Study of the s-process in low mass stars of Galactic

disc metallicity

Laura Husti

Roberto GallinoSara Bisterzo

2

Outline:Outline:

• Origin of the heavy elements. Origin of the heavy elements.

• Stellar Nucleosynthesis and Evolution.Stellar Nucleosynthesis and Evolution.

• S-process nucleosynthesis in AGB S-process nucleosynthesis in AGB starsstars at solar metallicity. at solar metallicity.

• Comparison with spectroscopic data of Comparison with spectroscopic data of intrinsic and extrinsic AGBsintrinsic and extrinsic AGBs

3

10-3

10-2

10-1

100

101

102

103

104

105

106

107

108

109

1010

1011A

bund

ance

rel

ativ

e to

10

6 sili

con

200150100500Mass number

Solar system abundances(at the time of solar system formation)

GH,He

(big bang)

Carbon (AGB stars)

Fe peak(mostly Type I SN)

N=82r-process peak

Te, Xe(Type II SN)

N=82s-process peak

Ba, La, Ce(AGB stars)

N-126r-process peak

Os,Ir,Pt(Type II SN)

N=126s-process peak

Pb,Bi(AGB stars)

ThUU,Th

(Type II SN)

-elements(mostly Type II SN)

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Formation of Heavy Elements

• Thermonuclear reactions A<56

• Neutron captures - slow (s-process)105-1011 free

neutrons/cm3

- rapid (r-process) 1022-1025 free

neutrons/cm3

5

s-process

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Stellar evolution of low mass stars• Core H burning• Core H exhaustion and

contraction of the star• H-shell ignition → core

contraction + envelope expansion

• First dredge-up• Core He burning• Core He exhaustion;

contraction of the C-O core

• Degenerate C-O core + alternate burning H and He shells + He intershell + extended convective envelope

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Nucleosynthesis in AGB Stars• Neutron sources:

– radiative 13C(α,n)16O– convective 22Ne(α,n)25Mg

• 22Ne formation: 14 N(α,γ)18F(β+,ν)18O(α,γ) 22Ne• 13C formation: 12C(p, γ) 13 N(β+,ν) 13C(p, γ) 14 N

Busso, Gallino &Wasserburg ARA&A 1999

8

Variuos 13C pocket efficiencies, same metallicity

ls(Sr, Y, Zr)

hs(Ba, La, Cs, Nd,Sm)

Pb

9

IRAS 0713+1005

10

Extrinsic AGBs

21 1 2

2

2

1

1

tAGB

envini

envobs MMM

diltAGB

envobs

M

M10

dilFe

X

Fe

XAGB

dilFe

XdilFe

XobsAGBini

101010110log

11

HD 123585

Allen & Barbuy 2006

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Conclusions

• comparison between theoretical models and spectroscopic observations for the element abundances of 30 barium stars and 12 post-AGB stars

• models succesfully reproduce the observational data