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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
Super-AGB stars & the ZAMS
AGB:
Super-AGB
low-mass &
intermediate-mass
massive
Mup
Mmas
(~ 7-9M⊙)
(~ 11-13M⊙)
MZAMS
MZAMS < Mup: unable to ignite core C-burn.
MZAMS ≥ Mmas: able to evolve through all nuclear burning stages
Super-AGB: evolution
(e.g. Garcia-Berro & Iben 1994 ApJ; Pumo & Siess 2007, ASPCS)
After H- & He-burn. → partial degenerate CO core
C-burn. (off-centre) → through a flash
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
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
Final fate
(Nomoto, 1984, ApJ)
Mfcore =MEC ~ 1.37 M⊙
Mfcore< MEC
collapsing electron
captures supernovae
NeO White Dwarf
Neutron star
Interplay between mass loss and core growth
(e.g. Woosley et al. 2002, ARA&A)
Mend,2
Mend,1
1.37 M⊙
Mend,2
NeO White Dwarf
mass loss so efficient
↓
envelop is lost before the core
has grown above ~ 1.37 M⊙
Mend,1
Neutron Star
The minimum initial mass for the formation of a neutron star is
usually referred to as MN (transition NeO WD / EC SN)
Existence of 2 “final” evolutionary channels
(e.g. Siess 2007; Pumo 2007, Pumo & Siess 2007, Poelarends et al. 2008)
• the less massive Super-AGBs
→ NeO WD
• the most massive Super-AGBs
→ SN EC





Adapted from Pumo, 2006, PhD thesis, Catania Univ.
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
“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
Super-AGBs
may be
progenitors
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)
Pumo, D’Antona & Ventura ApJ, 672, L25, 2008
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
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!
Thank you
SN triggered by EC
(Nomoto & co-workers 1980,1981, 1984, 1987)
EC reactions on:
24Mg and 24Na, 20Ne and 20F
MONe =MEC ~ 1.37 M⊙
M Ch  1.46 Ye / 0.5  Ye
2
where Ye   X i Z i Ai
i
Start and
acceleration of the
core collapse!
2
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)
Partial degeneracy of electrons
electrons in the spherical shell

4p 2
p2 
dpdV
f ( p)dpdV  ne
exp  
32
(2me kT )
 2me kT 
p, p  dp
(momentum space)
f ( p)dpdV 
8p 2
for p  pF
3
0
for p  pF
8p
1
f ( p)dpdV  3 
dpdV
E / kT 
 1 e
2
Computation method and numerical details

Stellar evolution code: STAREVOL (Siess, 2006, A&A)
with the differences reported
in Siess & Pumo 2006a,b
without ovsh. → Mini between 7 and 13 M⊙

2 Grids of stellar models:
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
M loss

  35,4000
M
core
Nuclear Network
52 nuclei+162 reactions (pp, CNO, -,-,-,p-,nreactions, 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
Reactions rates
 reaction rate r
(number of reactions per unit time and volume)
r
1
1   pT
N p NT v
v   ( v)( v) vdv
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
Treatment of convection
No overshooting: MLT (=1.75) + Schwarzschild
mean nuclear reaction rate
Y  Y 
 
t  t  nucl
1
con ij k 
mt  mb
mt
 ij  (m)dm
k
mb
Yes overshooting: upper edge of convective zone
nucleosynthesis shell by shell + diffusive mixing
1

vc l



2

Y


Y



3
4r 2  D
con D  

 


 2z
M (r ) 
 t  mix M (r ) 
 D0 exp
fH p



Instabilità dinamica: criterio di Schwarzschild
1
 dT 
 dT 
T1  T0  r 
  T0  r 

dr
dr

a

 rad
r
T0  T0
0
P   
T  P

P  T
 1

 dT 
 dT 
T1'  T0  T0  r 
  T0  r 

dr
dr

e

 ad
T1'  T1 
dT
dr

a
  1 
1  dT 
  1 1 dP
dT
 ln T  K ln  P   




 T  dr  ad
 P dr
dr



'
 rad   ad
dT
dr
ad

e
dT
dr

rad
dT
dr
ad
2  1 T dP
d ln T
; 
2 P dr
d ln P
Sottostima estensione zona convettiva

a 0
“convective overshooting”
ampliamento estensione
No inerzia
penetrazioni in regioni dinamicamente stabili
zona convettiva
 rad   ad
rad  ad
↻ ↻ ↻ ↻
↻ ↻ ↻ ↻
↻ ↻ ↻ ↻
rad  ad
r
Numerical treatment of the flame
time step:
t  
rprec. flame
max( vtheo , vreal )


with   0.1
Timmes et al. 1994 ApJ
t  10 5  50 yr , n_step  5000 - 10000
spacial zoning:
l kj1  l kj
l kj
 10%
rtheo  vtheot  2rCZb
(rCZb  shell width underneath CBCZ)
n_mesh - point prec,flame  50


shell extention  10-2  5 Km
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
Stage
Timescale
Teff (K)
L (L_sun)
Pre-MS
- 105
~ 5000
~ 103
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 -10-1 Lc,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)
 Lc behaviour similar to the one of mc
 m anti-correlated to Lc & mc
The C-burning nucleosynthesis
12C(12C,α)20Ne
20Ne
(~ 0.15-0.35),16O (~ 0.5-0.7), 23Na (~ 0.03-0.05)
12C(12C,p)23Na
+
p and α available for nucleosynthesis up to 27Al
16O(α,)20Ne
12C
(> 0.015) potential
trigger of explosion!
↓
Complete disruption of the
star
(Gutierrez et al. 2005 A&A)
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
Second dredge-up
features highly depend on Mini
Garcia-Berro & co-workers 1994,1996, 1997, 1999 ApJ (Z=0.02)
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-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.
Connessione MN – 2DUP
Evoluzione finale e massa MN