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Yields from single AGB stars Amanda Karakas Research School of Astronomy & Astrophysics Mt Stromlo...
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![Page 1: Yields from single AGB stars Amanda Karakas Research School of Astronomy & Astrophysics Mt Stromlo Observatory.](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649f475503460f94c6901d/html5/thumbnails/1.jpg)
Yields from single AGB stars
Amanda KarakasResearch School of Astronomy &
AstrophysicsMt Stromlo Observatory
![Page 2: Yields from single AGB stars Amanda Karakas Research School of Astronomy & Astrophysics Mt Stromlo Observatory.](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649f475503460f94c6901d/html5/thumbnails/2.jpg)
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?
![Page 3: Yields from single AGB stars Amanda Karakas Research School of Astronomy & Astrophysics Mt Stromlo Observatory.](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649f475503460f94c6901d/html5/thumbnails/3.jpg)
Basic Stellar Evolution
Main sequence: H Helium
Red Giant Branch: core contracts outer layers expand
E-AGB phase: after core He-burning star becomes a red giant for the second time
Z = 0.02 or [Fe/H] = 0.0
FDU
SDU
HBB, TDU
TP-AGB phase: thermal pulses start mass loss intensifies
![Page 4: Yields from single AGB stars Amanda Karakas Research School of Astronomy & Astrophysics Mt Stromlo Observatory.](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649f475503460f94c6901d/html5/thumbnails/4.jpg)
Asymptotic Giant Branch stars
Recent reviews: Busso et al. (1999),
Herwig (2005)
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The third dredge-up: carbon stars
![Page 6: Yields from single AGB stars Amanda Karakas Research School of Astronomy & Astrophysics Mt Stromlo Observatory.](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649f475503460f94c6901d/html5/thumbnails/6.jpg)
Example: 6.5 Msun, Z = 0.012
![Page 7: Yields from single AGB stars Amanda Karakas Research School of Astronomy & Astrophysics Mt Stromlo Observatory.](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649f475503460f94c6901d/html5/thumbnails/7.jpg)
Example: 6.5 Msun, Z = 0.012
![Page 8: Yields from single AGB stars Amanda Karakas Research School of Astronomy & Astrophysics Mt Stromlo Observatory.](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649f475503460f94c6901d/html5/thumbnails/8.jpg)
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
![Page 9: Yields from single AGB stars Amanda Karakas Research School of Astronomy & Astrophysics Mt Stromlo Observatory.](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649f475503460f94c6901d/html5/thumbnails/9.jpg)
Making carbon stars is easier at lower metallicity
M = 3, Z = 0.004, [Fe/H] ~ 0.7
![Page 10: Yields from single AGB stars Amanda Karakas Research School of Astronomy & Astrophysics Mt Stromlo Observatory.](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649f475503460f94c6901d/html5/thumbnails/10.jpg)
Example: 6.5Msun, Z = 0.02
Sodium production
Production of heavy Mg isotopes
Surface abundance evolution during TP-AGB
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A note on stellar models
• I’ve 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
![Page 12: Yields from single AGB stars Amanda Karakas Research School of Astronomy & Astrophysics Mt Stromlo Observatory.](https://reader035.fdocuments.net/reader035/viewer/2022062423/56649f475503460f94c6901d/html5/thumbnails/12.jpg)
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-12Z = 0.02 Z = 0.008
Z = 0.004
Legend:Black: my
modelsBlue: IzzardRed: Marigo
(2001)Pink: van den
Hoek & Groenewegen
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Nitrogen-14
Z = 0.02 Z = 0.008
Z = 0.004
Legend:Black: my modelsBlue: IzzardRed: Marigo (2001)Pink: van den Hoek & Groenewegen
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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?