Introduction to in

Amanda Karakas1 and John Lattanzio2 1) Research School of & Astrophysics Mt. Stromlo Observatory 2) School of Mathematical Sciences, Monash University Lecture Outline

1. Introduction to AGB stars, and prior to the AGB phase 2. Nucleosynthesis before the AGB phase 3. Evolution and nucleosynthesis of AGB stars 4. The slow- capture process in AGB stars 5. Low and zero- AGB evolution 6. Super-AGB stars and post-AGB objects Outline of this lecture

1. Introduction to AGB stars 2. Observational constraints 3. Brief overview of stellar modelling techniques 4. Evolution of low and intermediate-mass stars up to the AGB phase Useful Reference Texts

Some of these can be viewed using Google books: 1. Chapter 2 from “Asymptotic Giant Branch Stars”, 2004, eds. H. J. Habing and H. Olofsson 2. D. D. Clayton, 1983, “Principles of and nucleosynthesis” 3. D. Arnett, 1996, “Supernovae & Nucleosynthesis” 4. B. E.J. Pagel, 1997, “ and Chemical evolution of ” 5. C. Iliadis, 2007, “Nuclear of Stars” 6. M. Lugaro, 2004, “Stardust from Meteorites” 7. Available for download: Karakas (PhD thesis, 2003) and Simon Campbell (PhD thesis, 2007) Where are they on a HR diagram?

AGB stars Introduction to AGB stars

• The asymptotic giant branch (AGB) phase is the final nuclear burning phase for all stars with masses 0.8 to 8Msun • Brief! Lasts less than 1% of the main-sequence lifetime • Cool (~3000 K) evolved red giants with distended envelopes (~ few hundred solar radii) • Spectral types: M, MS, S, SC, C type • Many AGB stars are observed to be losing mass rapidly (~10-5 Msun yr-1) through slow outflows (~10 km/s) • Aer ejection of the envelope, the AGB phase is terminated leading to: AGB -> post-AGB -> PN -> WD • Various mixing episodes alter the surface composition • Most are long-period variables (Mira, semi-regular, irregular) • Recent reviews: Herwig (2005), van Winckel (2003) Asymptotic Giant Branch stars

Mass scale: Total mass = 3Msun, Core mass = 0.6Msun Envelope mass = 2.4Msun H-rich envelope Radial scale: If we scale the core to the size of a marble (few cms) then to reach the outer layers we have to travel ~ 500 metres!

H-exhausted core AGB stars

From Frank Timmes website A few definitions

• Low-mass stars: – Initial masses from 0.8 to ~2.5 solar masses • Intermediate-mass stars: – Initial masses from ~2.5 to 8Msun • These definitions for Z = 0.02; depend on Z • Some authors define stars with M < 0.8 Msun as low-mass • X = hydrogen mass fraction, Y = mass fraction, and Z = 1 - X - Y = “” • In the : X = 0.705, Y = 0.28, Z = 0.015

• [X/Y] = log10 (X/Y) - log10 (X/Y)sun ; in our Sun [Fe/H] = 0.0 by definition Birth statistics

From Frank Timmes website Stellar Lifetimes

Age of the ≈1.2 x 1010 years; ≈1.37 x 1010 years

Main sequence Total stellar Initial mass (M ) sun lifetime (Myr) lifetime (Myr)

25 6.7 7.5

15 11 13

5 78 102

2 8.7 x 102 1.2 x 103

1 9.2 x 103 1.2 x 104

0.8 2.0 x 104 3.2 x 105

From Woosley, Heger & Weaver (2002, Rev. Mod. Phys. 74, 1015) From my models (e.g. Karakas & Lattanzio 2007) The origin of the elements

• Lower mass stars (< 0.8Msun) are still on the main sequence fusing in their cores • Hence these stars have not contributed to the chemical evolution of our Galaxy • In terms of single stars, the most important are 1) massive stars that explode as Type II (core collapse) , and 2) stars that evolve through the asymptotic giant branch (AGB) phase • Relative lifetimes are different! SN are short-lived and contribute quickly (assumed instantaneously) • AGB stars more slowly (50Myr to few Gyr)

Aims of these lectures

• AGB stars are important! • So we need accurate observations of their physical properties (e.g. composition, masses, ) • Along with accurate stellar evolution models that can explain these properties • Naturally there are problems with all of the above! • In these set of lectures, I aim to teach you about the evolution and nucleosynthesis of AGB stars • From the perspective of a stellar modeller • Let’s start with an overview of the observational data -rich AGB stars

• Much of the information we have about the composition of AGB stars comes from their stellar spectra • Carbon stars have strong bands of carbon

compounds (e.g. CN, C2, CH) and no metallic oxide bands, caused by C/O > 1 in the atmosphere • Most C-rich stars are evolved giants • First discovered by Secchi (1868) • In 1952, Merrill discovered that Tc was present in the atmosphere of S-type stars (with enhanced C but C/O < 1) • Review by Knapp & Wallerstein (1998) Carbon-star spectra (from SDSS)

A-type:blue 7,500 to 11,000K G-type:white/yelllow 5,000 to 6,000K

M3-late type:red :red < 3,500K < 3,500K Carbon-star spectra (from SDSS)

A-type:blue 7,500 to 11,000K G-type:white/yelllow 5,000 to 6,000K

M3-late type:red Carbon star:red < 3,500K < 3,500K AGB stars are long-period variables

The Mbol-log(P) diagram for LMC long-period variables (Wood 1998)

Spectra of two variables. Upper is M-type and lower is C-type (Olivier & Wood 2003)

Graphite grain Murchison meteorite

Silicon carbide grain carbide (SiC) grains

Not SN

Low-mass AGB stars SN Type II

Nova grains?

