Accretion Induced Collapse of White Dwarfs

1. Accretion Induced Collapseof White Dwarfs Jordi Isern Institute for Space Sciences (CSIC-IEEC) In collaboration with E.Bravo (UPC-IEEC) & I. Domínguez (UGR) Anacapri, Naples May, 2009

2. Explosive sources of energy Gravitational collapse Thermonuclear explosion Neutron star Electron degenerate core {12C,16O}{56Ni} q ~ 7x1017 erg/g 1 Mo x q ~ 1051 erg K ~ 1051 erg Eem ~ 1049 erg Lmax ~ 1043 erg/s M ~ 1.4 Mo R ~ 106 cm M ~ 1.4 Mo R ~ 108-109 cm EG ~ 1053 erg K ~ 1051 erg Eem ~ 1049 erg Hoyle & Fowler (1960) Zwicky (1938)

3. Gamow picture of a core collapse supernovae Energy sources Crab Nebula

4. SNIa Observational constraints. I • H must be absent at the moment of the explosion • There are some evidences (weak) of H-lines before maximum or at late epochs • Progenitors should be long lived to account for their presence in all galaxies, including ellipticals • The explosion should produce at least ~ 0.3 M0 of 56Ni to account for the light curve and late time spectra • The short risetime indicates that the exploding star is a compact object • No remnant is left SNIa are caused by the explosion of a C/O white dwarf in binary systems

5. Third Uhuru Catalog (1973) # High—mass X—ray binaries: O – B star companion # Low—mass X—ray binaries: Low mass star, ~ 1 Mo

6. # The question posed by the LMXRs was: -Can the binary system resist the explosion of a massive star and the subsequent ejection of matter? # If not, two possible solutions: - Capture of a previously formed neutron star - “Non explosive” formation of a neutron star 1 • Non explosive means here without the ejection • of a huge amount of mass. The 1053 erg are always • present (Schatzman 1974).

7. Electron degeneracy At high densities e- are dominant If Even at T=0 electrons (and other fermions) are able to exert pressure! Zero temperature structures can exist

8. Hydrostatic Equilibrium Characteristic times Hydrodynamic time: HD 440 -1/2 Thermal time: 107 yr Nuclear time: 109 yr

9. Non relativistic electrons If electrons are non relativistic It is always possible to find an equilibrium structure The star only needs to contract Hydrostatic equilibrium: As the WD accretes matter it contracts and heats up M ~ R-1/3

10. Relativistic electrons If electrons are relativistic Hydrostatic equilibrium: It is not possible to find an equilibrium structure There is not a length scale If E < 0 R < 0 The star contracts If E > 0 R > 0 The star expands The ideal scenario for catastrophic events !

11. Accretion induced collapse • Idea: Shatzman (1974), Canal & Schatzman (1976), Ergma & Tutukov (1976) • A white dwarf accretes matter from a companion • Avoids the thermonuclear explosion and reaches the Chandrasekhar limit at a density that ensures the collapse • Small amount of mass is ejected and the binary survives • How to avoid the explosion? • Cooling the fuel: CO white dwarf case (Canal & Schatzman 1976; Canal & Isern 1979) • Less flamable fuel: ONeMg white dwarf case (Miyaji et al 1980)

12. # The energy losses by electron captures depend on the ignition density # The injected energy depends on the velocity of the burning front There is not a length scale If E < 0 R < 0 Star contracts If E > 0 R > 0 Star explodes Nuclear energy release Electron captures He cores always experiment a thermonuclear explosion CO cores can explode or collapse to a neutron star ONe cores always collapse to a neutron star Fe cores always collapse to a neutron star or black hole

13. Laminar Flame Woosley

14. Flame acceleration The laminar flame becomes turbulent: * Rayleigh-Taylor instability * Kelvin- Helmholtz Flame surface increases efective velocity increases Deflagration: subsonic velocity laminar flame: v ~ 0.01 cs Turbulent flame: v ~ 0.1 - 0.3 cs

15. The minimum velocity of the burning front is that of a laminar flame propagating conductively Minimum density for obtaining a collapse: 5.5 x 109 g/cm3 Bravo & Garcia—Senz (1999)

16. The CO white dwarf case Bravo et al 1996

18. Very high accretion rates ~ 10-5 Mo produce the off center ignition The critical issue is the maximum mass of CO white dwarfs

19. He-accreting white dwarfsmerging of CO+He white dwarfs; CO WD + Helium star (AM CVn stars) If 5x10-8 Mo/y  dMH/dt  10-9 Mo/yr. And MWD < 1.13 Mo an off center detonation forms • H-accreting white dwarfs(cataclysmic variables, symbiotic stars, supersoft X-ray sources) • dMH/dt < 10-9 Mo/yr.Nova explosions. Novae reduce the mass or produce a very • inefficient increase of the total mass, except MWD 1.2 Mo, but they are made of ONe • 10-6 Mo/y > dMH/dt > 10-9 Mo/yr. Hydrogen burns in flasshes, but produces He at a • rate that can ignite under degenerate conditions. • MEdd > dMH/dt > 10-6 Mo/yr. Formation of a red giant

20. Possible issues: rotation? The lifting effect of rotation allows the formation of more massive CO cores Domínguez et al’96

21. Possible issues: A change in the internal chemical composition?

22. ---- Oxygen ___ Carbon

23. Behavior upon crystallization Ts Ts 0 X2 1 0 X2 1

24. Change of the chemical profile because of solidification Plus a mixture of heavy species After solidification

25. Possible solutions: Changes in the 12C+12C rate? Gasques et al (2007) Reduction of the rate due to saturation: Gasques ety al (2007) Jiang et al (2007) # CO-WD easily colpase # Possibility to obtain more massive CO-WD

26. What about an increase of the reaction rate? Superbursts: Are long, energetic, rare explosions in LMXRBs, probably triggered by the 12C + 12C burning (Cumming & Bildstein 2001; Strohmayer & Brown 2002) • But this scenario has several problems (Cooper et al 2009) : • The ocean seems too cold for 12C ignition • Heavy fusion hindrance (Gasques et al 2007) reduces the reaction rate by a factor 2 • Triggering of superbursts demands large amounts of 12C. This not seems the case • Cooper et al (2009) have postulate a resonance near the Gamow peak (E ~ 1.5 MeV) similar to that found by Spillane et al (2007) • Secondary effects?

