Yields from single AGB stars

Amanda KarakasResearch School of Astronomy &

AstrophysicsMt 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

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. 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

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) – 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

Carbon-12Z = 0.02 Z = 0.008

Z = 0.004

Legend:Black: my

modelsBlue: IzzardRed: Marigo

(2001)Pink: van den

Hoek & Groenewegen

Nitrogen-14

Z = 0.02 Z = 0.008

Z = 0.004

Legend:Black: my modelsBlue: IzzardRed: Marigo (2001)Pink: van den Hoek & Groenewegen

The effect of mass loss on the yields

Yield of 23Na changes by more than 1 order of magnitude!

VW93

Reimers

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?