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52° CONGRESSO SAIT TERAMO, 4 - 8 MAGGIO 2008

52° CONGRESSO SAIT TERAMO, 4 - 8 MAGGIO 2008. The s-nucleosynthesis process in massive AGB and Super-AGB stars. M.L. Pumo CSFNSM - Università di Catania & INAF - Osservatorio Astrofisico di Catania. In collaboration with: P. Ventura, F. D’antona & R.A. Zappalà. AGB: low-mass &

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52° CONGRESSO SAIT TERAMO, 4 - 8 MAGGIO 2008

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  1. 52° CONGRESSO SAIT TERAMO, 4 - 8 MAGGIO 2008 The s-nucleosynthesis process in massive AGB and Super-AGB stars M.L. Pumo CSFNSM - Università di Catania & INAF - Osservatorio Astrofisico di Catania Incollaborationwith:P.Ventura,F.D’antona&R.A.Zappalà

  2. AGB: low-mass & intermediate-mass Super-AGB massive Mup Mmas MZAMS (~ 7-9M⊙) (~ 11-13M⊙) Super-AGB stars & the ZAMS MZAMS < Mup: unable to ignite core C-burn. MZAMS≥ Mmas: able to evolve through all nuclear burning stages

  3. Super-AGB: evolution (e.g. Garcia-Berro & Iben 1994 ApJ; Pumo & Siess 2007, ASPCS) After H- & He-burn. →partialdegenerate CO core C-burn. (off-centre) → through a flash Afterflash: • development of a flame that reaches the stellar centre, transforming the CO core into a NeO mixture • C-burn. proceeds outside the core before extinguishing, just leaving H- & He-burn. shell

  4. AGB Super-AGB • Structure is similar to the one of AGB stars, except that their cores are: • more massive (1-1.37M⊙) • made of Ne (15-30%) and O (50-70%) • After completion of C-burn., the core mass increases due to the H-He double burn. shell

  5. Final fate (Nomoto, 1984, ApJ) Mfcore< MEC Mfcore =MEC ~ 1.37 M⊙ collapsing electroncaptures supernovae NeO White Dwarf Neutron star

  6. Mend,2 NeO White Dwarf Mend,1 Neutron Star mass loss so efficient ↓ envelop is lost before the core has grown above ~ 1.37 M⊙ Mend,1 1.37M⊙ Mend,2 Interplay between mass loss and core growth (e.g. Woosley et al. 2002, ARA&A) The minimum initial mass for the formation of a neutron star is usually referred to as MN (transition NeO WD / EC SN)

  7. the less massive Super-AGBs → NeO WD • the most massive Super-AGBs → SN EC • Mass distr. of WDs • Neon-novae • Sub-luminous Type II SNe • Self-Enrichment in GCs • Trans-iron nucleosynthesis Adapted from Pumo, 2006, PhD thesis, Catania Univ. Existence of 2 “final” evolutionary channels (e.g. Siess 2007; Pumo 2007, Pumo & Siess 2007,Poelarends et al. 2008)

  8. Self-Enrichment in GCs & the Super-AGB stars “Blue” MSs in  Cen and NGC 2808 (Piotto et al. 2005, 2007) Peculiar HB morphology in NGC 6441 and NGC 6388 (Caloi & D’Antona 2007) No negligible fraction of stars (10-20%) having helium content Y ≳ 0.35 High helium population originated from the helium-rich ejecta of a previous stellar generation Progenitors having the required high helium abundance in their ejecta

  9. In case of no evidence for a global CNO enrichment, massive Super-AGBs evolve into EC SNe. high number of neutron stars (up to ~103), thanks to supernova natal kicks low enough not to be ejected by the GC (e.g. Ivanova et al. 2008) Super-AGBs may be progenitors Pumo, D’Antona & Ventura ApJ, 672, L25, 2008

  10. Trans-iron nucleosynthesis: s-process in massive AGB & Super-AGB stars (e.g. Ritossa et al. 1996, Abia et al. 2001, Busso et al. 2001, Siess & Pumo 2006) Main neutron source:22Ne(α, n)25Mg reaction Astrophysical environment: thermally pulsing AGB phase Efficiency is still uncertain

  11. Preliminary results (for a M=6M⊙ Z=0.02 model) Production of 87Rb is advantaged compared to the one of other nearby elements, such as Zr, Y and Sr. Rubidium–rich AGB stars in our galaxy (Garcia-Hernandez et al, Nature, 2006) The work is in progress: other studies are needed to confirm our hypothesis!

  12. Thank you

  13. EC reactions on: 24Mg and 24Na, 20Ne and 20F MONe =MEC ~ 1.37 M⊙ SN triggered by EC (Nomoto & co-workers 1980,1981, 1984, 1987) Start and acceleration of the core collapse!

