Transcript Supernovae (last updated 2005/6)

Supernovae
High Energy Astrophysics
[email protected]
http://www.mssl.ucl.ac.uk/
3. Supernovae: Stellar evolution, collapse
and energy release; Shock waves;
Neutrinos; Phases of shock expansion; Xray spectra
[3]
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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 the force of its own
gravitation.
• Energy set free by the collapse expels most of
star’s mass.
• Dense remnant, often a neutron star, remains.
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Nuclear binding
• M
Nuc
(A, Z) < ZM + (A - Z)M → Mass deficit
p
n
• MNuc (A, Z) = ZMp + (A - Z)Mn - (Eb /c 2)
• Life of a star is based on a sequence of nuclear
fusion reactions
• Heat produced counteracts gravitational attraction
and prevents collapse
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binding energy per nucleon
Binding energy and mass loss
A=total no. nucleons
Z=total no. protons
E b= binding energy
Change from X to Y emits
energy since Y is more
tightly bound per nucleon
than X.
Fusion
Fission
X Y
Fe
Y
X
A
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Stellar Evolution and Supernovae
• Stellar evolution – a series of collapses and
fusions
H => He => C => Ne => O => Si
• Outer parts of star expand to form opaque
and relatively cool envelope (red giant
phase).
• Eventually, Si => 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.
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Stellar Evolution Sequence
For large stellar mass – M > 8 M
1. H fusion to He  Red Giant/H-fusion shell
2. He fusion to C  “
“ /He- “
“
3. C fusion to Ne  “
“ /C- “
“
[for M < 8 M → C-flash/star explodes]
4. Ne fusion to O  “
“ /Ne- “
“
5. O fusion to Si  “
“ /O- “
“
6. Si fusion to Fe  “
“ /Si- “
“
[BUT with inert Fe core!]
Each step of the cycle is shorter than its predecessors due to
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the progressively reducing element abundances
Stellar Evolution Schematic
Complete Star a Red Supergiant
Nuclear Fusion Regions
near Inert Fe Core
103 R
core
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Stellar Mass Ranges for Supernovae
• Three possibilities:
2.0 < M star < 8 M
1.4 < M core < 1.9 M
8.0 < M star < 15 M
Mcore > 1.9 M
15 M < Mstar
Type I SN
Type II SN
(NS)
Type II SN
(BH)
• If the star has < 2 M or the core is < 1.4 M, it
undergoes a quiet collapse, shrinking to a stable
White Dwarf.
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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 < M
< 8 M → Disintegration/no Neutron
star
Star
Type II: More massive, so when Si-burning begins, star
shrinks very rapidly
8 < M star < 15 M → Neutron Star
15 M < Mstar → Black Hole
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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
i.
ii.
•
From this exploded material
Later from shock-heated interstellar material
Core may
i.
ii.
iii.
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
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• Outer parts of star require >10 J to form a
Supernova… how does the implosion lead
to an explosion?
• Once the core density has reached
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18
-3
10 - 10 kg m , further collapse impeded
by nucleons resistance to compression
• Shock waves form, collapse => explosion,
sphere of nuclear matter bounces back.
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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 >> 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 (or very little) mass (like
photons) and can traverse large depths without
being absorbed but they do interact at typical
stellar core densities r > 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 (Mcore < 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.
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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 9 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
that it impedes even neutrinos!!!
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Supernovae
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• Energy release ≤ 10 J in type I and II SN
• Accounts for v >10,000 km/s initial velocity of
expanding Supernova Remnant (SNR) shell.
• Optically the “star” brightens by more than 10
magnitudes in a few hours, then decays in
weeks - months
Explosive nucleosynthesis:
• Reactions of heavy nuclei produce ~ 1 M of
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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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Shock Expansion
• At time t=0, mass m 0 of gas is ejected with
velocity v0 and total energy E 0.
• This interacts with surrounding interstellar
material with density r0 and low temperature.
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
• System radiates (dE/dt) rad. Note E0 ~10
41-45
J 18
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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Summary of SNR Expansion Phases
I.
mo >> MISM
II.
mo < MISM - shock heated gas adiabatic
due to high temperature
III. mo < < MISM - gas cools radiatively at
constant momentum
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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.
4
3
m0 
r 0 R (t )
3
(1)
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• Since momentum is conserved:
4
3
m0 v0  (m0 
r 0 R (t )). v(t )
3
(2)
• Applying condition (1) to expression (2) shows
that the velocity of the shock front remains
constant, thus :
v(t) ~ v 0
R(t) ~ v0 t
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Supernova 1987A
• B3 I Star exploded in
February 1987 in Large
Magellanic Cloud (LMC).
SNR
• 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; HST Ha images as contours
• Low energy X-rays
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:
4
3
m0 
r 0 R (t )
3
(3)
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Thus (2) becomes :
4
3
m0 v0 
r 0 R (t )v(t ) since mo is small
3
4
dR (t ) (4)
3

r 0 R (t )
3
dt
Integrating:

