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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] 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 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. 3 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 4 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 5 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. 6 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 7 the progressively reducing element abundances Stellar Evolution Schematic Complete Star a Red Supergiant Nuclear Fusion Regions near Inert Fe Core 103 R core 8 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. 9 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 10 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 11 Energy Release in Supernovae 44 • 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 17 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. 12 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. 13 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 14 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. 15 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!!! 16 Supernovae 45 • 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 56 56 56 Ni which decays to Co and Fe in ~ months • Rate of energy release consistent with optical light curves (exponential decay; t ~ 50 - 100 d) 17 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 19 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 20 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) 21 • 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 22 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. 23 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. 24 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. 25 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 26 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) 27 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 28 • 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 29 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 30 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 31 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. 32 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 33 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) 34 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 35 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) 36 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. 37 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 38 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 39 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) 40 Crab Nebula Pulsar Exploded 900 years ago. Nebula is 10 light years across. 41 • 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 42 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 43 • 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. 44 SUPERNOVAE END OF TOPIC 45