GP COMThe fate of less than 1 in 2000 white dwarfs in our galactic disk. But none yet seen in other galaxies.

Ken Shen (UCSB), Nevin Weinberg (UCB) & Gijs Nelemans (U. Nijmegen)

Pure helium accretion onto a Carbon/Oxygen or Oxygen/Neon WD in an AM CVn binary leads to unstable burning and helium shell flashes:

• The last flash is likely an explosion and ejection of 0.02-0.1 solar masses of radioactive 56Ni, 52Fe, and 48Cr

• This yields a Type I supernova with absolute V magnitude of -15 to -17 with a 3-5 day rise time

• If every AM CVn has one such explosion, the .Ia rate in old stellar populations

(E/S0’s) would be 2-7% of Type Ia rate.

Faint (.Ia)* Thermonuclear Supernovae from AM CVn Binaries

*thanks to Chris Stubbs for name!

L.B., Shen, Weinberg & Nelemans 2007, Ap J Letters, 662, L95

Helium Burning

10 20 30 40 50 60 70 Orbital Period (Minutes)

GP Com

CP Eri

ES Cet

AM CVn

CE 315

HP Lib

RXJ0806?

CR Boo, KL Dra

V407 Vul

V803 Cen

He burning is thermally stable when donor mass is 0.2-0.27, with orbital periods of 2.5-3.5 minutes (Tutukov & Yungelson ‘96). As the accretion rate drops, the burning becomes unstable, and flashes commence.

Thick= Cold DonorsThin= Hotter Donors

Unstable Helium Burning of Interest

What Happens to the Accreted Helium?

• At early times, there will be many weak flashes, He novae

• At late times, there will be one last explosive flash with an ignition mass of 0.03-0.1 solar masses, set by the accreting WD mass

• This last He flash can detonate, leading to a new kind of event, a “.Ia” supernovae

• After this, Helium simply accumulates (Bildsten et al. 06)

Evolutio

n in tim

e

L. B., Shen, Weinberg & Nelemans ‘07

Helium Donor Mass

Ignition Masses

Path to Dynamical Helium ShellsThe 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.

L. B., Shen, Weinberg & Nelemans ‘07

Binary Evolution Naturally Yields Dynamics The intersection of

the ignition masses with that of the donor yields the hatched region, most of which lie above the dynamical event line ==>

• For WD masses >0.9, potential outcome is detonation

• For lower WD masses, the outcome may be less violent, further work needed

Must understand the dynamic outcome and nucleosynthetic yields from these low pressure detonations, a new regime.

Example

Shock goes down (blue arrow) into the C/O and the He detonation (red arrow) moves outward. The shocked C/O under the layer is not ignited. Underlying WD remains

Yields at this point in time (0.24 seconds) are 0.012 M_sun of 56Ni, 0.0071 of 48Cr, and 0.0076 of 52Fe, much helium still not burned, but it will later.

Nevin Weinberg ‘07

Thermonuclear Supernova Lightcurves

• Type Ia result from burning a solar mass of C/O to ~0.6 solar masses of 56Ni (rest burned to Si, Ca, Fe) and ejected at v=10,000 km/sec. • This matter would cool by adiabatic expansion, but instead is internally heated by the radioactive decay chain 56Ni=>56Co=>56Fe• Arnett (1982) (also see Pinto & Eastman 2000) showed that the peak in the lightcurve occurs when the radiation diffusion time through the ejected envelope equals the time since explosion, giving

• The luminosity at peak is set by the instantaneous radioactive decay heating rate… ==> can measure the 56Ni mass via the luminosity, yielding the 0.1-1.0 solar mass range

• Our small Helium ignition masses (0.02-0.1) only detonate helium, which leaves the WD at 10,000 km/sec, leading to rapid rise times.

• The radioactive decays of the fresh 48Cr (1.3 days), 52Fe (0.5 d) and 56Ni (8.8 days) will provide power on this rapid timescale!!

L. B., Shen, Weinberg & Nelemans ‘07

.Ia Supernovae

.Ia Lightcurves courtesy of Daniel Kasen (JHU)

(M_tot,M_Ni)56Ni Balls

56Ni/Si Balls

Delta M_15(B)>3, and sometimes 4 (typical Ia’s have 2 at most)

.Ia Supernovae Rates• Roelofs et al ‘07 space density implies an AM CVn birthrate

• If every AM CVn gives a .Ia SN, their rate would be 2-7 % of the Type Ia rate in an Elliptical Galaxy.

• .Ia Discoveries would reveal distant AM CVns, and may well have been missed in SN surveys due to rapid decline.

• Volume rate in the nearby universe says that upcoming optical transient surveys with rapid cadences should find many. . . Daily survey 1/2 sky (LSST) to V=24 would give up to 1000 .Ia’s per year.

In 10^11 solar masses of old stars (e.g. E/S0 galaxy), two WDs are made per year. The observed rates for thermonuclear events are:• 20 Classical Novae (Hydrogen fuel) per year, implying a CV birthrate (Townsley & LB 2005) of one every 250-500 years.

• One Type Ia Supernovae every 250 years, or one in 500 WDs explode!Predicted He rates are: • Helium novae (Eddington-limited) every 250

years (V445 Puppis)

• One large He explosion every ~5,000 years

=> .Ia Supernovae

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

M87 in Virgo

.Ia SN Rates

• Solid line shows number per year if .Ia’s have a volume rate of 1e-6 .Ia’s/yr Mpc^3 (5% of the Ia rate)

• Some events are lost in the light of the Elliptical host, which depends on .Ia brightness (-15,-17 shown)

• Lines show all-sky detection rate assuming L* ellipticals and V=24 (SNLS is 4 deg^2 => < 1 .Ia/yr).

• PS1 medium deep survey at V=24, 50 deg^2 (grizy every 4 days) with 1 arc-sec seeing gets 10 per year at -17, and 1 per year at -15

• SDSS SN is V=22.5, 280 deg^2, every 2 day (1.5 arc-sec) gives 7 per year at -17, and 0.5 per year at -15

L. B., Shen, Weinberg & Nelemans ‘07

Conclusions and Future Work• Binary evolution with helium donors

naturally lead to dynamical helium shell burning, likely too weak to detonate carbon/oxygen WD, but likely detonating He (more work needed here!)

• The combination of a lower ejecta mass and more rapidly decaying radioactive elements makes the .Ia supernovae bright enough to be studied in nearby galaxies.

• Much theoretical work remains to be done, including

1. Better calculations of Helium ignition masses for changing accretion rate, including 14N electron captures… etc..

2. Hydrodynamic calculations to give full yields and velocity profiles (how much energy is lost to shocking the C/O WD?)

3. Lightcurve calculations, more robust rate estimates . . . .

• Detection would provide a census of AM CVn binaries in distant galaxies.. And nuclear flashes are the only way to see them.