Supernovae High Energy Astrophysics [email protected]
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Transcript of Supernovae High Energy Astrophysics [email protected]
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3. Supernovae: Stellar evolution, collapse and energy release; Shock waves; Neutrinos; Phases of shock expansion; X-ray 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 (A, Z) < ZM + (A - Z)M → Mass deficit
• 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
Nuc p n
p n b2
Nuc
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Binding energy and mass lossA=total no. nucleonsZ=total no. protonsE = binding energy
bin
din
g en
ergy
per
nu
cleo
n
X XYY FeA
Change from X to Y emits energy since Y is more tightly bound per nucleon than X.
b
Fusion
Fission
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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 SequenceFor 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
the progressively reducing element abundances
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Stellar Evolution Schematic
Complete Star -a Red Supergiant
Nuclear Fusion Regionsnear Inert Fe Core
103 R
core
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2.0 < M < 8 M
1.4 < M < 1.9 M
8.0 < M < 15 M
M > 1.9 M
15 M < M
Type I SN
Type II SN (NS)
star
star
core
core
• If the star has < 2 M or the core is < 1.4 M, it
undergoes a quiet collapse, shrinking to a stable
White Dwarf.
Stellar Mass Ranges for Supernovae• Three possibilities:
starType II SN (BH)
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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
Type II: More massive, so when Si-burning begins, star shrinks very rapidly 8 < M < 15 M → Neutron Star
15 M < M → Black Hole
Stellar Mass Ranges (Cont.)
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 i. From this exploded material
ii. Later from shock-heated interstellar material
• Core may i. Disintegrateii. Collapse to a Neutron stariii. 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 >10 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 => explosion, sphere of nuclear matter bounces back.
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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 >> 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 > 1015 kg m-3
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• 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 < 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
Neutrinos (Cont.)
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• 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 that it impedes even neutrinos!!!
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Neutrinos (Cont.)
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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 Ni which decays to Co and Fe in ~ months • Rate of energy release consistent with optical
light curves (exponential decay; ~ 50 - 100 d)
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56 56 56
Supernovae
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ISM,
• At time t=0, mass m of gas is ejected with velocity v and total energy E .
• This interacts with surrounding interstellar material with density and low temperature.
• System radiates (dE/dt) . Note E ~10 J0
00
0
0
Shock front, ahead of ‘heated’ material
Shell velocity much higher than sound speed in ISM, so shock front of radius R forms.
R
rad 0
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Shock Expansion
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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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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.
)(3
4 300 tRm
(1)
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• 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
)()).(3
4( 3
0000 tvtRmvm (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.
SNR
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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 H 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:
)(3
4 300 tRm
(3)
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Thus (2) becomes :
)()(3
4 3000 tvtRvm
dt
tdRtR
)()(
3
4 30
)(3
4000 tRtvm
(4)
(5)Integrating:
R(t) = 4v(t).t v(t) = R(t)/4t
since mo is small
Substituting (4) for movo in (5)
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• Taking a full calculation for the adiabatic shock wave into account for a gas with = 5/3:
5
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1
0
017.1)( tE
tR
t
tRtv
)(4.0)( and
0Edtdt
dE
RAD
• Temperature behind the shock, T v2, remains high – little cooling
• Typical feature of phase II – integrated energy lost since outburst is still small:
2
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3v
k
mT
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N132D in the LMC
• 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
• SNR age ~ 3000 years
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
• Most material swept-up into dense, cool shell.
• Residual hot gas in interior emits weak X-rays.
vR 03
3
4 = constant
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XMM X-ray Observations: SNR DEM L71• Remnant in Large Magellanic Cloud (LMC): d = 52 kpc; diam → 10 pc; age → 104 yr
0.7 – 1.0 keV
Chandra X-ray image: shell & centre
• Just entering Phase III: vshock ~ 500 km/s; Tinterior ~ 15 MK, Tshell ~ 5 MK
Shell
Interior
XMM CCD Spectra
• Shell emission dominates (XMM CCD spectra)• Emission line spectrum from XMM RGS shows: - thermal nature of the plasma - element abundances like LMC
XMM Reflection Grating Spectrometer (RGS) spectrum
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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
R ~ 20pc v ~ 115 km/s (from Hfilaments)
Lifetime,
= 2 x 10 seconds = 65,000 years
now
now
sec1015.1
4.0103204.0
5
16
now
now
v
Rt ~
12
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Assuming v = 7 x 10 km/s and = 2 x 10 kg m
from (5) we find that m ~10 M
Density behind shock, , can reach 4is ISM density in front of shock)
Matter entering shock heated to:( = av. mass of particles in gas)
03
-21 -30
0
0
0
2
16
3v
k
mT
m
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For fully ionized plasma (65% H; 35% He)
Cygnus Loop: v ~ 10 m/s → 100 km/s
=> 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 H filaments are denser and slower than rest of the SNR structures.
251045.1 vT (6)
now5
5
6
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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) - high-T > 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 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
Exploded 900 years ago. Nebula is 10 light years across.
Pulsar
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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
also -rays detected up to 2.5x10 eV
Log flux density
-22
-32
Wat
ts p
er s
q m
per
Hz
8 10 16 20Log (Hz)
Radio
IR-optical
X-ray
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• Summarizing: B ~ 10 Tesla to produce X-rays ~ 10 Hz (i.e. peak occurs in X-rays) E ~ 3 x 10 eV
~ 30 years
• Also, expect a break at frequency corresponding to emission of electrons with lifetime = lifetime of nebula. Should be at ~10 Hz (~3000Angstroms). This and 30 year lifetime suggest continuous injection of electrons.
nebula-8
m18
e-13
syn
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SUPERNOVAE
END OF TOPIC
