Townsley and L. B., 2004, Ap. J., 600, 390 (Theoretical overview)

1. Diversity of Type Ia SNe: Challenges and Opportunities Until the final explosion of an accreting WD as a Type Ia supernovae, the brightest manifestations of mass transfer are thermonuclear ignitions of accreted Hydrogen and Helium. The current and upcoming transient and SNe surveys are bound to reveal new populations and puzzles. Townsley and L. B., 2004, Ap. J., 600, 390 (Theoretical overview) Townsley and L.B., 2005, Ap. J., 628, 395 (Classical Novae) Scannapieco and L.B., 2005, Ap. J., 629, L85 (Type Ia SN Rates) L.B., Townsley, Deloye & Nelemans 2006, Ap. J., 640, 466 (AM CVn) Shen and L. B. 2007, Ap J, 660, 1444 (Stable H/He Burning) L.B., Shen, Weinberg & Nelemans 2007, Ap J., 662, L95 (Faint .Ia SN) Piro and L.B., 2008, Ap J., 673, 1009 (Simmering and 56Ni in Ia) Shen and L.B., 2008, submitted to Ap J (Unstable H burning) L.B. 2008, in preparation (Survey Expectations)

2. Stars with < 6-8 solar masses make a Carbon/Oxygen white dwarf of mass 0.5-1.0 with radius ~ Earth and central density >106 gr/cm3 PN image from HST Kalirai et al ‘07 Ring Nebulae (M 57) 1.05 or so Kalirai et al ‘07 Stellar Lifetime (Myr) Young White Dwarf 500 100 50

3. White Dwarfs Accreting H or He ~1% of white dwarfs are in binaries where accretion occurs, releasing gravitational energy Donor star can be H/He or pure He 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. White Dwarf of Carbon/Oxygen Piro ‘05

4. In 1011 solar masses of old stars (e.g. Elliptical galaxy), two WDs are made per year. The observed rates for thermonuclear events are: Some numbers • 20 Classical Novae (Hydrogen fuel) per year, implying a white dwarf/main sequence contact binary birthrate (Townsley & LB 2005) of one every 400 years. • One Type Ia Supernovae every 250 years, or one in 500 WDs explode in an old galaxy! M87 in Virgo Predicted rates are: Helium novae every ~250 years, one large He explosion (.Ia; Bildsten et al. ‘07) every ~5,000 years. Double WD mergers every 200 years

5. Type Ia Supernovae from Accretion The ‘standard’ story (Nomoto, Thielemann & Yokoi ‘84) is 12C ignition in the core leading to a full explosion, implying that: 12C+12C ignition • The density must >109 gr/cm3 in the cold (~108 K) core to trigger C burning. This requires M>1.34 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.

6. Hydrogen Burning is Usually Unstable Townsley & Bildsten 2005 Supersoft Sources: Burn H Stably (van den Heuvel et al 1992), or weakly unstable Accumulated mass Cataclysmic Variables (CVs): undergo unstable burning, leading to Classical Novae. Accumulated mass appears to leave, observed CN rate reveals population.

7. AM CVn Binaries: Pure Helium Accretors! • Found by Humason and Zwicky (‘47) as faint blue stars, spectra by Greenstein & Matthews (‘57) only showed helium lines. • Later work found 17 minutes orbital period • The accretor is a C/O or O/Ne WD, where the donor is a degenerate Helium WD. • Giving an orbital period-donor mass relation, • and donor masses ranging from 0.006-0.12 Msun

8. The fate of 1 in 2000 white dwarfs in our galactic disk.But none yet seen in other galaxies. GP COM These are the brightest Sources for Space-Based Gravitational Wave Detectors (e.g. LISA)

9. Iben & Tutukov ‘89 He Ignition Mass • Early in the evolution, many helium flashes with masses less than 0.01 that would appear as long-lived Classical Novae (e.g. V445 Pup). • The last flash has the largest mass (0.03 in these cases), and occurs at 10-7-10-8 Msun/year (depending on the accreting WD mass) • Last flash is large enough to detonate and eject <0.1 of radioactive material, creating a faint supernovae (Bildsten et al. 2007) • If every AM CVn gives a .Ia, their rate would be 2-7 % of the Type Ia rate in an Elliptical Galaxy.

