The s-process paradigm after Straniero et al. 1995 (Gallino et al. 1998)

13C(a,n)16O

22Ne(a,n)25Mg

Modelling AGB stars and their nucleosynthesisO. Straniero, S. Cristallo, L. Piersanti

INAF - Teramo

• in low-mass AGB stars (1.5-2.5). s process controlled by the 13C(a,n)16O operating in a thin pocket (10-3 Mʘ) during the interpulse:

T=90 MK (8 KeV),

timescale=105 yr

neutron density = 107 cm-3

neutron exposure=0.4 mbarn-1

neutrons/seeds=from 1 (solar Z) up to >20 (at low Z)

• In massive AGB (M>4) the s process is dominated by the 22Ne(a,n)25Mgoperating at the base of the convective zone generated by a TP:

T>300 MK (30 KeV),

timescale<1 yr

neutron density >1011 cm-3 (up to 1013)

neutron exposure<0.1 mbarn-1

neutrons/seeds < 10

The 3 players of the s-process

poisons

seeds

neutrons

seeds

poisonsneutrons Ratios of the 3 s-process peaks

ls, hs, Pb

Cameron (1957)

Pb, Bi

Variations of metallicty

(seeds)

0.0001<Z<Z

FUNS results

Cristallo et al. 2009

& 2011

hs=Ba,La,Nd

ls=Y,Sr,Zr

Many observational confirmations of the low-mass AGB scenario: Low Rb/Sr in S and C stars (Lambert 1995, Abia 2001) Low 96Zr in S stars and SiC grains (Lambert 1995, Lugaro 2003)

Brancings discriminate between different scenarios

AGB nucleosynthesis is a complex interplay of several phenomena,

among which convection and nuclear burning are, perhaps, the most

important. Interferences may be constructive or destructive, so that the

resulting nucleosynthesis may be enhanced or suppressed.

The "bagnasciuga" (wet & dry)

The s-process problem, i.e., how to

model the transition zone between the

sea and the shore (where children play).

HHe

Convective layer

adradV

Transition layerstable radiative layer

adrad

stable radiative layer

0V 0V

adrad

p

oH

rVV

exp

No composition variationsdredge up

convective radiative

Hydrodynamical models of overshoot:

1) the penetration increases as the stability of the radiative layer below the convective zone decreases , e.g. Singh 1995

2) The velocity decays exponentially,e.g. Freytag 1996

Why an exponential decay of V?

oVt

Vdt

V

dVV

dt

dV 11

2

2 aaa

0

0,5

1

1,5

2

2,5

0 2 4 6 8 10

v/vo

; r

/a

time/avo

)1ln(1

r W

dW1 dr

1

1

1 and

tVtV

V

dt

dr

dtVdWtVWdt

drV

o

o

o

oo

aaaa

a

a

aa

)exp( 1)exp( rVVrtV oo aaa

Viscous dissipation of kinetic energy -> 2VVr a

pHa

1

Varying : effects on TDU and 13Ceff

Observational constraints: C/O observations in intermediate age GCs in Magellanic Couds s-procrss overabundances in intrinsic AGB stars.

From post-pocess to a more realistic description of the AGB nucleosynthesis

In low mass star, s-process dominated by the first few 13C neutron burst episodes. After 4-5 TPs, hs/ls and Pb/hs frozen in the He rich zone.

The extension of the 13C pocket decreases as the Mass Increase.

• It implies: less efficient 13C neutron source as the stellar mass increases1.

1.5<M<3 Similar Core Mass <-> Similar 13C pockets 3<M<5 Progressive reduction of the 13C pockets M>5 Negligible 13C pockets

1Mass limits depend on Z and Y

[Fe/H]=-1.7

Additional processes….• Instabilities induced by Rotation may modify the H profile

left by the third dredge up and, later on, the 13C and the 14N profile into the pocket.

• The bulk motion in the convective envelope generates gravity waves propagating inward . Turbulence may be generated by non-linear effects (Denussenkov 2003) or by interaction with rotation (Talon 2007). The consequent mixing may affect nucleosynthesis and angular momentum transport

• Magnetic field dissipates angular momentum (magnetic breaking, Sujis 2008), but may also induce magnetic buoyancy operating in the He-rich intershell (Trippellathis meeting).

Differential Rotation: ES + GSF instabilities during the interpulse

Sharp variations of w and jleft by the TDU drives GSF

Meridional Circulation is always active in rotating stars, because of the von Zeipel effect, but it is inhibited by a m gradient.

See, e.g. Langer et al. 1999, Siess et al. 2004, Herwig et al. 2003, Piersanti et al. 2013

\\\ convection

||| X(13C)>10-3

ES

GSF

Development of rotation-induced instabilities between the 2nd and the 3rd TDU (M=2 Mʘ [Fe/H]=0)

The rotation paradigm

+

Turbulent convection at TDU:a proton profile forms at the top

of the He-rich zone

Rotation induced instabilities during the interpulse: redistribution of

protons and, later on, of 13C -14N on a larger area. Same neutrons, more

seeds and more poisons

AS a result: lowerseeds

poisonsneutrons

From Piersanti et al. 2013

Rotation as a possible explanation of the hs/ls observed spread

Intrinsic C stars only

Includes extrinsic and post AGB

s-rich stars in Globular Cluster M22

Rotation could do this job

Straniero et al. 2014

Tables of Yields available on FRUITY

Keep in touch with FRUITY Newslettersee S. Cristallo poster

Mass loss and mixing-length

- Mass loss affects number of TPs- Mixing length affects the deepness of a TDU

Both affect the total amount of material dredged up, but the relative abundance ratios are almost insensitive.

Vassiliadis Wood 1993, Groenewegen1997 (2009), Straniero 2006, Wood 2007