Yields from single AGB stars

1. Yields from single AGB stars Amanda Karakas Research School of Astronomy & Astrophysics Mt Stromlo Observatory

2. 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?

3. 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

4. Asymptotic Giant Branch stars Recent reviews: Busso et al. (1999), Herwig (2005)

5. The third dredge-up: carbon stars

6. Example: 6.5 Msun, Z = 0.012

7. Example: 6.5 Msun, Z = 0.012

8. 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

9. Making carbon stars is easier at lower metallicity M = 3, Z = 0.004, [Fe/H] ~  0.7

10. Example: 6.5Msun, Z = 0.02 Surface abundance evolution during TP-AGB Sodium production Production of heavy Mg isotopes

11. 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

12. 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

13. 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

14. 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

15. The effect of mass loss on the yields Yield of 23Na changes by more than 1 order of magnitude! VW93 Reimers

16. 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)

17. 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?