10. 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 10,000 km/sec. • This matter would cool by adiabatic expansion, but instead is heated by the radioactive decay chain 56Ni=>56Co=>56Fe • Arnett (1982) (also Pinto & Eastman 2000) showed that the peak in the lightcurve occurs when the radiation diffusion time through the 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 peak luminosity, yielding 0.1-1.2 solar masses.

11. “Super” luminous 1991T Bolometric LCs Subluminous 1991bg Contardo et al. ‘00, A&A, 359, 876

12. .Ia Supernovae* • The small ignition masses (0.02-0.1) only burns the helium, which leaves the WD at 10,000 km/sec, leading to rapid rise times. • The radioactive decays of the fresh 48Cr (1.3 d), 52Fe (0.5 d) and 56Ni (8.8 d) will provide power on this rapid timescale!! L. B., Shen, Weinberg & Nelemans ‘07 x10 *Thanks to Chris Stubbs for the name

13. Thermonuclear Lightcurves • 2003fg (Howell et al) was a very bright event (2006gz plotted here) • 1991bg is the prototype subluminous SN • The .Ia’s are calculation from Kasen (priv. comm.) • 1991T is the bright class seen only in star-forming galaxies (young ages!) • 1992A is ‘typical’ Bildsten ‘08

14. The sub-luminous Ia’s fit within the continuum of the Phillip’s relation, extending down by nearly 2.5 mags. • Most prevalent in E/S0 galaxies (Howell ‘01, van den Bergh et al ‘03) • Still other odd ones (2002cx)! Phillips Relations 2006gz 2002cx Garnavich et al ‘00

15. Topical Because of New Surveys! Sloan Digital Sky Survey (Dilday et al 2008) Pan-Starrs1 (2008) Current survey (V=22.5, 258 deg2) will find 7 .Ia per year at -17, and 0.5 per year at -15. Total duration = 9 months…. so maybe one in sample Medium deep survey (V=24, 50 deg2) gets10 .Ia per year at -17, and 1 per year at -15

16. Even More! ROTSE (now) SkyMapper (2008) Palomar Transient Factory (2008)

17. LSST (2014) Large Synoptic Survey Telescope (LSST) is a proposed 8.4-meter, 10 square-degree-field telescope that will provide V=24 imaging across the entire sky every night. Cerra Pachon, Chile. Daily survey 1/2 sky would ~1000 .Ia’s per year.

18. Supernovae and Transients IIP’s .Ia’s Kulkarni et al (2007)

19. The boxes plot the volume rate * duration for Type Ia (30 d), Type IIp (100 d), .Ia (5 d), M85-OT (60 d), CN (30 d), and last thermal pulses (2 years) • SDSS SN survey (solid line) and PS1 Med. Deep (dashed line) are shown. The line is 1 event visible per “exposure”. • Heavy solid line is LSST at V=24 with 20,000 deg2 • What to do with 10,000 SNe? Survey Volumes and Discovery! Bildsten, in prep. SNLS (24.3, 4 deg2) 1 per exposure (SDSS & PS1) LSST Bildsten ‘08

20. Type Ia Supernovae Dependence on Galaxy Type and Cosmic Rates Observed trends in Ia properties with galaxy type (no evidence yet for metallicity effects) will hopefully identify progenitors: Brightest (e.g. 1991T) events occur preferentially in young stellar environments (hence mostly spiral and irregular galaxies) Sub-luminous (and peculiar, eg. 1991bg) Ia’s dramatically prefer old stellar populations . . (Elliptical and S0 Galaxies) Rates track BOTH the stellar mass and the star formation rate The odd 2002cx-likes are only seen in star forming galaxies Ia’s are clearly the result of old and young stellar populations and motivated our (Scannapieco & LB, 2005) simple explanation for the observed cosmic Ia rate. Sullivan et al. (2007) used CFHT SNLS data to go much further!

21. Recent CFHT and SDSS Results Neill et al (2007) astroph-0701161 Dilday et al. astroph 0801.3297 SDSS The Type Ia rate was higher in the past by at least a factor of 3, pointing to rapid channels of explosion.

22. Clear puzzles remain in connecting accreting WDs to their exploding Type Ia counterparts • .Ia SN expected from AM CVn binaries (which came from double WDs) and should be revealed in upcoming surveys. • The current and about to commence SNe surveys should yield explicit dependence of Ia properties with host galaxy types, hopefully informing us about the progenitor(s) and mechanism(s) that yield at least 4 distinct classes. Conclusions Clearly much more to learn about thermonuclear events on accreting white dwarfs!!