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

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?

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

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

The third dredge-up: carbon stars

Example: 6.5 Msun, Z = 0.012

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. 12 C, 19 F, s-process elements Intermediate-mass AGB stars (3 to 8Msun) –Hot bottom burning occurs alongside the TDU –Results in enhancements of 4 He, 14 N –Destruction of 12 C and possibly 16 O

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

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

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

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) – 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

Carbon-12 Z = 0.02Z = Z = Legend: Black: my models Blue: Izzard Red: Marigo (2001) Pink: van den Hoek & Groenewegen

Nitrogen-14 Z = 0.02 Z = Z = Legend: Black: my models Blue: Izzard Red: Marigo (2001) Pink: van den Hoek & Groenewegen

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

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)

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?