Supernovae

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Supernovae

• High Energy Astrophysics
• jlc_at_mssl.ucl.ac.uk
• http//www.mssl.ucl.ac.uk/

2
Introduction

• Supernovae occur at the end of the evolutionary
• history of stars.
• Star must be at least 2 M? core at least 1.4
  M?.
• Stellar core collapses under force of its own
• gravitation.
• Energy set free by collapse expels most of stars
  mass.
• Dense remnant, often a neutron star, remains.

3
Nuclear binding

• M (A, Z) lt ZM (A - Z)M
• M (A, Z) ZM (A - Z)M - (E /c )
• Life of a star is based on a sequence of nuclear
  fusion reactions.
• Heat produced counteracts gravitational
  attraction and prevents collapse.

p
n
nuc
2
p
n
b
4
Binding energy and mass loss
Atotal no. nucleons Ztotal no. protons E
binding energy
Change from X to Y emits energy since Y is more
tightly bound per nucleon than X.
b
binding energy per nucleon
Fusion
Fission
A
X
X
Y
Y
Fe
5

• Stellar Evolution and Supernovae
• Series of collapses and fusions
  
• H gt He gt C gt Ne gt O gt Si
• Outer parts of star expand to form opaque and
  relatively cool envelope (red giant phase).
• Eventually, Si gt Fe most strongly bound of all
  nuclei
• Further fusion would absorb energy so an inert Fe
  core formed
• Fuel in core exhausted hence star collapses.

6
Stellar Evolution Schematic
Complete Star - a Red Supergiant
Nuclear Fusion Regions near Inert Fe Core
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Stellar Mass Ranges for Supernovae

• 2.0 lt M lt 8 M?
• 1.4 lt M lt 1.9 M?
• 8.0 lt M lt 15 M?
• M gt 1.9 M?
• 15 M? lt M

• Three possibilities

star
Type I SN
core
star
Type II SN
core

• If the star has lt 2 M? or the core is lt 1.4 M?,
  it
• undergoes a quiet collapse, shrinking to a
  stable
• White Dwarf.

Type II SN
star
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Stellar Mass Ranges (Cont.)

• Type I Small cores so C-burning phase occurs
  catastrophically in a C-flash explosion and star
  is disrupted
• 2.0 lt M lt 8 M? ? Disintegration/no
  Neutron Star
• Type II More massive, so when Si-burning begins,
  star shrinks very rapidly
• 8 lt M lt 15 M? ? Neutron Star
• 15 M? lt M ? Black Hole

star
star
star
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Stellar Collapse and Supernova Summary

• Stars with a defined mass range evolve to produce
  cores that can collapse to form Neutron Stars
• Following nuclear fuel exhaustion, core collapses
  gravitationally this final collapse supplies the
  supernova energy
• Collapse to nuclear density, in few seconds,
  is followed by a rebound in which the outer parts
  of the star are blown away
• The visible/X-ray supernova results due to
  radiation from this exploded material and later
  from shock-heated interstellar material
• Core may
• Disintegrate
• Collapse to a Neutron star
• Collapse to a Black Hole
• according to its mass which in turn
  depends on the mass of the original evolved star

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Energy Release in Supernovae

• Outer parts of star require gt10 J to form a
  Supernova how does the implosion lead to an
  explosion?
• Once the core density has reached 10
  - 10 kg m , further collapse impeded by
  nucleons resistance to compression
• Shock waves form, collapse gt explosion, sphere
  of nuclear matter bounces back.

44
17
18
-3
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Shock Waves in Supernovae

• Discontinuity in velocity and density in a flow
  of matter.
• Unlike a sound wave, it causes a permanent
  change in the medium
• Shock speed gtgt sound speed - between 30,000 and
  50,000 km/s.
• Shock wave may be stalled if energy goes into
  breaking-up nuclei into nucleons.
• This consumes a lot of energy, even though the
  pressure (nkT) increases because n is larger.

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Importance of Neutrinos

• Neutrinos carry energy out of the star
• They can
• - Provide momentum through collisions to
• throw off material.
• - Heat the stellar material so that it
  expands.
• Neutrinos have no mass (like photons) and
  can
• traverse large depths without being
  absorbed
• but they do interact at typical stellar
  core
• densities r gt 1015 kg m-3

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Neutrinos (Cont.)

