Shocks, Particle Acceleration and Supernovae Flashcards
How do particles get accelerated by shocks?
Through Fermi Acceleration or diffusive shock acceleration
What is a Shock Wave?
A discontinuous change in the fluid flow variables which occurs when there is motion faster than the sound speed of gas
What is and is not conserved in a shock?
Momentum is since the difference in pressure between up and downstream which generates momentum.
Energy is not conserved as pressure does work on fluid as it flows.
What are the 3 criteria to determine if a shock is strong?
- It is hypersonic
- Large pressure jump
- Ram pressure ie ρ_0*u_0^2»_space; 1
Describe an adiabatic shock.
There is a paradox where the shock compresses the gas, expecting the pressure ratio to be 4^(5/3) and that the process is adiabatic. However, the shock is hypersonic therefore pressure ratio»_space;1. The solution is that the compression is irreversible and so not adiabatic. Entropy is then generated through viscous dissipation of the shock.
Describe the acceleration of relativistic particles by shocks.
Particles only change direction when they undergo a scattering event. They are travelling much faster than the shock and so can come back across.
In the shock frame, the particle is scattered off pre-shock and then is hit by the fluid moving towards it, resulting in a change in energy. It then gets scattered off receding downstream gas, losing energy
As u_0>u_1, the particle will gain more energy than it loses, so over the 2 collisions it gets accelerated.
Describe a supernova.
Outer layers of O & B stars unstable to radiation pressure. Momentum is then transferred to material that intercepts photons which can be high enough to overcome gravity.
The star can then lose mass at a rate of 10^-6 Msolar/yr. The high velocity gas drives a shockwave into the interstellar medium. A more dramatic input is that supernovae propel material into the interstellar medium at 0.01c
What are the properties of a Type 2 supernova?
They show Hydrogen emission lines, occur at the end of a star’s life.
What are the properties of a Type Ia supernova?
Very homogeneous. Characteristic light curve, fall for 30 days. The favoured model is a White Dwarf that has accreted matter from a companion star. No H line. Standard candle
What are the properties of Type Ib and Ic supernovae?
They lack a hydrogen line, but not a standard curve. Believed to be caused by massive stars at the end of their evolution.
How are shocks involved with the late stage evolution of massive star cores?
Core collapses, neutron degeneracy sets in and the core stiffens causing infalling material from envelope to rebound in outward shockwave.
This shockwave drives material from envelope outward, compressing and heating as it moves through.
In a few milliseconds, the amount of matter swept up by SW=amount of matter in SW, and the shock stalls from competition between infalling and outflowing gas
Dense layer produced is believed to be able to trap small fractions of neutrinos pouring out of the core.
Momentum transfer from these re-generates outward SW after 300ms
Regenerated blast wave then smashes outwards through star causing a supernova.
What are the phases of the end of a star’s life?
Blast-wave phase, radiative phase and the momentum conserving (snowplough) phase
Why does the sun contain metals?
It has been formed by gas which has been enriched by SN explosions.
