52° CONGRESSO SAIT TERAMO, 4 - 8 MAGGIO 2008 The s-nucleosynthesis process in massive AGB and...
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Transcript of 52° CONGRESSO SAIT TERAMO, 4 - 8 MAGGIO 2008 The s-nucleosynthesis process in massive AGB and...
52° CONGRESSO SAIT
TERAMO, 4 - 8 MAGGIO 2008
The s-nucleosynthesis process The s-nucleosynthesis process in massive AGB and Super-AGB in massive AGB and Super-AGB
starsstarsM.L. PumoM.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 Super-AGB stars & the ZAMSZAMS
Mup Mmas MZAMS (~ 7-9M⊙) (~ 11-13M⊙)
AGB:low-mass &
intermediate-massSuper-AGB massive
MZAMS < Mup: unable to ignite core C-burn.
MZAMS ≥ Mmas: 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: evolutionSuper-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
Mfcore =MEC ~ 1.37
MM⊙⊙ Mf
core< MMECEC
collapsing electroncaptures supernovae
Neutron star
NeO White Dwarf
Final fateFinal fate(Nomoto, 1984, ApJ)
Interplay between mass loss Interplay between mass loss and core growthand core growth
1.37 M⊙
Mend,2
Mend,1
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⊙
The minimum initial mass for the formation of a neutron star is
usually referred to as MN (transition NeO WD / EC SN)
(e.g. Woosley et al. 2002, ARA&A)
Existence of 2 “final” Existence of 2 “final” evolutionary channelsevolutionary 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 Self-Enrichment in GCs & the Super-AGB starsSuper-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 & D’Antona 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 ~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
Super-AGBs may
be progenitor
s
Trans-iron nucleosynthesis: Trans-iron nucleosynthesis: s-process in massive AGB & s-process in massive AGB &
Super-AGB starsSuper-AGB stars
Main neutron source: 22Ne(α, n)25Mg 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=6MPreliminary results (for a M=6M⊙⊙ Z=0.02 model)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 youThank you
SN SN triggeredtriggered by EC by EC
MONe =MEC ~ 1.37 MM⊙⊙ EC reactions on:
24Mg and 24Na, 20Ne and 20F
iiiie
eeCh
AZXY
YYM
where
5.0/46.122
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 ~ 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 electronsPartial degeneracy of electrons
2
exp)2(
4)(
space) (momentum dppp, shell spherical in the electrons
2
23
2
dpdVkTm
p
kTm
pndpdVpf
eee
for 0
for 8
)(3
2
F
F
pp
ppp
dpdVpf
1
18 )(
/3
2
dpdVe
pdpdVpf
kTE
Computation method and Computation method and numerical details 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. → 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
4000,35 core
loss
M
M
52 nuclei+162 reactions (pp, CNO, -,-,-,p-,n-reactions, 12C+12C, 12C+16O)
Nuclear Network 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 ratesReactions rates
= rQ/
reaction rate r
(number of reactions per unit time and volume)
Ni = number density of interacting speciesv = relative velocity(v) = velocity distribution in plasma(v) = reaction cross section (10-9 - 10-12 barn)
typical units: MeV g-1 s-1
energy production rate
vNNr TppT
1
1 vdv)v()v(v
No overshooting: MLT (=1.75) + SchwarzschildSchwarzschild
mean nuclear reaction ratemean nuclear reaction rate
Yes overshooting:Yes overshooting: upper edge of convective zone upper edge of convective zone
nucleosynthesis shell by shell + diffusive mixingnucleosynthesis shell by shell + diffusive mixing
Treatment of convectionTreatment of convection
t
b
m
m
kbt
knucl
dmmmmt
Y
t
Y)(ij
1ijcon
p
c
mixfH
zD
lvD
rM
YDr
rMt
Y2
exp
3
1
con )(
4)(
0
22
Instabilità dinamica: criterio di Instabilità dinamica: criterio di SchwarzschildSchwarzschild
ade
rada
dr
dTrT
dr
dTrTTT
dr
dTrT
dr
dTrTT
000'
1
001
1
0 00 TT
r
Pd
Td
dr
dP
P
T
dr
dT
dr
dP
Pdr
dT
TPKTPT
TP
P
adad ln
ln ;
1111lnln
2
2
1'
1
adradea dr
dT
dr
dT
dr
dT
dr
dTTT 1
'1
adrad
Sottostima estensione zona Sottostima estensione zona convettivaconvettiva
0a
No inerzia
“convective overshooting”
penetrazioni in regioni dinamicamente stabili
ampliamento estensione
zona convettiva
r
adrad adrad
↻ ↻ ↻ ↻ ↻ ↻ ↻ ↻ ↻ ↻ ↻ ↻
adrad
time step:
spacial zoning:spacial zoning:
Numerical treatment of the flameNumerical treatment of the flame
Km 510extention shell
50point-n_mesh
CBCZ) underneath width shell(
2
%10
2-
flameprec,
1
b
btheotheo
kj
kj
kj
CZ
CZ
r
rtvr
l
ll
10000-5000n_step ,5010
1.0 with ),max(
5
.
yrt
vv
rt
realtheo
flameprec
Timmes et al. 1994 ApJ
Riscaldamento del core
Esaurimento del
combustibile
Contrazione del core
Bruciamento nucleare
c > 2.4· 10-8 µeT3/2g
cm-3
Core degenere
inerte
~ 5000
0.65 – 0.7
0.08-0.1
0.03
Tcore (109 K)
Pre-MS
C burning
He burning
H burning
Stage
~ 103
106 – 107
103
10
Density (g cm-
3)
- 105
10-103 / 102-103
106
107 – 10 8
Timescale (yr)
Stage Timescale Teff (K) L (L_sun)
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
C-burning: evolution
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)
(Siess & Pumo 2006a,b)
Lc behaviour similar to the one of mc
m anti-correlated to Lc & mc
The C-burning The C-burning nucleosynthesisnucleosynthesis
12C(12C,α)20Ne
12C(12C,p)23Na
16O(α,)20Ne
12C (> 0.015) potential trigger of explosion!
↓
Complete disruption of the star
(Gutierrez et al. 2005 A&A)
20Ne (~ 0.15-0.35),16O (~ 0.5-0.7), 23Na (~ 0.03-0.05)
+
p and α available for nucleosynthesis up to 27Al
Nucleosynthesis in the NeO coreNucleosynthesis in the NeO core
22Ne(α,n)25Mg
n: 16O, 20Ne, 23Na, 25Mg → 17O, 21Ne, 24Mg, 26Mg
22Ne(α,)26Mg
α particle:
protons:
26Mg(p,)27Al
23Na(p,α)20Ne
23Na(p,)24Mg
Second dredge-upSecond dredge-upfeatures highly depend on Mini
Garcia-Berro & co-workers 1994,1996, 1997, 1999 ApJ (Z=0.02)
Mini~ Mup
(3.46·107 yr) (3.50·107yr)
Mini~ Mmas
(1.67·107 yr) (1.77·107yr)
(3.35·107 yr) (3.36·107yr)
Mini < Mmas
Second dredge-outSecond 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 Evoluzione finale e massa Mmassa MNN