SN Ia Rates: Theory, Progenitors, and Implications

1. SN Ia Rates: Theory, Progenitors, and Implications CIfAR Stanford 2008

2. SN Ia progenitors • Why important? • Among the most powerful explosions in the Universe (next to GRBs) • SNe Ia and cosmology • Role in chemical evolution and gas dynamics • Scenario: exploding CO white dwarf near 1.4 Msun. • Energy released (~0.5Msun CO --> 56Ni) • No H in spectrum • Light curve shape (radioactive decay) • Presence in old stellar pops (what else could they be?) CIfAR Stanford 2008

3. Double Degenerate - 2 white dwarfs (Mtot >= 1.4 Msun at explosion) Single Degenerate - white dwarf + evolving secondary (M ~ 1.4 Msun at explosion) SN Ia Progenitors - 2 Broad Classes Key point: white dwarf max mass = 1.4 Msun (Chandra- sekhar mass) CIfAR Stanford 2008

4. Two Basic Questions • What is the “delay time distribution” of SNe Ia? • What is the main sequence mass of SNe Ia progenitors? • By what evolutionary path(s) do white dwarfs become SNe Ia? Basic questions, but no clear answers … CIfAR Stanford 2008

5. SN Ia rate depends on SFR SNLS - Sullivan et al 2006 Mannucci et al 2006 CIfAR Stanford 2008

6. Scannapieco and Bildsten 2005 SNR/M SNR/M M SFR Sullivan et al 2006 CIfAR Stanford 2008

7. 2 different B values Scannapieco & Bildsten 2005 passive Sullivan et al 2006 active CIfAR Stanford 2008

8. SFR½ SN Ia rate depends on SFR CIfAR Stanford 2008

9. SNR = A٠M + B٠SFRSNR/M = A + B (SFR/M) • Does this imply two paths to SNeIa? … • … or is there a simple unifying picture that can be used to understand the A+B prescription for the SNIa rate? • Continuum of delay times – more natural? • Why do the A and B values have the values that are observed? • Why ~√SFR dependence rather than ~SFR? • Why is fit so poor in the SNR/M -- SFR/M plane? CIfAR Stanford 2008

10. Model • Single degenerate scenario • Delay time depends on evolutionary timescale of secondary CIfAR Stanford 2008

11. “Rate” vs time • Rate at which stars leave main sequence • white dwarf formation rate • distribution of delay times for a burst • rate from a starburst decreases with time as ~ √t • Factor of ~100x in mean stellar age (100Myr – 10Gyr) gives factor of ~10x in SN Ia rate, as observed starburst rate~√t CIfAR Stanford 2008

12. Rate vs time • Simple SFR(t) ~ t-η to allow for range of ages +1 -1 -1 +1 CIfAR Stanford 2008

13. 4 different  values CIfAR Stanford 2008

14. age Models vs Observations • Locus of WD formation rates independent of SFR(t) - includes passive galaxies • 1% of WD’s become SNeIa • 1% agrees with models (roughly) • 1% agrees with MW (roughly) • [Disagrees with clusters (10-20%)] • Note that 1% eff is constant from active to passive galaxies! CIfAR Stanford 2008

15. Meaning • Single component model – not A+B • Single free parameter normalization - fSNIa • Continuous distribution of delay times • Rate in active and passive galaxies both explained naturally • Only physics is evol-utionary timescales CIfAR Stanford 2008

16. Normalization Fraction 0.01 of all stars in the mass range 1-9 Msun become SNeIa. (1e10 Msun) <[Fe/H]>=-0.5 “cum grano salis” 1e10 Msun Salpeter mass fcn 1-9 Msun for SNIa 0.6 Msun Ni56 per SNIa 6 x 106 Msun Fe peak X fSNIa CIfAR Stanford 2008

17. Efficiency vs mass (SD) • 1 Msun main sequence stars find it very difficult to get to the Chandra mass and make a Type Ia SN • Close binaries with primary < 2Msun make a He WD, not a C+O WD • Mass arguments: 1 Msun on the m.s. makes a 0.5 Msun WD, hard to imagine 2 x 1 Msun making a 1.4 Msun WD • Most of companions to 1 Msun stars haven’t evolved yet • binary frequency lower for low mass objects (?) Therefore fraction of WD’s that make SNeIa should be much lower at low masses (>10x). CIfAR Stanford 2008

18. Effects of efficiency • Normalized at high mass (short timescale) end • Assume efficiency drops by 10x from M=3 to 1 Msun (conservative) Single degenerate model cannot explain all SNeIa. Some other mechanism must be involved for at least some SNeIa. CIfAR Stanford 2008

19. DD Scenario Han & Podsiadlowski2004 CIfAR Stanford 2008

20. z=0.24, star-forming host Most luminous SNIa ever discovered (MV=-20.0, 10 billion Lsun) Lies off the stretch-L relation - too bright for its stretch s=1.13 by 4.4 sigma SNLS-03D3bb(Howell et al. 2006) CIfAR Stanford 2008

21. 03D3bb • Requires 1.3 Msun of 56Ni to power light curve, 2Msun total mass • “normal” SNIa – 0.6 Msun of 56Ni • 03D3bb is 2.2x brighter, therefore has 2.2x Ni mass • Detailed calculation using Arnett models agrees well • Mass > Chandra mass of 1.4 Msun! CIfAR Stanford 2008

22. Cosmic SFR(z) Hopkins and Beacom 2006 CIfAR Stanford 2008

23. SNR predictions from SFR(z) • SFR(z) gives SFR(t) per Mpc^3 • Normalization somewhat arbitrary • SN rate very sensitive to exact SFR(z) CIfAR Stanford 2008

24. SNR predictions from SFR(z) • Solid=model, dashed=A+B (Sullivan 2006) Kuznetsova et al 2008 Dilday et al 2008 CIfAR Stanford 2008

25. Conclusions • SNIa rate depends on SFR • “natural” explanation in terms of evolutionary timescales • 1% of white dwarfs become SNeIa • Single degenerate model cannot explain all SNeIa • one parameter model fits active and passive • excellent fit to data – better than A + B SFR/M • A and B naturally explained • Based on stellar evolutionary timescales • Continuous delay time distribution • Predictions: • SNIa rate will correlate with mean age from population models • SNII/SNIa • z distributions • Chem evol … CIfAR Stanford 2008

26. CIfAR Stanford 2008