How to Ignite a White Dwarf !!

1. How to Ignite a White Dwarf !! Lars Bildsten Kavli Institute for Theoretical Physics and Dept of Physics University of California, Santa Barbara Jesusita Fire, May 2009 Photo: K. Paxton

2. Stars with < 6-8 M make 0.5-1.0 M Carbon/Oxygen white dwarfs with radius ~ Earth and central densities >106 gr/cm3 that simply cool with time. PN image from HST Ring Nebulae (M 57) 1.05 M Kalirai et al ‘07 Kalirai et al ‘07 Stellar Lifetime(Myr) Young White Dwarf 500 100 50

3. How to Ignite a White Dwarf • Single stellar evolution does not appear to cause thermonuclear explosions • We need to provoke a thermonuclear runaway that proceeds at such a rate that the matter releases an energy per gram in excess of the gravitational binding energy on a timescale shorter than the Kelvin Helmholtz time => Becomes unbound! • White Dwarfs have lots of fuel (He/C/O) and gravitational binding energies per gram less than that of nuclear burning: 1 MeV per baryon Use accretion in a Binary as the trigger

4. Drawing from Classics

5. Disclaimer I was asked by the organizers to give a pedagogical talk…. so my referencing of all the vast literature is not thorough. Please don’t be offended if I don’t reference YOUR most excellent paper. Trust me, I read your paper and I loved it!!

6. Accreting White Dwarfs in our Galaxy <1% of white dwarfs are in binaries where accretion occurs, releasing gravitational energy Donor star Whereas nuclear fusion of HHe or HeC releases This contrast is further enhanced when the white dwarf stores fuel and burns it rapidly, making these binaries detectable in distant galaxies during thermonuclear events. White Dwarf Piro ‘05

7. Two WDs are made per year in a 1011 Melliptical galaxy. The observed rates for thermonuclear poweered objects are: Some numbers: M87 in Virgo • 20 Classical Novae (Hydrogen fuel) per year, implying a white dwarf/main sequence contact binary birthrate of one every 400 years. • One Type Ia Supernovae every 250 years, or one in 500 WDs explode! • Supersoft Sources? Predicted rates are: Helium novae (Eddington-limited) every ~250 years, one large He explosion every ~5,000 years, and WD-WD mergers every 200 years

8. The Accretion Matrix Type of accreting WD Type of Donor Red= Resulting Accreting Binary as Observed Black= Resulting Accreting Binary as Predicted

9. Hydrogen Burning is Usually Unstable Supersoft Sources: Burn H Stably (van den Heuvel et al 1992), or weakly unstable. Accretion phase ~10 Myrs Townsley & LB ‘05 Accumulated mass Cataclysmic Variables: unstable burning leads to Classical Novae. Whether the mass stays or goes is uncertain, but WDs are not massive enough!

10. Recurrent Novae Imply Massive WDs Recurrence times of 12-80 years, implying massive WDs Not known if all the accreted matter is expelled during the event. If not, then the WD mass increases Accretion rate onto the WD Core is likely 10-7 M/yr => 3 Myrs to add 0.3 M, of interest as a way to ignite core. Shen & LB 2009

11. Type Ia Supernovae This motivates the “standard story” of unstable C ignition in the core from a single degenerate H donor. . . . • The density must >109 gr/cm3 in the cold (~108 K) core to trigger C burning. This requires M>1.33M and accumulation of mass during accretion. . . • Challenge is the outcome of H and He burning, and how mass accumulates to trigger C ignition in the core, leading to MANY progenitor scenarios. 12C+12C ignition Nomoto, Thielemann and Yokoi 1984

12. Heat Transport in the White Dwarf Core • The behavior of the WD core depends on the accretion time, • compared to the time it takes for heat to flow from the hotter surface set by the temperature from H or He burning: • where K is the conductivity and CP the heat capacity of the WD (Hernanz et al. 1988; Nomoto 1982) Townsley & LB 2004

13. Carbon Ignition, NOT M>Mch If cold (T<3x108 K or so) and ‘low’ accretion rate, ignition is from high densities.. which only occurs for massive white dwarfs.. Yakovlev et al ‘07

14. Carbon ignites => 1000 yrs of Simmering Before Dynamics Sets in!! Nomoto et al. 1984; Woosley & Weaver 1986 th = 10tdyn th = 1hr Central trajectory th = 1day Carbon ignition curves (Yakovlev et al. ‘06) Piro & LB ‘08

15. Outcomes when dynamical burning occurs are actively debated. .

16. Rapid C/O Accretion from Mergers Accretion of C/O at a high rate leads to: Nomoto and Iben 1985 Adiabatic compression of the core Ignition at the outer edge, where there is a larger density change from accretion Adiabat

17. Rapid C/O Accretion (Cont.) Rapid accretion results in an off-center ignition that likely leads to burning C/O to O/Ne and maybe NS formation, The accretion rate needs to be <10-6 M/yr to have ignition start in the core. ~70 Myr ~Gyr

18. Helium Accreting White Dwarfs Angular momentum loss is gravitational wave emission, setting accretion rates! • P>60 minutes, the donors are Hydrogen rich main sequence stars. • H-rich stars have a minimum radius of 0.1R so that P<60 min. implies He-rich donors !!

19. Helium Ignition on C/O Cores • Just as in AGB stars, the accretion of helium leads to thermally unstable flashes • These are mass and accretion rate dependent • Squares (triangles) are for 0.6 (0.8) M WDs, triangles for >1.0 M Shen and LB ‘09

20. The radial expansion of the convective region allows the pressure at the base to drop. For low shell masses, this quenches burning. For a massive shell, however, the heating timescale set by nuclear reactions: Path to Dynamical Helium Shells will become less than the dynamical time, So that the heat cannot escape during the burn, potentially triggering a detonation of the helium shell. This condition sets a minimum shell mass.

21. Minimum Requirements for Dynamic Onset => Explosion • For a He burning star donor (Star); Savonije et al 86; Ergma & Fedorova ‘90), He ignition masses >0.2M occur on 0.6M WDs and were studied as double detonations (Nomoto ‘82, Livne ‘90, Woosley et al ‘86, Woosley & Weaver ‘94). • The AmCVn systems have much lower ignition masses, opening up .Ia SNe options and/or core C/O ignitions Bildsten et al. ’07, Shen and LB ‘09

22. Fink shock plots Fink, Hillebrandt and Ropke 2007

23. Fink shock plots Fink, Hillebrandt and Ropke 2007

24. Questions?