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

PN image from HST Ring Nebulae (M 57) Young White Dwarf Stars with 10 6 gr/cm 3 that simply cool with time. Kalirai et al ‘ M  Stellar Lifetime(Myr)

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

Drawing from Classics

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!!

Piro ‘05 Accreting White Dwarfs in our Galaxy <1% of white dwarfs are in binaries where accretion occurs, releasing gravitational energy 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. Donor star White Dwarf

M87 in Virgo Some numbers: 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? Two WDs are made per year in a M  elliptical galaxy. The observed rates for thermonuclear poweered objects are: Helium novae (Eddington-limited) every ~250 years, one large He explosion every ~5,000 years, and WD-WD mergers every 200 years Predicted rates are:

The Accretion Matrix He WDC/O WDO/Ne WD H/He from Low Mass MS star H/He from MMS H/He from RGB/AGB Novae/CV Supersoft/Merge r ?? Novae/CV Supersoft Symbiotics Novae/CV Supersoft Symbiotics Helium WD Helium Burning Star Unstable? AmCvn/RCorBor Helium Rich CV AmCvn/RCorBor Helium Rich CV C/O WDNot likelyMergers.. Type of accreting WD Type of Donor Red= Resulting Accreting Binary as Observed Black= Resulting Accreting Binary as Predicted

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

Recurrent Novae Imply Massive WDs Shen & LB 2009 Recurrence times of 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 M  /yr => 3 Myrs to add 0.3 M , of interest as a way to ignite core.

Type Ia Supernovae This motivates the “standard story” of unstable C ignition in the core from a single degenerate H donor.... The density must >10 9 gr/cm 3 in the cold (~10 8 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. 12 C+ 12 C ignition Nomoto, Thielemann and Yokoi 1984

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 C P the heat capacity of the WD (Hernanz et al. 1988; Nomoto 1982) Heat Transport in the White Dwarf Core Townsley & LB 2004

Carbon Ignition, NOT M>M ch If cold (T<3x10 8 K or so) and ‘low’ accretion rate, ignition is from high densities.. which only occurs for massive white dwarfs.. Yakovlev et al ‘07

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

Outcomes when dynamical burning occurs are actively debated..

Accretion of C/O at a high rate leads to: 1.Adiabatic compression of the core 2.Ignition at the outer edge, where there is a larger density change from accretion Rapid C/O Accretion from Mergers Nomoto and Iben 1985 Adiabat

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

Helium Accreting White Dwarfs 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 !! Angular momentum loss is gravitational wave emission, setting accretion rates!

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

Path to Dynamical Helium Shells 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: 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.

Minimum Requirements for Dynamic Onset => Explosion Bildsten et al. ’07, Shen and LB ‘09 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

Fink shock plots Fink, Hillebrandt and Ropke 2007

Fink shock plots Fink, Hillebrandt and Ropke 2007

Questions?