1. Nuclear-burning white dwarfs: Type Ia supernova progenitors? Theory and Observational Signatures A tale in two parts: Rosanne Di Stefano and Jeno Sokoloski

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

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

5. 1572: “Tycho’s supernova”

6. 1604: “Kepler’s star”

7. Foretelling calamity?

8. Kepler: No, it predicts greater sales of books.

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

10. 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.”

11. Forget about SNIa…for a moment. Consider phenomena that must occur to WDs, whether or not they ever become Type Ia supernovae.

12. Start with binary stars. Some become binaries in which a white dwarf has acompanion. 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

13. The key issue: Does genuine mass retention occur?

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

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

16. 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 WDsthat gain some mass. Some do reach M_c. There are uncertainties about the exactnumbers.

17. To derive the rate: calculations The first population synthesis calculations indicatedthat, in principle, there are enough close binaries inwhich the WD can achieve M_c to account for therate. (Rappaport, Di Stefano & Smith 1994) The addition of binary evolution calculations clarifiesthe issues that determine the rates (esp. winds;DiStefano et al. 1996, 1997, Di Stefano & Nelson1996) Large range of models (Di Stefano 1996; Hachisu,Kato, & Nomoto 1996)

18. See Nelson 2007.

20. 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.

21. The nuclear- burning WDs are bright.

22. 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)

23. 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).

24. 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.

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

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

31. 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.

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

33. Numbers observed M101 M83 M51 M104 NGC447 2 NGC469 7 SSS: 42 SSS: 28 SSS: 15 SSS: 5 SSS: 4

34. 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?

35. Numbers observed M101 M83 M51 M104 NGC447 2 NGC469 7 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

36. 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.

37. 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

38. We must look for bright binaries. UV and IR are both potentially important. (UVperhaps more for close binaries and IR for widebinaries.) Similar to situation for symbiotics, where thediscrepancy is ~2 orders of magnitude.(Indeed, some symbiotics may be progenitorsof Sne Ia.)

39. We must also quantify the interconnectionsbetween those binaries that are Type Iaprogenitors 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

40. ModelAICsBHsSne Ia 0.031.1e4 0 0 0.062.3e41.6e3 0 0.093.1e47.7e34.4e3 0.123.5e41.6e41.3e4 0.153.7e42.2e42.4e4 0.183.9e42.9e43.5e4 0.213.9e43.2e44.8e4 0.244.0e43.3e46.0e4 0.274.1e43.4e47.3e4 Per 10^8 solar masses of stars formed. RD and Harris

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

42. The progenitors must be very bright binaries. Holds for single-degenerate and double-degeneratemodels, at different times prior to explosion. They are not supersoft sources during most of thetime when they are bright. We must seek them at other wavelengths. There are a wide range of systems and processes thatwe can and should use for calibration and crosschecks.

43. 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.