How are neutrinos able to cause a supernova explosion?

Question

I was trying to understand the type II supernovae's core collapse mechanism from Wikipedia.

As the core's density increases, it becomes energetically favorable for electrons and protons to merge via inverse beta decay, producing neutrons and elementary particles called neutrinos.

So far, everything's fine.

Because neutrinos rarely interact with normal matter, they can escape from the core, carrying away energy and further accelerating the collapse, which proceeds over a timescale of milliseconds. As the core detaches from the outer layers of the star, some of these neutrinos are absorbed by the star's outer layers, beginning the supernova explosion.

How do neutrinos initiate the supernova explosion? Don't they rarely interact with matter?

The newly formed neutron core has an initial temperature of about 100 billion kelvins, 10⁴ times the temperature of the Sun's core. Much of this thermal energy must be shed for a stable neutron star to form, otherwise the neutrons would "boil away". This is accomplished by a further release of neutrinos.

These 'thermal' neutrinos form as neutrino-antineutrino pairs of all flavors, and total several times the number of electron-capture neutrinos. The two neutrino production mechanisms convert the gravitational potential energy of the collapse into a ten-second neutrino burst, releasing about 10⁴⁶ joules (100 foe).

We have two mechanisms of neutrinos production here. One is due to inverse beta decay leading to production of neutrons and neutrinos. The other is thermal neutrinos. What is their production due to? Heat of hot neutron core?

What is their role? They sap away thermal energy from hot neutron core, and convert gravitational potential energy into neutrino bursts? What does that mean?

My main question is: How are these neutrinos able to cause shockwave supernovae explosions?

Answer 1

Here is an enjoyable explanation:

Ok so I got doubt with neutrinos here that, how are they initiating the supernova explosion. Don't they interact rarely with matter?

There are two things to point out. It is true that neutrinos rarely interact with matter, but on the other hand the neutrino pulse is very intense. There are just so many of them that the rare interactions can now happen frequently, at least for the few seconds that the pulse lasts.

Second, the the chance of an interaction between an outgoing neutrino and the matter surrounding the core goes up with increasing density. Because the core bounces, an immense shock wave is created traveling outward from the core. All waves are denser than the material they are traveling through, so the escaping neutrinos will preferentially interact with the matter in the shock front, rather than the less dense matter ahead of or behind it. The shock front is dense enough to absorb not a tiny fraction of a percent of the neutrinos, but maybe as many as 10% of them. It can gain an incredible amount of energy from those neutrinos, more than enough to power through the infalling matter and explode the entire star. (Of course if the star is big enough then even that won’t be enough and the infalling matter stifles the explosion, and ultimately a black hole is formed.)

The other is thermal neutrinos, whose production is due to ? heat of hot neutron core? I really couldn't understand how they formed.

Calculating the number of neutrinos created is quite hard. Of course there will be one neutrino for every proton converted to a neutron in the core, which is a lot, but then the Urca process creates many times this number of neutrinos as the core cools. At its simplest, imagine that a proton absorbs an electron to create a neutron. This neutron is carried by convection away from the interior of the core to a region of lower electron degeneracy. There it undergoes beta decay becoming a proton and electron again. Since both of those reactions release a neutrino (or antineutrino), and because the highly turbulent convection within the core and gain region can turn matter over many times per second, the number of neutrinos released will be quite large indeed.

(I am no expert myself, so I welcome any corrections!)

Answer 2

Supernovae are powered by gravitational energy. Specifically, the change in gravitational potential energy when an electron-degenerate core, the mass of the Sun and radius of the Earth, collapsed rapidly ($<1$ s) to a radius of $\sim 10$ km.

Where does this energy go? Initially it is shared between escaping neutrinos and making the the dense core extremely hot ($10^{11}$ K). However, as the central core achieves neutron star densities, the collapse is abruptly halted by the repulsion between closely packed nucleons and the consequent "bounce" drives a shockwave outwards.

The shock wave is not powerful enough to power the supernova. Instead, the driving force at play is the increased opacity of the hot, dense inner regions to neutrinos.

A basic rule of thumb is it takes a light year of lead to stop a neutrino. But neutron star densities are about $10^{13}$ times that of lead, so the equivalent is around 10 km of neutron star material. It is the absorption of the energy and momentum of just a small fraction (a few percent) of the neutrinos produced in the hot, but rapidly cooling, core, that drives the supernova explosion and throws off the outer envelope of the star.

Now to your specific question:

The hot core produces neutrinos chiefly via the URCA process. These are cycles of beta decay and inverse beta decay. $$ n \rightarrow p + e + \bar{\nu}$$ $$ p + e \rightarrow n + \nu $$ At very high temperatures, these processes can come into thermal equilibrium (ordinarily, the second process is energetically unfavorable). The resultant (anti) neutrinos have a range of energies of order $k_B T \sim 10$ MeV and would be called "thermal neutrinos".

Electron and positrons produced by pair production at high energies can also annihilate to produce neutrino/anti neutrinos pairs. Again, the energies of the neutrinos reflect the thermal energies of the electrons and positrons. There is also a scattering interaction between nucleons that can produce neutrino/anti neutrino pairs called "neutrino bremsstrahlung".

These thermal neutrinos are a coolant. They take away energy from the core, allowing it to cool by an order of magnitude in seconds by emitting $\sim 10^{57}$ such neutrinos (the thermal energy of the core divided by 10 MeV). However, in these first few seconds after core collapse, the region just outside where these neutrinos are produced is able to absorb some of them, which drives the explosion.