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
Stars with < 6-8 M make 0.5-1.0 M Carbon/Oxygen
white dwarfs with radius ~ Earth and central densities >106
PNcool
image
from HST
gr/cm3 that simply
with time.
Ring Nebulae (M 57)
1.05 M
Kalirai et al ‘07
Kalirai et al ‘07
Young White Dwarf
Stellar Lifetime(Myr)
500
100
50
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!!
Accreting White Dwarfs in our Galaxy
Donor star
<1% of white dwarfs are in
binaries where accretion occurs,
releasing gravitational energy
Whereas nuclear fusion of
HHe or HeC releases
White Dwarf
Piro ‘05
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.
Some numbers:
M87 in Virgo
Two WDs are made per year in a 1011 M
elliptical galaxy. The observed rates for
thermonuclear poweered objects are:
• 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
The Accretion Matrix
Type of Donor
Type of accreting WD
H/He from Low Mass
MS star
H/He from MMS
H/He from RGB/AGB
He WD
C/O WD
O/Ne WD
Novae/CV
Novae/CV
Novae/CV
Supersoft/Merge
r
Supersoft
Supersoft
Symbiotics
Symbiotics
??
Helium WD
Unstable?
AmCvn/RCorBor
AmCvn/RCorBor
Helium Burning Star
Unstable?
Helium Rich CV
Helium Rich CV
C/O WD
Not likely
Mergers. .
Mergers . .
Red= Resulting Accreting Binary as Observed
Black= Resulting Accreting Binary as Predicted
Hydrogen Burning is Usually Unstable
Townsley & LB ‘05
Accumulated mass
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!
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
Shen & LB 2009
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.
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.
Nomoto, Thielemann
and Yokoi 1984
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:
Townsley & LB 2004
where K is the conductivity and CP
the heat capacity of the WD
(Hernanz et al. 1988; Nomoto 1982)
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
Carbon ignites => 1000 yrs of Simmering Before
Dynamics Sets in!!
Nomoto et al. 1984; Woosley & Weaver 1986
th = 10tdyn
th = 1hr
th = 1day
Central
trajectory
Carbon ignition curves
(Yakovlev et al. ‘06)
Piro & LB ‘08
Outcomes when dynamical
burning occurs are actively
debated. .
Rapid C/O Accretion from Mergers
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
Nomoto and Iben 1985
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
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 !!
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 shock plots
Questions?