Supernovae: If this isn’t “high energy,” then I don’t know what is.

1. Supernovae:If this isn’t “high energy,” then I don’t know what is. High Energy Astrophysics 9/28/2012 Image courtesy NASA/JPL-Caltech

2. Supernovae were originally classified spectroscopically. Type II further divided by light curve characteristics (Plateau or Linear), or characteristics of hydrogen lines (Narrow) or eventual lack thereof (IIb)

3. Type Ia spectrum (Matheson et al. 2008)

4. Type II Spectrum Note the P Cygni profile of the Ha line. (Filippenko. 1997)

5. Of course, this classification scheme isn’t too useful, physically. Type Ia = white dwarf progenitor Core Collapse = massive star progenitor

6. So apart from their spectra, how can we tell that these are entirely separate beasts? SN 1994D (Ia) All types of supernovae can be found in spiral galaxies We only find Type Ia in elliptical galaxies SNe Ia show no preferences Image courtesy STScI

7. Supernovae are the catastrophic release of energy at the end of a star’s life. Luminosity ~109 L⨀ (generally higher for Ia) E ~ 1046 J (room for variance) Most energy in neutrinos, rest in kinetic energy and photons (Cadonau. 1987) Note: 1987A is a bit of an oddball Thanks Niel

8. Type Ia supernovae are surprisingly uniform, but not perfectly so. Branch & Miller 1993 showed that majority of known SNe Ia were not peculiar (recent work suggests higher rate)Small variance suggests common, consistent mechanismLeading candidate: thermonuclear explosion of an accreting white dwarf (Riess et al. 1996)

9. Because no Ia progenitors have been observed, we can only speculate on their nature. From (Livio. 2000), main questions are Composition of WD? Mass of WD? Where ignition occurs? Method of mass accretion?

10. Accreting white dwarf is most likely a carbon-oxygen white dwarf He WD: Typically smaller than 0.45 M⨀, ignite He around 0.7 M⨀. Produce He, 56Ni, decay products O-Ne WD: Form from MS stars ~10 M⨀, probably too uncommon. Likely to collapse to neutron star from accretion (could explain subluminous SNe Ia?) C-O WD: Form from MS stars ≲ 10 M⨀, masses between 0.8 and 1.2 M⨀. Expected to explode as SNe Ia with right accretion rate

11. Chandrasekhar mass carbon ignitors most likely to produce observed explosions. Ignite at center from compressional heating Strengths: right energy production and mechanism, ejecta composition, relative homogeneity, spectra fit in great detail. Weaknesses: accretion to 1.4 M⨀ difficult, higher initial mass WDs (~1.2 M⨀) prone accretion induced collapse, late time spectra disagree with models, rise times difficult to reproduce.

12. Sub-Chandrasekhar mass helium ignitors are deemed unlikely progenitors. Helium accretes, ignites off-center, pressure wave ignites C-O core (Indirect Double Detonation) Strengths: detonate at lower masses – easy to achieve proper statistics, better agreement with late time spectra, adequate light curves (fast rising) Weaknesses: early spectra do not agree, highest velocity ejecta have wrong composition, expect different luminosity function due to potential mass range

13. Single Denegerate accretion more likely to produce SNe Ia than Double Degenerate Statistics not a likely limitation on DD Rather, merger is more likely to produce neutron star (responsible for 91T events?) SD can reach 1.4 M⨀ accreting at high rate (≥ 10-8 M⨀ /yr) Too slow = recurrent novae

14. The winner: single carbon-oxygen white dwarf accreting from post-main sequence star to Chandrasekhar mass.

15. SNe Ia are useful as standardizable candles for distance indicators in cosmology Phillips (1993) showed correlation between peak brightness and decline rate Riess et al. (1995), picked this up and ran with it, gathering more data, fine-tuning correlation

16. Clearly they were on to something. (Riess et al. 1998)

17. Let’s not forget that we can get supernovae from massive stars as well >8 M⨀ stars can fuse up to iron in coreUndergo shell-burning to maintain equilibrium

18. Why stop at iron? A = 56

19. Eventually, we build up a degenerate iron core, because all it can do is sit there. As the core reaches the Chandrasekhar mass, we run into trouble. H → He He → C C→ O O → Ne Ne → Mg Mg → Si Si → Fe Inert Iron

20. In the degenerate, high-temperature environment, there is no way to radiate energy. A number of endothermic processes act to cool the core. • Photodissociation of iron (and the resulting helium). 56Fe + g → 13 4He + 4n 4He + g → 2p+ + 2n • Electron capture onto free protons. • Thermal neutrino production g + g → ne+ ne Sudden loss of energy and degeneracy pressure in core results in a very rapid collapse of inner layers (occurs in roughly 1 second)

21. Innermost layers collapse onto rigid proto neutron star and bounce outward, encountering still-oblivious outer layers and creating a massive, high-velocity shock. Proto-neutron star

22. Shockwave’s energy is sapped by dissociation of nuclei and attempting to plow through infalling material. Stalls about 100-200 km out from the PNS Proto-neutron star

23. Neutrinos to the rescue! PNS emits A LOT of neutrinos (En ≈ 100KE). Neutrinos interact with, and energize, material behind stalled shockwave. Makes supernova possible. PNS cools, and preferentially emits ne due to ne + n → p+ + e- This creates a neutron rich environment at the shock front via ne + p+ → n + e+ Possible site for the r-process.

24. Neutrinos deposit energy in the “gain region” just interior to the stalled shockThe shock need only wait seconds before neutrino luminosity becomes high enough. ne ne ne Proto-neutron star

25. Neutrinos allow shock to continue outwards and achieve breakout (ejects outermost layer of mass) Proto-neutron star

26. Shock breakout can be one source of high-energy radiation from core collapse supernovae X-ray flash marked beginning of SN 2008D, corresponding to shock breakout and first photon emissionLasted 7 minutes before fading to a UV/Optical afterglow (Soderberg et al. 2008)

27. We also see x-ray emission from the remnant of SN 1987A blowing into the ISM. X-ray emission increases as remnant expands Colliding with and heating surrounding medium Sadly, I could not find a spectrum associated with this emission directly, so we can’t see exactly how the material is emitting. (Larsson et al. 2011)

28. X-ray emission also associated with emission from highly ionized nuclei (Zhekov et al. 2005)

29. And, (because Peter Meszaros would retroactively fail me if I didn’t mention this), supernovae are believed to be the culprits for producing long GRBs. Long GRBs thought to result from supernovae with massive enough progenitors that they collapse to black holesBurst could result from accretion on to black hole