From José et al. (2004) Stellar modelling

• With these observational constraints in mind, we’ll have a look at how we make models • First, we model the interior structure • That gives us the , as a function of the interior mass, at each time step • Start with a zero-age main sequence model of the mass and composition we want • By model, we mean a snapshot in time of a star in • The ZAMS model is evolved (or moved forward in time) by solving the equations at each mass-mesh point within the star at each time step 1 ZAMS model

16O

12C

14N Then we evolve forward in time…

Modeller’s view of an Hertzsprung-Russell diagram: show the change in effective and as a function of time

t = 0 Stellar modelling

• Evolve from the main sequence to the AGB • We include 6 species (H, 3,4He, C, N and O) involved in the main -generating reactions • AGB phase is computationally demanding: – Prior to the AGB: max ~10,000 time-steps, avg ~ 2000 – During the AGB: max ~1.2 million!, avg ~ 100,000 • We stop the calculation when the envelope mass is lost • Or, convergence difficulties cause the calculation to cease (more common!) • Then this structure is used as input into a “post- processing nucleosynthesis code” Output from evolution code Post-processing nucleosynthesis

• Require as input the structure of the star as a function of time • This tells us how hot each burning region is, how extended the convective zones (in mass), how many mixing episodes… • Then, in the nucleosynthesis code we re-solve for the abundances in the star, as a function of interior mass and time • For many (74 to ~200) • Require as input the initial abundances and reaction rates • We assume that the energy from these extra reactions does not change the structure of the star! 74 species nuclear network Output from nucleosynthesis code

Composition as function of mass at a given time-step: Output from nucleosynthesis code

Composition as function of mass at a given time-step:

Composition at the surface, as as function of time Output from nucleosynthesis code

Composition as function of mass at By integrating the surface abundances over a given time-step: the star’s lifetime, we get yields:

Composition at the surface, as as function of time Basic Stellar Evolution

Prior to reaching the AGB, the stars evolve through core H and He-burning

Main sequence: H to Helium τ ~ 1010 yrs for 1 ~ 108 yrs for 5 Branch: core contracts outer layers expand E-AGB phase: aer core He-burning star becomes a red giant for the second time Core H-burning and beyond: 1Msun

Movies from John Lattanzio’s website: http://www.maths.monash.edu.au/~johnl/StellarEvolnV1/ Core H-burning and beyond: 5Msun Evolution prior to the AGB phase

• Aer core H-burning has ceased, the envelope expands and the core begins to contract • A hydrogen-shell burning is established in a shell around the contracting He-core • This provides most of the surface luminosity 2 4 • At this point (owing to L = 4πσR Teff ) Teff drops owing to increasing L and R • The envelope becomes convective, and moves inward into regions partially processed by previous H-burning (first dredge-up) • Following a period of core He-burning, the star becomes a giant for the second time (AGB) Core H-burning and beyond: 1Msun Core H-burning and beyond: 1Msun Core H-burning and beyond: 5Msun The first dredge-up: 1Msun The first dredge-up: 5Msun Core helium ignition: m < 2.5

• As stars ascend the giant branch, the He core continues to contract and heat • Once the temperature inside the core reaches about 108 K, core He ignition takes place • Low-mass stars need to contract substantially before reaching this temperature, causing the central regions to become -degenerate • energy losses from the core cause the temperature maximum to move outward • Eventually, the triple alpha reactions are ignited at the point of maximum temperature • E.O.S only slightly dependent on T, leading to a thermonuclear runaway: The core He flash Core He-flash Core He-flash Core helium burning

• Will be discussed in more detail in Lecture 2 • Following core He-ignition, there is a stable period of core helium fusion • The coulomb repulsion is larger for He than for H, hence more energy is required to fusion to occur • This means higher burning temperatures and because energy generation ∝ T40, shorter lifetimes! • Typical He-burning lifetimes are ~100 million years for low-mass stars (~1Msun), compared to 1010 for H-burning • Whereas core He-burning lasts about 20 million years for the 5Msun, compared to 80 million years for H-burning Structure during second dredge-up

Results for a 5 Msun, Z = 0.02 model: The second dredge-up: 5Msun Summary of 1st lecture

• All stars with masses ~0.8 to 8 Msun will pass through the AGB phase • This phase is brief, lasting less than 1% of the main sequence lifetime • The richest nucleosynthesis occurs there • Observational constraints come from observations of stars and from meteorites data • AGB phase is computationally demanding • Low and intermediate-mass stars go through central H and He-burning before reaching the AGB • Experience the first and/or second dredge-up which alters their surface composition