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Maurizio Paolillo » 12.Evolution of Shell-type Supernova remnants


Phases of Shell-type SNRs

  • Supernova explosion: fast; ejecta with speed v ~ 104 km/s.
  • Free expansion: hundred of years; ejecta mass > swept up mass.
  • Adiabatic or Sedov: 10,000-20,000 years; swept-up mass > eject mass.
  • Snow-plow or Cooling: Few 100,000 years; shock front cools, interior also cools.
  • Disappearence: Up to millions of years; remnant slows to speed of the random velocities in the surrounding medium, merges with ISM.

Shock Formation

At time t=0, a mass m0 of gas is ejected with velocity v0 and total kinetic energy E0.

This interacts with surrounding interstellar material (ISM) with density ρ0 and low T.

The shell velocity much higher than the sound speed in ISM, so a shock front of radius R forms.

The ejecta from the supernova explosion interact with the surrounding ISM

The ejecta from the supernova explosion interact with the surrounding ISM


Free Expansion

The shell of swept-up material in front of shock does not represent a significant increase in mass of the system.
The ISM mass previously within the swept-up sphere of radius R is still small compared to the ejecta mass:

(4\pi /3)\rho R^3 << m_0

  • Since momentum is conserved:

m_0 v_0=(m_0+(4\pi /3)\rho_0 R^3)v

  • As long as swept-up mass << ejecta mass, the velocity of the shock front remains constant and Rs(t) ~ v0t
  • The temperature decreases can be calculated assuming adiabatic expansion, given the relatively short duration of this phase compared to other characteristic thermal timescales: T\propto R^{-3(\gamma -1)}

Sedov Phase

The dynamics can be described by location of shock front versus time.

Without discussing all the details of the Sedov soulution let’s just look for a self-similar solution, in which the dynamics can be reduced to one variable = Rtl.

Note that dynamics are determined by initial energy of explosion, E, and the density of ISM, ρ0.

Consider quantity E/ρ0. It has units of (length)5(time)-2.

Therefore, (E/ρ0)(t2/R5) is a dimensionless quantity which describes the dynamics of the expansion.

The solution requires R(t) = k(E/ρ0)1/5 t2/5 and v(t) = 2R/5t.

This solution describes the expansion of SNR pretty well.

Sedov phase: the Shock Jump

  1. Mass flux: ρ1ν1 = ρ0ν0
  2. Momentum flux:

P1 + ρ1ν12 = P0 + ρ0ν02

  1. Energy flux:

½ρ1ν13+ Pν1γ/(γ-1) = ½ρ0ν03+ Pν0γ/(γ-1)

where ρ is density, P is pressure, γ is the adiabatic index.

Introduce the Mach number M = v0/c0 where c0 = sqrt(γP0/ρ0) is the sound speed upstream, and find in the limit of large M.

ρ1/ρ0= (γ+1)/(γ-1) and T1/T0 = 2γ(γ-1)M2/(γ+1)2

For γ = 5/3, find ρ1/ρ0 = 4 and T1/T0 = (5/16)M2

Get large increase in temperature for large M.

The shock discontinuity in reference frame where shock is stationary, v0 = shock speed

The shock discontinuity in reference frame where shock is stationary, v0 = shock speed


Sedov Solution

In Sedov solution, find for downstream material:
pressure = (3/4) ρ0v2
temperature = (3m/16k) v2 where m is the mean mass per particle downstream (including electrons) and k is Boltzmann’s constant.

Temperature ~ (10 K)v2 for v in km/s,

For v ~ 1000 km/s, have T ~ 107 K which means gas is heated to X-ray producing temperatures.

N132D in the LMC

The precursor to this supernova is thought to have been 25 times the mass of our Sun.

This region measures 50 light years across. The SN is located in the Large Magellanic Cloud, 170,000 light years away.

Stellar material is moving out at velocities of about 2,000 km/s, creating shock fronts.

Shock fronts from the original SN have been reflected from dense ISM clouds. As the stellar material passes through in filaments, they glow. The dense ISM clouds have been heated and crushed by the SN shocks.

Gas is heated by shock to X-ray emitting temperatures.

Although gas glows in X-rays, the loss of energy due to radiation is relatively unimportant to the dynamics of the expansion, i.e. cooling time is longer than age of SNR.

The SNR N132D in the Large Magellanic Cloud

The SNR N132D in the Large Magellanic Cloud


Radiative Cooling

Eventually, the shock slows down. We can define the end of adiabatic phase as when half of energy has been radiated away.

Typically, at this stage the shock speed is about 200 km/s (with dependence on initial energy and ISM density).

Without the high temperature behind the shock, there is no high pressure to drive it forward through the ISM. The shock front is now `coasting’ with constant radial momentum, ie. All the material in the shell moves outwards with total momentum given by the equation shown.

(4\pi /3)R^3 \rho_0 v

Most of the swept-up material is compressed into a dense, relatively cool shell (a temperature of approx. 104 K).

There is some X-ray emission from the residual gas in the interior, but this is much weaker than before.

R=R_{rad}\left (\frac{8t}{5t_{rad}}-\frac{3}{5}\right )^{1/4}

Disappearance

When shock velocity drop to ~20 km/s, the expansion becomes subsonic and the SNR merges with the ISM.

However, this is an oversimplification since the ISM has magnetic fields and may present inhomogeneities. Moreover, the pressure of cosmic rays must be taken into account. Both of these factors affect the expansion of the SNR.

The magnetic field may help the coupling of the ejected matter with the ISM so that a shock wave can develop. Inhomogeneities of the ISM are also important, since the shock velocity and radiation intensity are both a function of density; as the shock enters a dense cloud, the velocity decreases and the radiation increases.

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