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Supernovae High Energy Astrophysics

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Presentation on theme: "Supernovae High Energy Astrophysics"— Presentation transcript:

1 Supernovae High Energy Astrophysics

2 2 3.Supernovae: Stellar evolution, collapse and energy release; Shock waves; Neutrinos; Phases of shock expansion; X- ray spectra [3]

3 3 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 stars mass. Dense remnant, often a neutron star, remains.

4 4 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 Nucpn pnb 2

5 5 Binding energy and mass loss A=total no. nucleons Z=total no. protons E = binding energy binding energy per nucleon XXYYFe A Change from X to Y emits energy since Y is more tightly bound per nucleon than X. b Fusion Fission

6 6 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.

7 7 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 the progressively reducing element abundances

8 8 Stellar Evolution Schematic Complete Star - a Red Supergiant Nuclear Fusion Regions near Inert Fe Core 10 3 R core

9 9 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 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: star Type II SN (BH)

10 10 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

11 11 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.Disintegrate ii.Collapse to a Neutron star iii.Collapse to a Black Hole according to its mass which in turn depends on the mass of the original evolved star

12 12 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 kg m, further collapse impeded by nucleons resistance to compression Shock waves form, collapse => explosion, sphere of nuclear matter bounces back

13 13 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.

14 14 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 > kg m -3

15 15 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.)

16 16 Neutrinos detected consistent with number expected from supernova in LMC in Feb 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!!! 9 Neutrinos (Cont.)

17 17 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; ~ d) Supernovae

18 18 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 J Shock front, ahead of heated material Shell velocity much higher than sound speed in ISM, so shock front of radius R forms. R rad Shock Expansion

19 19 Supernova Remnants Development of SNR is characterized in phases – values are averages for end of phase Phase I II III Mass swept up (M ) Velocity (km/s) Radius (pc) Time (yrs) 90 22, ,000 Phase IV represents disappearance of remnant

20 20 Summary of SNR Expansion Phases I.m o >> M ISM II.m o < M ISM - shock heated gas adiabatic due to high temperature III.m o < < M ISM - gas cools radiatively at constant momentum

21 21 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)

22 22 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

23 23 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 v o ~ 3,000 km/s. SNR

24 24 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.

25 25 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.

26 26 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

27 27 Phase II - adiabatic expansion Radiative losses are unimportant in this phase - no exchange of heat with surroundings. Large amount of ISM swept-up: (3)

28 28 Thus (2) becomes : (4) (5) Integrating: R(t) = 4v(t).t v(t) = R(t)/4t since m o is small Substituting (4) for m o v o in (5)

29 29 Taking a full calculation for the adiabatic shock wave into account for a gas with = 5/3: and Temperature behind the shock, T v 2, remains high – little cooling Typical feature of phase II – integrated energy lost since outburst is still small:

30 30 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

31 31 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

32 32 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. = constant

33 33 XMM X-ray Observations: SNR DEM L71 Remnant in Large Magellanic Cloud (LMC): d = 52 kpc; diam 10 pc ; age 10 4 yr 0.7 – 1.0 keV Chandra X-ray image: shell & centre Just entering Phase III: v shock ~ 500 km/s; T interior ~ 15 MK, T shell ~ 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

34 34 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)

35 35 Example – Nature of Cygnus Loop - passed the end of phase II - radiating significant fraction of its energy R ~ 20pc v ~ 115 km/s (from H filaments) Lifetime, = 2 x 10 seconds = 65,000 years now t ~ 12

36 36 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 4 is ISM density in front of shock) Matter entering shock heated to: ( = av. mass of particles in gas)

37 37 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. (6) now 5 5 6

38 38 Young SNRs Marked similarities in younger SNRs. Evidence for two-tempertaure thermal plasma - low-T 5 keV (T = 1.45 x 10 v K) Low-T - material cooling behind shock High-T - bremsstrahlung from interior hot gas -52

39 39 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

40 40 Crab Nebula First visible/radio object identified with a cosmic X-ray source lunar occultation => identification and extension Well-studied and calibration source (has a well known and constant power-law spectrum)

41 41 Crab Nebula Exploded 900 years ago. Nebula is 10 light years across. Pulsar

42 42 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

43 43 Spectrum of the Crab Nebula also -rays detected up to 2.5x10 eV Log flux density Watts per sq m per Hz Log (Hz) Radio IR-optical X-ray 11

44 44 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 m 18 e-e- 13 syn 15


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