52° CONGRESSO SAIT TERAMO, 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. Dantona & R.A. Zappalà

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

After H- & He-burn. partial degenerate CO core C-burn. (off-centre) through a flash Super-AGB: evolution After flash: 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 (e.g. Garcia-Berro & Iben 1994 ApJ; Pumo & Siess 2007, ASPCS )

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 AGB Super-AGB

M M f core =M EC ~ 1.37 M M EC M f core < M EC collapsing electron captures supernovae Neutron star NeO White Dwarf Final fate (Nomoto, 1984, ApJ)

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

Existence of 2 final evolutionary channels (e.g. Siess 2007; Pumo 2007, Pumo & Siess 2007, Poelarends et al. 2008) Adapted from Pumo, 2006, PhD thesis, Catania Univ. 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

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

In case of no evidence for a global CNO enrichment, massive Super-AGBs evolve into EC SNe. high number of neutron stars (up to ~10 3 ), thanks to supernova natal kicks low enough not to be ejected by the GC (e.g. Ivanova et al. 2008) Pumo, DAntona & Ventura ApJ, 672, L25, 2008 Super-AGBs may be progenitors

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

Preliminary results (for a M=6M Z=0.02 model) Production of 87 Rb 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!

Thank you

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

Sub-luminous Type II-P SNe H lines with P-cygni profiles Explosion energy ~ erg (5-10 · normal Type II SN) ~ 3-5 M v Low 56 Ni ( M, 0.1M in normal Type II SN)

Partial degeneracy of electrons

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: without ovsh. M ini between 7 and 13 M Z in the range to 0.04 with ovsh. M ini 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

52 nuclei+162 reactions (pp, CNO, -, -, -,p-,n- reactions, 12 C+ 12 C, 12 C+ 16 O) Nuclear Network 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

Reactions rates = rQ/ reaction rate r (number of reactions per unit time and volume) N i = number density of interacting species v = relative velocity (v) = velocity distribution in plasma (v) = reaction cross section ( barn) typical units: MeV g -1 s -1 energy production rate

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

Instabilità dinamica: criterio di Schwarzschild 1010

Sottostima estensione zona convettiva No inerzia convective overshooting penetrazioni in regioni dinamicamente stabili ampliamento estensione zona convettiva r

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

Riscaldamento del core Esaurimento del combustibile Contrazione del core Bruciamento nucleare c > 2.4· µ e T 3/2 g cm -3 Core degenere inerte ~ – T core (10 9 K) Pre-MS C burning He burning H burning Stage ~ – Density (g cm -3 ) / – 10 8 Timescale (yr) Stage TimescaleT eff (K)L (L_sun)

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

L c behaviour similar to the one of m c m anti-correlated to L c & m c

The C-burning nucleosynthesis 12 C( 12 C,α) 20 Ne 12 C( 12 C,p) 23 Na 16 O(α, ) 20 Ne 12 C (> 0.015) potential trigger of explosion! Complete disruption of the star (Gutierrez et al A&A) 20 Ne (~ ), 16 O (~ ), 23 Na (~ ) + p and α available for nucleosynthesis up to 27 Al

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

Second dredge-up features highly depend on M ini Garcia-Berro & co-workers 1994,1996, 1997, 1999 ApJ (Z=0.02) M ini ~ M up (3.46·10 7 yr) (3.50·10 7 yr) M ini ~ M mas (1.67·10 7 yr) (1.77·10 7 yr) (3.35·10 7 yr) (3.36·10 7 yr) M ini < M mas

Second dredge-out M ini value depends on Z and mixing treatment M ini = 9.5 – 10.8M if Z = M ini ~ 7.5M with ovsh.

Connessione M N – 2DUP

Evoluzione finale e massa M N