposit that energy into the temperature and kinetic energy of the gas, which might be possible if it becomes convectively unstable.

So I think your summary is quite accurate (except for the bit about "boiling neutrons" which I don't quite follow), but @KyleKanos is also correct that this is all a matter of serious debate and research at the moment.

See also Why does a supernova explode