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Understanding the Impact of Core Mass on Carbon Ignition in Intermediate-Mass Stars

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iorwen
31 diapositives · 438 Views · Aug 22, 2014

This study explores the critical core mass (Mup) for carbon ignition in stars with initial masses between 5 and 9 Mʘ, an essential factor in stellar evolution. By examining the physics behind carbon ignition and the initial-to-final mass relation, we highlight how variations in composition (Z and Y) and convective mixing influence Mup and ultimately lead to significant astrophysical consequences. We discuss scenarios involving super-AGB stars, the formation rates of Type Ia supernovae, and the implications for core-collapse supernova progenitors, providing insights into massive star evolution.

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Understanding the Impact of Core Mass on Carbon Ignition in Intermediate-Mass Stars
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  1. Mup Oscar Straniero & Luciano Piersanti --------- INAF - Osservatorio di Teramo

  2. where: Zo=0.02 Yo=0.28 Becker and Iben 1979

  3. Status of the art

  4. CLASSICAL MODELS (bare Schwarzschild criterion) In order of appearance: Siess 2007 versus Becker Iben formula 1979 (same Z,Y than Siess)

  5. n-cooling H-rich He CO Conv. Env. H burning He burning Beyond the core He burning: the early-AGB

  6. The golden rule • It exists a critical-core mass for the C ignition. • It is MH~1.08 Mʘ (with the “current” physics). To evaluate Mup, we have to take under control: • The physics of the C ignition: neutrinos, 12C+12C, amount of C left (12C+a), thermodynamics of a dense plasma (EOS)… . • The initial-to-final mass relation for an intermediate mass stars (5 - 9 Mʘ), which depends on: • The extension of the H-exhausted core at the end of the MS phase. • The duration of the core-He burning phase (when H is burned in shell). • The efficiency of the 2nd dredge up.

  7. Classical Overshoot Semi Convection Varying the convective scheme

  8. ≈0.9 Mʘ≈ Classical versus Semiconvection (core-He burning) In order of appearance: Siess 2007 versus Straniero et al. 2003 , see also Dominguez et al. 1999

  9. ≈1.6 Mʘ Siess07 OV ≈1.7 Mʘ? Classical versus Overshoot In order of appearance: Siess 2007 (squares) versus Girardi 2000 (triangles) Both: OV (red) noOV (black)

  10. Semiconvection versus overshoot In order of appearance: Straniero et al. 2003 (semiconv.) versus Siess 2007 (overshoot) ≈0.7 Mʘ

  11. Varying the composition (Y) • Larger He larger m  larger TC  larger CC  larger final MH and, then: SMALLER MUP Z=0.04, Bono et al. 2000

  12. Varying the composition (Z) • Larger Z  larger (external) k and more expanded and cooler structures BUT • Larger Z  more CNO and more efficient H-burning Z> 10-3 Z MUP 10-5<Z< 10-3 Z MUP Z< 10-5 Z MUP

  13. Varying the composition (Z)

  14. Approaching the C ignition enuclear=|en|

  15. Varying the neutrino rate (en) In order of appearance: Esposito et al. 2003 (same for Haft et al 1995 or Itoh et al. 1996) versus Munakata et al. 1986

  16. Varying en or X(12C) enx5 en/5 Equivalent to a reduction/multiplication of X(12C) by sqrt(5) (0.1 to 0.6), the range of uncertainty implied by the 12C+a16O see Straniero et al. 2003)

  17. Varying the 12C+12C • Laboratory measurements available down to about 3 MeV (the Gamow peak for Mup is 1.5 MeV). • Several evidences of “molecular” structure producing narrow resonances at low energy (see Wiescher 2007, Spillane et al. 2007, Cooper et al. 2009). • A (possible) resonance near the Gamow peak would significantly increase the rate, thus reducing both the critical MH and, in turn, Mup

  18. SNe Ia Core burning What a resonance at 1.5 MeV would imply M=7M Z=Z  (dashed line)

  19. Mup Mup reduces of ~2 M 

  20. Astrophysical Consequences of a variation of Mup(numbers will be given for a dMup~2 Mʘ)

  21. Astrophysical consequences I • The number of super-AGB (ONeMg WD or electron-capture SNe) is larger. • By adopting a (Salpeter like) power-low IMF (b=2.35), SAGB would be 2/3 of the stars with M>M_up’ (the progenitors of “normal” core collapse SNe). 1.5 MeV resonance, semiconv., classical mod.

  22. Astrophysical Consequences II • Massive CO WDs cut off: MWD max reduced down to 0.95 Mʘ). BUT ROTATION MAY HELP see Dominguez et al 1996

  23. Massive CO WD: the lifting effect of rotation delays the II dredge up, allowing more massive MH(Dominguez et al. 1996) Convective envelope He-rich zone CO core 6.5 Mʘ Z= Zʘ

  24. Astrophysical Consequences III • Less Massive AGB  less space left for Hot Bottom Burning

  25. Astrophysical Consequences IV • SN Ia rates, both scenarios, Single Degenerate and Double Degenerate, 4 times less frequent!! • Prompt SNIa suppressed • First SNe Ia delayed (see Piersanti et al. 2010)

  26. Astrophysical consequences V • It is more easy to produce low mass core collapse SNe, down to ~6 Mʘ (electron capture), down to ~8 Mʘ (normal core collapse) • Somewhat in agreement with recent progenitor mass estimations: e.g. Smartt et al. 2008 found a minimum mass at 8.5 ± 1.5 Mʘ

  27. Astrophysical Consequences VI • Carbon Burning in massive stars and SAGB. More extended convective zones should be favoured by a larger 12C+12C rate. • Consequences for the final mass-radius relation, explosion energy release and related nucleosynthesis

  28. M=11.0 M Z=0.0149 Y=0.2645

  29. ‘ The value of Mup CF88 M=8.5 M Z=0.0149 Y=0.2645 NEW RATE

  30. Summary • Combining present uncertainties, Mup is known no better than dM=2 Mʘ (conservative error estimate). • The (many) astrophysical/observational consequences have been illustrated.