  14. Sub-luminous Type II-P SNe H lines with P-cygni profiles Explosion energy ~ 1051 erg (5-10 · 1051 ‘normal Type II SN’) ~ 3-5 Mv ↓ Low 56Ni (0.001-0.006 M⊙, 0.1M⊙ in ‘normal’ Type II SN)

  15. Partial degeneracy of electrons

  16. without ovsh. → Mini between 7 and 13 M⊙ Z in the range 10-5 to 0.04 with ovsh. → Mini between 5 and 10.5 M⊙ Z =10-4 and 0.02 Once calculated the stellar models up to the end of the C-burn. phase Subsequent NeO core mass evolution Computation method and numerical details • Stellar evolution code: STAREVOL (Siess, 2006, A&A) with the differences reported in Siess & Pumo 2006a,b • 2 Grids of stellar models:

  17. Nuclear Network 52 nuclei+162 reactions (pp, CNO, -,-,-,p-,n-reactions, 12C+12C, 12C+16O) Nucleosynthesis of elements with con Z<17 + ‘Neutron sink nucleus’ Rates from NetGen (Aikawa et al. 2006, A&A) with screening factor from Graboske et al. 1973, ApJ

  18. Reactions rates • reaction rate r (number of reactions per unit time and volume) Ni = number density of interacting species v = relative velocity (v) = velocity distribution in plasma (v) = reaction cross section (10-9 - 10-12 barn) • energy production rate   = rQ/ typical units: MeV g-1 s-1

  19. Treatment of convection No overshooting: MLT (=1.75) + Schwarzschild mean nuclear reaction rate Yes overshooting: upper edge of convective zone nucleosynthesis shell by shell + diffusive mixing

  20. Instabilità dinamica: criterio di Schwarzschild 1 0

  21. “convective overshooting” penetrazioni in regioni dinamicamente stabili ampliamento estensione zona convettiva No inerzia ↻ ↻ ↻ ↻ ↻ ↻ ↻ ↻ ↻ ↻ ↻ ↻ r Sottostima estensione zona convettiva

  22. Timmes et al. 1994 ApJ Numerical treatment of the flame time step: spacial zoning:

  23. Core degenere inerte c > 2.4· 10-8 µeT3/2g cm-3 Contrazione del core Riscaldamento del core Esaurimento del combustibile Bruciamento nucleare Stage Timescale (yr) Tcore (109 K) Density (g cm-3) H burning 107 – 10 8 0.03 10 He burning 106 0.08-0.1 103 C burning 10-103 / 102-103 0.65 – 0.7 106 – 107 Timescale Teff (K) L (L_sun) Stage Pre-MS - 105 ~ 5000 ~ 103

  24. C-burning: evolution (Siess & Pumo 2006a,b) 1) Convective flash: Lc= maximum expansion of the core quenching of the convective instability Core contraction 2) Convective flame: Lc~ 5·10-2 -10-1Lc,flash Smaller expansion no quenching of the convective instability Confirmation: Garcia-Berro & Iben 1994 ApJ (Z=0.02) Siess 2006 A&A (Z=0.02) Gil-Pons et al. 2005 A&A (Z=0)

  25. Lc behaviour similar to the one of mc • m anti-correlated to Lc & mc

  26. The C-burning nucleosynthesis 12C(12C,α)20Ne 12C(12C,p)23Na 16O(α,)20Ne 20Ne (~ 0.15-0.35),16O (~ 0.5-0.7), 23Na (~ 0.03-0.05) + p and α available for nucleosynthesis up to 27Al 12C (> 0.015) potential trigger of explosion! ↓ Complete disruption of the star (Gutierrez et al. 2005 A&A)

  27. Nucleosynthesis in the NeO core α particle: 22Ne(α,n)25Mg n: 16O, 20Ne, 23Na, 25Mg → 17O, 21Ne, 24Mg, 26Mg 22Ne(α,)26Mg protons: 26Mg(p,)27Al 23Na(p,α)20Ne 23Na(p,)24Mg

  28. Mini~ Mup Mini~ Mmas Mini < Mmas (3.46·107 yr) (3.50·107yr) (1.67·107 yr) (1.77·107yr) (3.35·107 yr) (3.36·107yr) Second dredge-up features highly depend on Mini Garcia-Berro & co-workers 1994,1996, 1997, 1999 ApJ (Z=0.02)

  29. Second dredge-out Mini value depends on Z and mixing treatment Mini = 9.5 – 10.8M⊙ if Z =10-5 - 0.02 Mini~ 7.5M⊙ with ovsh.

  30. Connessione MN – 2DUP

  31. Evoluzione finale e massa MN

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