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