Nuclear-burning white dwarfs: Type Ia supernova

progenitors?

Theory and Observational Signatures

A tale in two parts: Rosanne Di Stefano and Jeno Sokoloski

"Over the state it appears, it presages military action, death and countrywide famine so that the population must seek refuge from their homes.” (1006)

"Over the state it appears, it presages military action, death and countrywide famine so that the population must seek refuge from their homes.” (1006)

"It...brings prosperity to the state over which it appears."

1572: “Tycho’s supernova”

1604: “Kepler’s star”

Foretelling calamity?

Kepler: No, it predicts greater sales of books.

Some thought that atoms had come together to form a new star.

Kepler: “If a pewter dish, leaves of lettuce, grains of salt, drops of water, vinegar, oil and slices of egg had been flying around in the air for all eternity, it might at last happen by chance that a salad would result.”

Forget about SNIa…for a moment.

Consider phenomena that must occur to WDs, whether or not they ever become

Type Ia supernovae.

• Start with binary stars.

• Some become binaries in which a white dwarf has a companion.

• Some of the white dwarfs accrete.

This is interesting in itself and has a range of observable

consequences.

• A wide range of rates are expected.

• Suppose there is some maximum WD mass, M_c.

• Do any of the WDs reach M_c?

S

The key issue: Does genuine mass retention occur?

Calculations by several groups say “yes”: Nomoto, Iben, Shen & Bildsten.

And some SSSs are steady. Could they be the systems we seek?

• There are many accreting WDs in the Galaxy.

This is interesting in itself and has a range

of observable consequences.

• There are thousands of nuclear-burning WDs that gain some mass.

• Some do reach M_c.

• There are uncertainties about the exact numbers.

To derive the rate: calculations

• The first population synthesis calculations indicated that, in principle, there are enough close binaries in which the WD can achieve M_c to account for the rate. (Rappaport, Di Stefano & Smith 1994)

• The addition of binary evolution calculations clarifies the issues that determine the rates (esp. winds; DiStefano et al. 1996, 1997, Di Stefano & Nelson 1996)

• Large range of models (Di Stefano 1996; Hachisu, Kato, & Nomoto 1996)

See Nelson 2007.

There could be enough nuclear-burning white dwarfs to supply the full rate of Type Ia

supernovae.

• But it is complicated!

• Several important uncertainties remain.

The nuclear- burning WDs are bright.

We can turn to pre-explosion observations

(1) of nearby galaxies.

(2) of the Milky Way. (~dozens of progenitors and many near-progenitors within a kpc)

• Nuclear-burning is required for Type Ia supernovae.

• This is true for single-degenerate *and* double degenerates. (RD 2010a, 2010b.)

• Nuclear-burning WDs are bright, as are WDs that accrete at high rates.

• Hot WDs may appear as luminous supersoft x-ray sources (SSSs).

• A word about double-degenerates.

• WDs will merge, whether or not they produce Type Ia supernovae.

• Those mergers that achieve M_c will experience long intervals at high-accretion rates prior to the common envelope that brings the two WDs close to each other. (RD 2010b)

• These may also appear as SSSs.

Supersoft X-ray Sources• Discovered in the early 1990s.• They have luminosities, temperatures and radii expected of hot

white dwarfs.

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L : 1036 −1038 erg s−1

k T : 10 −100 eV

€

L : 1036 −1038 erg s−1

k T : 10 −100 eV

€

€

€

L : 1036 −1038 erg s−1

k T : 10 −100 eV€

€

L : 1036 −1038 erg s−1

k T : 10 −100 eV

They are difficult to detect in the Milky Way, but can be seen in other galaxies.

In both SD and DD models, we expect >1000 nuclear-burning white dwarfs on their way to supernova-hood

—and many more that will never make it to M_c.

We can obtain a good census of these sources in nearby galaxies.

Numbers observed

• M101• M83• M51• M104• NGC44

72• NGC46

97

• SSS: 42• SSS: 28• SSS: 15• SSS: 5• SSS: 5• SSS: 4

QSSs

• Exhibit some of their emission above 1 keV.

• Little or no emission above 2 keV.

• Effective values of kT:

100-350 eV.

There are a significant number of such sources.

Can other x-ray sources help?

Numbers observed

• M101• M83• M51• M104• NGC44

72• NGC46

97

• SSS: 42; QSS: 21; other: 65

• SSS: 28; QSS: 26: other: 74

• SSS: 15; QSS: 21; other: 56

• SSS: 5; QSS: 17; other: 100

• SSS: 5; QSS: 22; other: 184

• SSS: 4; QSS: 15; other: 72

• Nuclear-burning is required for Type Ia supernovae.

• This is true for single-degenerate *and* double degenerates.

• Nuclear burning is associated with SSSs.

• Chandra/XMM show that there are not enough SSSs.

• This does not falsify the progenitor models.

• Instead it tells us that the progenitors must be bright at other wavelengths.

The low duty cycle of SSS behavior in nuclear-burning WDs is supported by the paucity of “supersoft-source nebulae”. If “on” even 10% of the time, each SSS can

ionize a ~10 pc region of the surrounding ISM. Only one such nebula has been detected.

Alicia Soderberg

• We must look for bright binaries.

• UV and IR are both potentially important. (UV perhaps more for close binaries and IR for wide binaries.)

• Similar to situation for symbiotics, where the discrepancy is ~2 orders of magnitude. (Indeed, some symbiotics may be progenitors of Sne Ia.)

• We must also quantify the interconnections between those binaries that are Type Ia progenitors and other binaries.

• These include: AIC (new surveys will help) DD scenarios Novae and recurrent novae Self Lensing (Kepler makes it possible) Blue straggler evolution and observation

Model AICs BHs Sne Ia0.03 1.1e4 0 00.06 2.3e4 1.6e3 00.09 3.1e4 7.7e3 4.4e30.12 3.5e4 1.6e4 1.3e40.15 3.7e4 2.2e4 2.4e40.18 3.9e4 2.9e4 3.5e40.21 3.9e4 3.2e4 4.8e40.24 4.0e4 3.3e4 6.0e40.27 4.1e4 3.4e4 7.3e4

Per 10^8 solar masses of stars formed.

RDandHarris

WDs pass in front of and either block or lens their companions.

• The progenitors must be very bright binaries.

• Holds for single-degenerate and double-degenerate models, at different times prior to explosion.

• They are not supersoft sources during most of the time when they are bright.

• We must seek them at other wavelengths.

• There are a wide range of systems and processes that we can and should use for calibration and cross checks.

The wide-binary symbiotic channel

• Is relatively simple, because no common envelope occurs.

• • Predicts a wide range of interesting

phenomena.

• Has implictions for globular clusters.

• May predict an even more interesting future for

RS Ophiuchi

There are reasons to be hopeful.