• Thus a stalled shock wave is revived by neutrino
  heating.
• Boundary at 150 km
• inside ? matter falls into core
• outside ? matter is expelled.
• After expulsion of outer layers, core forms
  either
• neutron star (M lt 2.5 M?) or
• black hole (depends on gravitational field which
  causes further compression).
• Neutrino detectors set up in mines and tunnels -
  must be screened from cosmic rays.

core
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Neutrinos (Cont.)

• Neutrinos detected consistent with number
  expected from supernova in LMC in Feb 1987.
• Probably type II SN because originator was
  massive B star (20 M?)
• Neutrinos are rarely absorbed so energy changed
  little over many x 10 years (except for loss
  due to expansion of Universe) thus they are very
  difficult to detect.
• However density of collapsing SN core is so high
  however that it impedes even neutrinos!!!

9
15
Supernovae
45

• Energy release 10 J in type I and II SN
• Accounts for v gt10,000 km/s initial velocity of
  expanding Supernova Remnant (SNR) shell.
• Optically the star brightens by more than 10
  mag in a few hours, then decays in weeks -months
• Explosive nucleosynthesis
• Reactions of heavy nuclei produce 1 M? of
• Ni which decays to Co and Fe in
  months.
• Rate of energy release consistent with optical
  light curves (exponential decay t 50 - 100 d)

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56
56
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Shock Expansion

• At time t0, mass m of gas is ejected with
  velocity v and total energy E .
• This interacts with surrounding interstellar
  material with density r and low temperature.
• System radiates (dE/dt) . Note E 10 J

0
0
0
0
Shock front, ahead of heated material
R
Shell velocity much higher than sound speed in
ISM, so shock front of radius R forms.
ISM, r
0
41-45
rad
0
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Supernova Remnants

• Development of SNR is characterized in phases
  values are averages for end of phase
• Phase I II
  III
• Mass swept up (M?) 0.2 180 3600
• Velocity (km/s) 3000 200
  10
• Radius (pc) 0.9 11
  30
• Time (yrs) 90 22,000
  100,000

Phase IV represents disappearance of remnant
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SNR Development - Phase I

• Shell of swept-up material in front of shock does
  not represent a significant increase in mass of
  the system.
• ISM mass within sphere radius R is still small.

(1)
19

• Since momentum is conserved
• Applying condition (1) to expression (2) shows
  that the velocity of the shock front remains
  constant, thus
• v(t) v
  
• R(t) v t

(2)
0
0
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Supernova 1987A

• B3 I Star exploded in February 1987 in Large
  Magellanic Cloud (LMC).

• Shock wave now 0.13 parsec away from the
  star, and is moving at vo 3,000 km/s.

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Dusty gas rings light up

• Two sets of dusty gas rings surround the star in
  SN1987A, thrown off by the massive progenitor.

• These rings were invisible before light from
  the supernova explosion has lit them up.

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Shock hits inner ring
The shock has hit the inner ring at 20,000 km/s,
lighting up a knot in the ring which is 160
billion km wide.
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Chandra X-ray Images of SN 1987A

• X-ray intensities (0.5 8.0 keV) in colour with
  HST Ha images as contours

• Low energy X-rays are
• well correlated with
• optical knots in ring
• dense gas ejected by
• progenitor?

• Higher energy X-rays
• well correlated with radio
• emission fast shock
• hitting circumstellar H II
• region?

• No evidence yet for
• emission from central
• pulsar

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Phase II - adiabatic expansion

• Radiative losses are unimportant in this phase -
  no exchange of heat with surroundings.
• Large amount of ISM swept-up

(3)
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• Thus (2) becomes

since mo is small
(4)
Integrating
(5)
Substituting (4) for movo in (5)
R(t) 4v(t).t v(t) R(t)/4t
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• Taking a full calculation for the adiabatic shock
  wave into account for a gas with g 5/3

and

• Temperature behind the shock, T ? v2, remains
• high little cooling
• Typical feature of phase II integrated
  energy
• lost since outburst is still small

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N132D in the LMC

• SNR age 3000 years

• Ejecta from the SN slam into the ISM at
  more than 2,000 km/s creating shock
  fronts.

