Introduction to nucleosynthesis in asymptotic giant branch stars Amanda Karakas1 and John Lattanzio2 1) Research School of Astronomy & Astrophysics Mt. Stromlo Observatory 2) School of Mathematical Sciences, Monash University Lecture Outline 1. Introduction to AGB stars, and evolution prior to the AGB phase 2. Nucleosynthesis before the AGB phase 3. Evolution and nucleosynthesis of AGB stars 4. The slow-neutron capture process in AGB stars 5. Low and zero-metallicity 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 stellar evolution and nucleosynthesis” 3. D. Arnett, 1996, “Supernovae & Nucleosynthesis” 4. B. E.J. Pagel, 1997, “Stellar Nucleosynthesis and Chemical evolution of Galaxies” 5. C. Iliadis, 2007, “Nuclear Physics 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) • A7er ejection of the envelope, the AGB phase is terminated leading to: AGB -8 post-AGB -8 PN -8 WD • Various mixing episodes alter the surface composition • Most are long-period variables 1Mira, semi-regular, irregular) • Recent reviews: Herwig 12005), van Winckel 12003) Asymptotic Giant Branch stars Mass scale: Total mass 9 3Msun, Core mass 9 0.6Msun Envelope mass 9 2.4Msun H-rich envelope Radial scale: If we scale the core to the size of a marble 1few cms) then to reach the outer layers we have to travel 4 500 metres! H-exhausted core AGB stars From Frank Timmes website A few de2nitions • Low-mass stars: : Initial masses from 0.8 to 42.5 solar masses • Intermediate-mass stars: : Initial masses from 42.5 to 8Msun • These de2nitions for Z 9 0.02; depend on Z • Some authors de2ne stars with M ; 0.8 Msun as low-mass • X 9 hydrogen mass fraction, Y 9 helium mass fraction, and Z 9 1 - X - Y 9 +metals, • In the Sun: X 9 0.705, Y 9 0.28, Z 9 0.015 • [X6Y] 9 log10 1X6Y)star - log10 1X6Y)sun ; in our Sun [Fe6H] 9 0.0 by de2nition Birth statistics From Frank Timmes website Stellar Lifetimes Age of the galaxy >1.2 x 1010 years; Universe >1.37 x 1010 years Main sequence Total stellar Initial mass 1M ) sun lifetime 1Myr) lifetime 1Myr) 25 6.0 0.5 15 11 13 5 08 102 2 8.0 x 102 1.2 x 103 1 ..2 x 103 1.2 x 104 0.8 2.0 x 104 3.2 x 105 From Woosley, Heger & Weaver 12002, Rev. Mod. Phys. 04, 1015) From my models 1e.g. Karakas & Lattanzio 2000) The origin of the elements • Lower mass stars 1; 0.8Msun) are still on the main sequence fusing hydrogen 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 1core collapse) supernova, and 2) stars that evolve through the asymptotic giant branch 1AGB) phase • Relative lifetimes are di?erent! SN are short-lived and contribute quickly 1assumed 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 1e.g. composition, masses, luminosities) • 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 Carbon-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 1e.g. CN, C2, CH) and no metallic oxide bands, caused by C6O 8 1 in the atmosphere • Most C-rich stars are evolved giants • First discovered by Secchi 11868) • In 1952, Merrill discovered that Tc was present in the atmosphere of S-type stars 1with enhanced C but C6O ; 1) • Review by Knapp & Wallerstein 11998) Carbon-star spectra 1from SDSS) A-type:blue 7,500 to 11,000K G-type:white6yelllow 5,000 to 6,000K M3-late type:red Carbon star:red ; 3,500K ; 3,500K Carbon-star spectra 1from SDSS) A-type:blue 7,500 to 11,000K G-type:white6yelllow 5,000 to 6,000K M3-late type:red Carbon star:red ; 3,500K ; 3,500K AGB stars are long-period variables The Mbol-log1P) diagram for LMC long-period variables 1Wood 1..8) Spectra of two variables. Upper is M-type and lower is C-type 1Olivier & Wood 2003) Presolar grains Graphite grain Murchison meteorite Silicon carbide grain Silicon carbide 1SiC) 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 density, temperature 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 hydrostatic equilibrium • The ZAMS model is evolved 1or moved forward in time) by solving the stellar structure equations at each mass-mesh point within the star at each time step 1 solar mass ZAMS model 16O 12C 14N Then we evolve forward in time… Modeller’s view of an Hertzsprung-Russell diagram: show the change in effective temperatures and luminosity as a function of time t 9 0 Stellar modelling • Evolve from the main sequence to the AGB • We include 6 species 1H, 3,4He, C, N and O) involved in the main energy-generating reactions • AGB phase is computationally demanding: : Prior to the AGB: max 410,000 time-steps, avg 4 2000 : During the AGB: max 41.2 million!, avg 4 100,000 • We stop the calculation when the envelope mass is lost • Or, convergence diAculties cause the calculation to cease 1more 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 1in 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 isotopes 10' 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 4 1010 yrs for 1 4 108 yrs for 5 Red Giant Branch: core contracts outer layers expand E-AGB phase: a7er 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 • A7er 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 1owing to L 9 4πσR Te? ) Te? drops owing to increasing L and R • The envelope becomes convective, and moves inward into regions partially processed by previous H-burning 12rst dredge-up) • Following a period of core He-burning, the star becomes a giant for the second time 1AGB) Core H-burning and beyond: 1Msun Core H-burning and beyond: 1Msun Core H-burning and beyond: 5Msun The 2rst dredge-up: 1Msun The 2rst 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 electron-degenerate • Neutrino 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 5ash Core He(5ash Core He(5ash Core helium burning • Will be discussed in more detail in Lecture 2 • Following core He-ignition, there is a stable period