The story of the 13C pocket and other fairy tales

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The story of the 13C pocket and other fairy tales

• Oscar Straniero
• (INAF-Osservatorio AstronomicoTeramo)

"The Origin of the Elements Heavier than Fe and
the 70th birthday of Roberto Gallino Torino,
24-28 September 2008
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Once upon a time..the origin (second half of XIX
century)
Padre Angelo Secchi on the roof of S. Ignazio in
Roma (1865)
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Observational and theoretical advances (1951-1973)

• Bidelman Keenan (1951). The parallel
  spectral sequence of giant stars, which includes
  S, R, N types as well as a new possible Ba II
  class, is characterized by anomalously large
  abundances of certain heavy elements (ApJ 114,
  473).
• Merrill (1952). Discovery of Tc in S stars (ApJ
  116, 21).
• Sandage Schwarzschild (1952). First models of
  Red Giant Stars (ApJ 116, 463).
• Salpeter (1952). aa ? 8Bea ?12C, the key
  process for the He burning (ApJ 115, 326).
  Stellar evolution of off main sequence stars was
  born.

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First hypothesis for nucleosynthesis in giants
stars.

• Cameron (1955). The 13C(a, n)16O and neutron
  captures as a solution for the Anomalous
  Abundances of the Elements in Giant Stars (ApJ
  121, 144).
• Fowler, Burbidge2 (1955). Just 1.4 neutrons per
  iron nucleus with the 13C (and 14N) in the
  ashes of the CNO burning (ApJ 122, 271).
• Cameron (1957). At the He ignition in the core,
  mixing of a few protons into the H-exhausted core
  and carbon dredge up remove these difficulties
  (AJ 62,138). Proton recycling by 14N(n, p)14C .

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s-process the beginning
From Cameron (1957) PASP 69, 408. On this
ground, Clayton (1988) predicts an increase of
the neutron exposure at lower Z. Busso et al
(1995), no need of a strong component to
explain the solar lead..
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Further developments.

• Cameron (1960). 22Ne(a,n)25Mg, a possible
  alternative neutron source (AJ 65, 485).
• Clayton et al. (1961). A superposition of neutron
  exposures is required to reproduce the
  experimentally observed abundance distribution
  for the s-process isotopes (Ann. Phys. 12, 331
  also SFC 1965 ApJS 11, 121).
• Weigert (1966), Schwarzschild Harm (1967).
  Thermal pulses occurs during the AGB phase (Z.Ap
  64, 395 ApJ 150, 961).
• Ulrich (1973). The partial overlap of successive
  convective shells leads to an exponential
  distribution of the neutron exposures (in
  Explosive Nucleosnthesis, Univ. Texas, Austin)

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A simple mathematical law is appealingThe
s-process paradigm in between 1973-1993
The s-process (main component) site was
identified in the successive convective shells
generated by thermal pulses in AGB stars and the
search for the neutron source was concentrated on
this site.
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The 22N(a,n) 25Mg scenario

• Iben (1975) ApJ 196, 525. Sugimoto Nomoto
  (1975), PASJ 27,197, Truran Iben (1977), ApJ
  216, 797.
• When the thermal pulse starts, 22Ne is
  synthesized from the 14N left behind by the
  H-burning during the interpulse after 2 a
  captures and a b decay.
• At maximum size, the He convective shell extents
  from the position of highest He-burning rate to
  just below the XY discontinuity, no further.
• Enough neutrons are released, if T3.5x108 K.
  This only occurs in massive AGB (Mgt5M?).
• Following the disappearance of the convective
  shell, the base of the convective envelope
  extends down to the outer portion of the region
  previously contained into the convective shell
  the third dredge up.

The 22N(a,n) 25Mg doesn't work in low mass AGB
stars. Typically, less than 1 neutron per 56Fe!
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Low mass AGB stars the
13C(a,n)16O scenario

• Sackmann 1980. Suggestion of a promising possible
  mixing of protons into the He convective shell at
  its maximum size (ApJ 235,554). However, Iben
  (1982), no mixing of protons during the TP due to
  the entropy barrier built up by the H burning .
• Iben Renzini 1982. Semiconvection driven by
  (assumed) enhanced C opacity during the TDU ? few
  protons (10-6 M? ? formation of 13C pocket during
  the IP ? engulfment into the convective shell and
  burning at relatively high temperature. 26
  neutrons per 56Fe (ApJ, 259, L79 263, L118, se
  also Hollowell Iben 1987). However, no
  semiconvection found after OPAL 1994.
• Busso Gallino 1988. First (quai)
  self-consistent s-process calculation based on
  stellar models inputs (from Hollowell and Iben
  1987).