• Dense ISM clouds are heated by the SNR shock and
  glow red. Stellar debris glows blue/green

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SNR N 132D XMM CCD Image and Spectrum

• X-ray image gives a more
• coherent view of the SNR

• Lower ion stages (N VII,
• C VI) show T 5 MK gas
• in ISM filaments at limb

• Higher ion stages (Fe XXV)
• show T 40 50 MK gas
• more generally distributed

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Phase III - Rapid Cooling

• SNR cooled, gt no high pressure to drive it
  forward.
• Shock front is coasting
• Most material swept-up into dense, cool shell.
• Residual hot gas in interior emits weak X-rays.

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XMM X-ray Observations SNR DEM L71

• Remnant in Large Magellanic Cloud (LMC)
• d 52 pc diam ? 10 pc age ? 104 yr

• Just entering Phase III
• vshock 500 km/s Tinterior 15 MK, Tshell
  5 MK

• Shell emission dominates (XMM CCD spectra)

• Emission line spectrum from XMM RGS shows
• - thermal nature of the plasma
• - element abundances characteristic of LMC

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Phase IV - Disappearance

• ISM has random velocities 10 km/s.
• When velocity(SNR) is 10 km/s, it merges with
  ISM and is lost.
• Oversimplification!!!
  - magnetic field (inhomogeneities in ISM)
  - pressure of cosmic rays

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Example Nature of Cygnus Loop

• - passed the end of phase II
• - radiating significant fraction of its energy
  R 20pc
  v 115 km/s (from
  Ha)
• lifetime,
• 2 x 10 seconds 65,000 years

now
now
t
12
33
3

• Assuming v 7 x 10 km/s
  and r 2 x 10 kg m ,
• from (5) we find that m 10 M?
• Density behind shock, r, can reach 4r , (r is
  ISM density in front of shock.
• Matter entering shock heated to
• ( av. mass of particles in gas)

0
-21
-3
0
0
0
0
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• For fully ionized plasma (65 H 35 He)
• Cygnus Loop v 10 m/s
  gt T 2 x 10 K (from (6))
• But X-ray observations indicate T 5 x 10 K
  implying a velocity of 600 km/s. Thus Ha
  filaments more dense and slower than rest of SNR.

(6)
5
now
5
6
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Young SNRs

• Marked similarities in younger SNRs.
• Evidence for two-temp thermal plasma -
  low-T lt 5 keV (typically 0.5-0.6 keV)
  - high-T gt 5 keV
  (T 1.45 x 10 v K)
• Low-T - material cooling behind shock High-T -
  bremsstrahlung from interior hot gas

-5
2
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Older SNRs

• A number of older SNRs (10,000 years or more) are
  also X-ray sources.
• Much larger in diameter (20 pc or more)
• X-ray emission has lower temperature -
  essentially all emission below 2keV.
• Examples Puppis A, Vela, Cygnus Loop - all
  Crab-type SNRs.

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Crab Nebula

• 1st visible/radio object identified with cosmic
  X-ray source.
• 1964 - lunar occultation gt identification and
  extension
• Well-studied and calibration source (has a well
  known and constant power-law spectrum)

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Crab Nebula
Exploded 900 years ago. Nebula is 10 light years
across.
39

• No evidence of thermal component
• Rotational energy of neutron star provides energy
  source for SNR
  (rotational energy gt radiation)
• Pulsar controls emission of nebula via release of
  electrons
• Electrons interact with magnetic field to
  produce synchrotron radiation

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Spectrum of the Crab Nebula
Log flux density

• also g-rays detected up to 2.5x10 eV

Radio
IR-optical
X-ray
11
41

• Summarizing
  B 10 Tesla to produce
  X-rays n 10 Hz (ie. peak occurs in
  X-rays) E 3 x 10 eV
  t 30 years
• Also, expect a break at frequency corresponding
  to emission of electrons with lifetime lifetime
  of nebula. Should be at 10 Hz
  (l3000Angstroms). This and 30 year lifetime
  suggest continuous injection of electrons.

-8
nebula
18
m
13
e-
syn
15
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SUPERNOVAE

• END OF TOPIC