4
(5)
m0 v0t  r 0 R (t )
3
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:
1
5 2
5
R (t )
 E0 
R(t )  1.17  t and v(t )  0.4
t
r
 0
• Temperature behind the shock, T  v2, remains
high – little cooling
3 m 2
T
v
16 k
• Typical feature of phase II – integrated energy
lost since outburst is still small:
 dE 
dt

E


0
  dt  RAD
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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
Progenitor
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SNR N 132D XMM-Newton 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, => no high pressure to drive it
forward.
• Shock front is coasting
4 3
R r 0 v = constant
3
• Most material swept-up into dense, cool
shell.
• Residual hot gas in interior emits weak Xrays.
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XMM X-ray Observations: SNR DEM L71
• Remnant in Large Magellanic Cloud (LMC):
0.7 – 1.0 keV
d = 52 kpc; 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
Chandra X-ray image: shell & centre
- element abundances like LMC
Shell
Interior
XMM Reflection Grating Spectrometer (RGS) spectrum
XMM CCD Spectra
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Phase IV - Disappearance
• ISM has random velocities ~10 km/s.
• When velocity (SNR) is ~ 10 km/s, it merges with the
ISM and is ‘lost’.
------------------------------------------------------• Entire four-phase model represents an
oversimplification!!!
- magnetic field (inhomogeneities in ISM)
- pressure of cosmic rays
- shock interacts with interstellar clouds
(velocity and temperature decrease and radiation increases)
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Example – Nature of Cygnus Loop
- passed the end of phase II
- radiating significant fraction of its energy
Rnow ~ 20pc
v
~ 115 km/s (from Ha filaments)
now
Rnow 20 3 10  0.4
Lifetime, t ~ 0.4

sec
5
vnow
1.1510
16
= 2 x 10
12
seconds
= 65,000 years
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3
Assuming v0 = 7 x 10 km/s
and r = 2 x 10 -21 kg m-3
0
from (5) we find that m0 ~10 M
Density behind shock, r, can reach 4r 0
(r is ISM density in front of shock)
0
3 m 2
v
Matter entering shock heated to: T 
16 k
( m = av. mass of particles in gas)
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For fully ionized plasma (65% H; 35% He)
5
T  1.4510 v
2
(6)
5
Cygnus Loop: vnow ~ 10 m/s → 100 km/s
5
=> T ~ 2 x 10 K (from (6))
6
But X-ray observations indicate T ~ 5 x 10 K
implying a velocity of 600 km/s.
Thus Ha filaments are denser and slower
than rest of the SNR structures.
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Young SNRs
• Marked similarities in younger SNRs.
• Evidence for two-tempertaure thermal plasma
- low-T < 5 keV (typically 0.5 - 0.6 keV)
-5 2
- high-T > 5 keV (T = 1.45 x 10 v K)
• Low-T - material cooling behind shock
• High-T - bremsstrahlung from interior hot gas
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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 2 keV.
• Examples : Puppis A, Vela, Cygnus Loop
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Crab Nebula
• First visible/radio object identified with a cosmic
X-ray source.
• 1964 - lunar occultation => identification and
extension
• Well-studied and calibration source (has a well
known and constant power-law spectrum)
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Crab Nebula
Pulsar
Exploded 900 years ago. Nebula is 10 light years across.
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• No evidence of thermal component
• Rotational energy of neutron star provides
energy source for SNR
(rotational energy => 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
Watts per sq m per Hz
Log flux density
Radio
-22
IR-optical
X-ray
Log n (Hz)
-32
8
10
16
also g-rays detected up to
20
2.5x1011 eV
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• Summarizing:
B nebula ~ 10 -8 Tesla to produce X-rays
n m ~ 1018 Hz (i.e. peak occurs in X-rays)
13
Ee ~ 3 x 10 eV
tsyn ~ 30 years
• Also, expect a break at frequency
corresponding to emission of electrons with
lifetime = lifetime of nebula. Should be at
15
~10 Hz (l~3000Angstroms). This and 30
year lifetime suggest continuous injection of
electrons.
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SUPERNOVAE
END OF TOPIC
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