In all cases, the 13C is assumed to burn in the
convective shell generated by the Thermal Pulse!
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The crisis

• Smith Lambert (1986), Malaney Lambert (1988),
  Busso et al. (1995), Lambert et al. (1995), Abia
  et al. (2001). Branchings are sensitive to the
  neutron density. The lack of 96Zr and yje low Rb
  imply rnlt108 neutrons/cm-3 in MS, S, N and Ba
  stars, incompatible with 22Ne source and only
  marginally compatible with 13C burning in the
  convective shell.
• Bazan Lattanzio (1993) investigate the
  energetic feedback occurring when the 13C pocket
  is engulfed into the convective shell and the
  s-process takes place. Radical alterations of the
  TP and the related nucleosyntesis are predicted
  (ApJ 409, 762). In particular, high neutron
  density (1010 cm-3) . Overproductions of both Rb
  and 96Zr are expected.

The (convective) 13C burning provides a good
neutron exposure, but the neutron density is too
high!
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The renaissance

• Straniero et al. (1995). 13C burns during the
  interpulse, in radiative condition and before the
  onset of the TP. The neutron density is only
  106-107 cm-3 and the timescale 104 yr (ApJ
  440,L85). The Rb and 96Zr drawbacks, as well as
  the energetic feedback problem, are removed.
• Gallino et al. (1998). Nucleosynthesis in the
  radiative 13C pocket, based on new models of low
  mass AGB star (1.5ltM/M?lt3, Straniero et al.
  1997). The chemical profile within the 13C pocket
  is still taken according to Hollowel and Iben
  (1987) and the amount of 13C is a free parameter.
  ST case (4x10-6 M? of 13C) gives the best
  reproduction of the solar (s-only) distribution
  (ApJ, 497, 388).
• Busso et al. (2001) Abia et al. (2002). hs/ls
  in good agreement with observation. A (moderate)
  spread in the 13C mass seems to be required.

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The formation of the 13C pocket

• Several (exotic) mechanisms have been proposed
  convective overshoot (Herwig 1997), mixing
  induced by rotation (Langer et al. 1999), weak
  turbulence induced by gravity waves (Tout
  Denissenkov 2003).
• Independently on the physical mechanism, if a
  diffusive mixing scheme is adopted the resulting
  13C pocket is definitely too small (10-7 M?,,
  Herwig 2000).
• Recent Hydrodinamical simulations (e.g. Arnett,
  2008, IX Torino workshop, Perugia), show that
  convection is not a diffusive process and mixing
  depends linearly on dr.

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Dredge up and the unstable equilibrium of the
convective boundary layer.

• When the convective envelope extends down to the
  H-depleted region, the boundary layer becomes
  unstable Castellani, Chieffi Straniero (1991),
  Frost Lattanzio (1996), Marconi, Castellani
  Straniero (1997), Molawy (1999).
• In any case, if the third dredge up would leave a
  XY discontinuity, it should be smoothed away by
  atomic diffusion. Iben (1982, ApJ, 260, 821),
  showed that the diffusion timescale is comparable
  to the duration of the post-flash deep.

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Removing the instability consequence for the
dredge up and the s process nucleosynthesis

• we assume (Straniero, Gallino Cristallo 2006
  (Nucl.PhysA 777,311)
• An exponential decay of the average convective
  velocity at the boundary.
• A time dependent mixing, The degree of mixing
  between two mesh points is proportional to

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The triple pocket.

• Maximum envelope
• penetration (TDU)
• 12C(p,?)13N(ß-)13C and 13C(p,?)14N
• 22Ne(p,?)23Na
• full shell H burning settles on

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13C pocket and dredge up as a function of b
(Cristallo et. 2008).Third TP of 2 M?
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Full network (700 isotopes) stellar model.
First formation of the 13C-pocket
ACTIVATION OF THE 13C(a,n)16O reaction
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Final AGB composition for 0.0001ltZltZ ?Cristallo
et al. 2008 (submitted to the ApJ)
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Comparison with galactic and extragalactic
INTRINSIC C stars at different metallicity
Data from Abia et al. 2002, 2008 de Laverny et
al. 2006
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Thank you Carlos for handling such a crazy spectra
Abia et al. 2001
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Thank you Roberto for giving us such a sandwich
of science and fun