Nucleosynthesis in single AGB stars

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Nucleosynthesis in single AGB stars

• Amanda Karakas
• Research School of Astronomy Astrophysics
• Mt Stromlo Observatory

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Introduction

• The asymptotic giant branch (AGB) is the final
  nuclear burning phase before stars become PN
• The composition of PN are determined (in part) by
  AGB nucleosynthesis
• Mixing episodes occur during the stars life that
  alter the surface composition
• How accurately do model compositions reflect the
  observed? Need stellar yields!
• Can we use PN compositions to constrain the
  amount of mixing in the stellar models?

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Basic Stellar Evolution
Z 0.02 or Fe/H 0.0
Main sequence H ? Helium
HBB, TDU
Red Giant Branch core contracts outer
layers expand
SDU
E-AGB phase after core He-burning star
becomes a red giant for the second time
FDU
TP-AGB phase thermal pulses start mass loss
intensifies
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Asymptotic Giant Branch stars
Recent reviews Busso et al. (1999), Herwig
(2005)
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The third dredge-up carbon stars
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Example 6.5 Msun, Z 0.012
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Example 6.5 Msun, Z 0.012
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Summary of AGB nucleosynthesis

• Low-mass AGB stars (1 to 3 Msun)
• The third dredge-up may occur after each thermal
  pulse (TP)
• Mixes He-burning products to the surface e.g.
  12C, 19F, s-process elements
• Intermediate-mass AGB stars (3 to 8Msun)
• Hot bottom burning occurs alongside the TDU
• Results in enhancements of 4He, 14N
• Destruction of 12C and possibly 16O

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Making carbon stars is easier at lower metallicity
M 3, Z 0.004, Fe/H ? 0.7
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Example 6.5Msun, Z 0.02
Surface abundance evolution during TP-AGB
Sodium production
Production of heavy Mg isotopes
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A note on stellar models

• Ive shown results from detailed, 1D stellar
  structure computations
• By detailed I mean that we solve the equations of
  stellar structure (for the L, T, rho, P) over a
  mass grid that represents the interior of the
  star
• Many AGB yield calculations come from synthetic
  AGB models (e.g. Marigo 2001, van den Hoek
  Groenewegen 1997, Izzard et al. 2004)
• These use fitting formula derived from the
  detailed models (e.g. core-mass luminosity)
• Synthetic models are only as good as the fitting
  formula they are based upon

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Stellar Yields

• Synthetic models Renzini Voli (1981), van den
  Hoek Groenewegen (1997), Marigo (2001), Izzard
  et al. (2004)
• Detailed models Ventura et al. (2001), Karakas
  Lattanzio (2003, 2007), Herwig (2004), Stancliffe
  Jeffery (2007)
• http//www.mso.anu.edu/akarakas/stellar_yields/
• Combination of both Forestini Charbonnel
  (1997)
• Preferable to use detailed models - if available
• PN compositions represent last 2 TPs whereas
  most yields integrated over whole stellar lifetime

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Carbon-12
Z 0.02
Z 0.008
Legend Black my models Blue Izzard Red Marigo
(2001) Pink van den Hoek Groenewegen
Z 0.004
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Nitrogen-14
Z 0.008
Z 0.02
Legend Black my models Blue Izzard Red Marigo
(2001) Pink van den Hoek Groenewegen
Z 0.004
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The effect of mass loss on the yields
Yield of 23Na changes by more than 1 order of
magnitude!
VW93
Reimers
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Stellar Modelling Uncertainties

• Mass loss model calculations use simple
  parameterized formulae which are supposed to be
  an average of what is observed
• Convection 1D models mostly use mixing-length
  theory. Also numerical problem of treating
  convective boundaries
• Extra-mixing? When and where to apply! What are
  the physical processes that produce it?
• Reaction rates large uncertainties remain for
  many important reactions
• Opacities stellar models should use molecular
  opacities that reflect the composition of the
  star (Marigo 2002)

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Conclusions

• AGB nucleosynthesis helps determine the
  composition of PN
• Yields of AGB stars are shaped by the TDU for
  low-mass objects
• Or a combination of HBB and the TDU for
  intermediate-mass objects
• Substantial model uncertainties are still present
  in all models (synthetic, detailed)
• Can we use the composition of post-AGB and PN
  objects to help constrain the models?