27. The onset of the thermonucearrunaway # The instability starts when nuclear reactions overwhelm the thermal netrino emission (typically T ~ 2x108 K) # A grrowing convective carbon burning core forms (C ~ 10-100 s < nuc; typically T < 7—8 x 108 K) # At this temperatures the characteristic time equilibrates and the thermonuclear runaway starts # Flame propagation is mediated by heat diffusion (deflagration) or shocks (detonation). Both T > 2 x 109 K) T 0 radius The duration of this phase is ~ 1000 yrs Burning of 0.05 Mo causes en expansion by a factor 3

28. Log of carbon fusion rate (cm3 s-1) Ignition line flames and detonations carbon simmering Influence of an additional resonance “a la Cooper et al” (E = 1.5 MeV; strength 0.13 meV) cm3s-1

29. Carbon ignition • The thermal structure of the white dwarf at carbon ignition (nuc~0.1-1 s) sets the initial conditions of the explosion  results of 3D simulations of SNIa depend on the initial distribution of hot spots (García-Senz & Bravo 2005, Röpke et al 2006, Meakin et al 2009) Carbon ignition occurs ~ 4.3 x 108 K

30. Implications of the hypothetical resonance at 1.5 MeV: • The ignition temperature is lowered down to T0~4.5108 K (the non-resonant value is ~8108 K) • nuc varies more steeply with T • less hot spots reach ignition conditions before the flame is born • smaller thermal contrast between a hot spot and the background temperature  it is easier for a hot bubble to survive dispersion due to Rayleigh-Taylor instability (Iapichino et al 2006) • multipoint ignition is hampered

31. Implications of the hypothetical resonance at 1.5 MeV (bis): • Smaller diference in binding energy from crossing the ignition line up to reaching the ignition temperature • less 12C has to be consumed before reaching the ignition temperature  less neutronization during simmering (by a factor ~ 0.4) .a fA large ratio of /p would imply a further reduction of the neutronization by a factor ~0.5 (at low Z)actor ~0.4)

32. The O-Ne case • Miyaji et al (1980) showed that stars in the range 8—12 Mo could develop ONeMg cores • Later on Woosley et al (1980), Nomoto (1984) reduced the range to 8—10 Mo S-AGB). The massive ones can not avoid the entire burning process. • Some of these S-AGBs end as O-Ne WDs and some of them enter in the process of electron captures and collapse to a neutron star (electron-capture supernovae) • The structure of the core of such S-AGBs is very different from the one of those arboring an Fe-core: • Very steep density gradient in the outer layers • Surrounded by an extremely extended loosely bound H/He envelope • Kitaura et al (2006) and Burrows et al (2007) have found successful explosions without invoking any acoustic mechanism

33. Electron captures

34. Thermal balance of electron captures Electron captures are endothermic only when electron comes from well above the thermal tail This equations need to include the chemical potentials of the involved nuclei. The threshold for e-capture increases

35. A fundamental ingredient: Electron captures create a thermal gradient that induces convection and tends to inhibit the ignition but also a chemical composition gradient that tends to inhibit convection and favors ignition)!

36. Evolution of O-Ne-Mg cores Gutierrez et al (1996)

37. Two critical points: 1) The abundance of 24Mg The off-center ignition is due to the energy trnsfered and the cntraction induced by electron captures Mg cannot trigger the explosion unless ~ 25%

38. Two critical points: 2) The presence of residual 12C Models suggest incomplete C-burning Xc </~ 1% Domínguez et al’93 Ritossa et al’96 The C-reaction rate at low temperatures is critical!

40. Spectrum at maximum light • Peak: absorption • CII OI SiII • SI CaII MgII • Incomplete burning 10000  15000 km/s Near-IR: SiII CaII MgII Fe peak Hatano et al. 1999

41. Observational constraints. II • Intermediate elements must be present in the outer layers to account for the spectrum at maximum light • The burning must be subsonic. It can be supersonic only if ρ < 107 g/cm3 The abundances of the iron peak elements (54Fe, 58Ni, 54Cr) must be compatible with the Solar System abundances after mixing with gravitational supernova products • Neutron excesses have to be avoided: • Post-burning e- -captures • Neutrons stored as 22Ne • Decrease ignition density • Decrease 22Ne content • Reduce the SNIa galactic contribution

42. Observational constraints. III • Homogeneity? • Differences in brightness: Overluminous (SN 1991T), underluminous (SN1991bg) • Differences in the expansion velocity (vexp ~ 10,000-15,000 km/s) • Two points of view: • There is a bulk of homogeneous supernovae plus some peculiars • SNIa display a continuous range of values • Is there a unique scenario & unique mechanism able to accommodate the normal behavior plus that of dissidents? • Is there a mechanism able to produce a continuous range of situations? • Can both mechanisms coexist? Anything able to explode eventually do it !!!