Presentation on theme: "Supernovae Supernova Remnants Gamma-Ray Bursts. Summary of Post-Main-Sequence Evolution of Stars M > 8 M sun M < 4 M sun Subsequent ignition of nuclear."— Presentation transcript:
Summary of Post-Main-Sequence Evolution of Stars M > 8 M sun M < 4 M sun Subsequent ignition of nuclear reactions involving heavier elements Fusion stops at formation of C,O core. Fusion proceeds; formation of Fe core. Supernova
Fusion of Heavier Elements Final stages of fusion happen extremely rapidly: Si burning lasts only for ~ 2 days. 12 6 C + 4 2 He → 16 8 O + 16 8 O + 4 2 He → 20 10 Ne + 16 8 O + 16 8 O → 28 14 Si + 4 2 He Onset of Si burning at T ~ 3x10 9 K → formation of S, Ar, …; → formation of 54 26 Fe and 56 26 Fe → iron core
Observations of Supernovae Supernovae can easily be seen in distant galaxies. Total energy output: E e ~ 3x10 53 erg (~ 100 L 0 t life,0 ) E kin ~ 10 51 erg E ph ~ 10 49 erg L pk ~ 10 43 erg/s ~ 10 9 L 0 ~ L galaxy !
Type I and II Supernovae Core collapse of a massive star: Type II Supernova Collapse of an accreting White Dwarf exceeding the Chandrasekhar mass limit → Type Ia Supernova. Type I: No hydrogen lines in the spectrum Type II: Hydrogen lines in the spectrum Type Ib: He-rich Type Ic: He-poor Type II P Type II L Light curve shapes dominated by delayed energy input due to radioactive decay of 56 28 Ni
The Famous Supernova of 1987: SN 1987A BeforeAt maximum Unusual type II Supernova in the Large Magellanic Cloud in Feb. 1987 Progenitor: Blue supergiant (denser than normal SN II progenitor) 20 M 0 lost ~ 1.4 – 1.6 M 0 prior to SN Evolved from red to blue ~ 40,000 yr prior to SN
The Remnant of SN 1987A Ring due to SN ejecta catching up with pre-SN stellar wind; also observable in X-rays. v ej ~ 0.1 c Neutrinos from SN1987 have been observed by Kamiokande (Japan) Escape before shock becomes opaque to neutrinos → before peak of light curve provided firm upper limit on e mass: m e < 16 eV
Remnant of SN1978A in X-rays Color contours: Chandra X-ray image White contours: HST optical image
Supernova Remnants The Cygnus Loop The Veil Nebula The Crab Nebula: Remnant of a supernova observed in a.d. 1054 Cassiopeia A Optical X-rays
Electrons are accelerated at the shock front of the supernova remnant: N e ( ,t) - q+1 -q Synchrotron Spectra of SNR shocks (I) ∂N e /∂t = -(∂/∂ )( N e ) + Q( ,t). Q( ,t) = Q 0 -q cc Uncooled Cooled
Resulting synchrotron spectrum: I - q Opt. thin, uncooled Opt. thick 5 Synchrotron Spectra of SNR shocks (II) -q Opt. thin, cooled sy,c = sy ( c ) Find the age of the remnant from t = ( c / [ c ]).
Gamma-Ray Spectra of SNRs SNR shocks may accelerate protons and electrons to ultrarelativistic energies → Two possible models for -ray emission: 1.Leptonic Model: Compton scattering of CMB photons by ultrarelativistic electrons 2.Hadronic model: 0 decay following p-p collisions of ultrarelativistic protons.
Multiwavelength Spectra of SNRs HESS J1640-465 e - synchrotron bremsstrahlung Compton 0 decay
Gamma-Ray Bursts (GRBs) Short (sub-second to minutes) flashes of gamma-rays
GRB Light Curves Long GRBs (duration > 2 s)Short GRBs (duration < 1 s) Possibly two different types of GRBs: Long and short bursts
General Properties Random distribution in the sky Approx. 1 GRB per day observed No repeating GRB sources
Afterglows of GRBs Most GRBs have gradually decaying afterglows in X-rays, some also in optical and radio. X-ray afterglow of GRB 970228 (GRBs are named by their date: Feb. 28, 1997) On the day of the GRB3 days after the GRB
Optical afterglow of GRB 990510 (May 10, 1999) Optical afterglows of GRBs are extremely difficult to localize: Very faint (~ 18 – 20 mag.); decaying within a few days. 1 day after GRB2 days after GRB
Optical Afterglows of GRBs Optical afterglow of GRB 990123, observed with Hubble Space Telescope (HST/STIS) Long GRBs are often found in the spiral arms (star forming regions!) of very faint host galaxies Host Galaxy Optical Afterglow
Energy Output of GRBs Observed brightness combined with large distance implies huge energy output of GRBs, if they are emitting isotropically: E ~ 10 54 erg L ~ 10 51 erg/s Compactness argument (homework set 5, problem 3) implies that the gamma-rays must be relativistically boosted with > 100
Blast-Wave Model of GRBs Impulsive electromagnetic energy release Entrainment of ions – conversion of EM to kinetic energy Coasting phase – internal shocks (prompt GRB) Deceleration phase – external shock (afterglow) time Lorentz factor ext 0 M 0 c 2 = E BW 00 M sw = M 0 Relativistic Mass swept-up from the external medium (in co-moving frame)
Beaming Evidence that GRBs are not emitting isotropically (i.e. with the same intensity in all directions), but they are beamed: E.g., achromatic breaks in afterglow light curves. GRB 990510
Models of GRBs (I) Hypernova: Leading model for long GRBs: Supernova explosion of a very massive (> 25 M sun ) star Iron core collapse forming a black hole; Material from the outer shells accreting onto the black hole Accretion disk => Jets => GRB!
Models of GRBs (II) Black-hole – neutron-star merger: Black hole and neutron star (or 2 neutron stars) orbiting each other in a binary system Neutron star will be destroyed by tidal disruption; neutron star matter accretes onto black hole => Accretion disk => Jets => GRB! Model works probably only for short GRBs.