Introduction to nucleosynthesis in asymptotic giant branch stars

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1 Introduction to nucleosynthesis in asymptotic giant branch stars Amanda Karakas 1 and John Lattanzio 2 1) Research School of Astronomy & Astrophysics Mt. Stromlo Observatory 2) School of Mathematical Sciences, Monash University

2 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

3 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

4 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)

5 Where are they on a HR diagram? AGB stars

6 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 -> 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)

7 Asymptotic Giant Branch stars H-rich envelope Mass scale: Total mass = 3Msun, Core mass = 0.6Msun Envelope mass = 2.4Msun 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

8 AGB stars From Frank Timmes website

9 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 = helium mass fraction, and Z = 1 - X - Y = metals In the Sun: X = 0.705, Y = 0.28, Z = [X/Y] = log 10 (X/Y) star - log 10 (X/Y) sun ; in our Sun [Fe/H] = 0.0 by definition

10 Birth statistics From Frank Timmes website

11 Stellar Lifetimes Age of the galaxy 1.2 x years; Universe 1.37 x years Initial mass (M sun ) Main sequence lifetime (Myr) x x x 10 4 Total stellar lifetime (Myr) x x x 10 5 From Woosley, Heger & Weaver (2002, Rev. Mod. Phys. 74, 1015) From my models (e.g. Karakas & Lattanzio 2007)

12 The origin of the elements Lower mass stars (< 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 (core collapse) supernova, 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)

13

14 Aims of these lectures AGB stars are important! So we need accurate observations of their physical properties (e.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

15 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 (e.g. CN, C 2, 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)

16 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 < 3,500K Carbon star:red < 3,500K

17 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 < 3,500K Carbon star:red < 3,500K

18 AGB stars are long-period variables The M bol -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)

19 Presolar grains Graphite grain Murchison meteorite Silicon carbide grain

20 Silicon carbide (SiC) grains Not SN Low-mass AGB stars SN Type II Nova grains? From José et al. (2004)

21 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 (or moved forward in time) by solving the stellar structure equations at each mass-mesh point within the star at each time step

22 1 solar mass ZAMS model 16 O 12 C 14 N

23 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 = 0

24 Stellar modelling Evolve from the main sequence to the AGB We include 6 species (H, 3,4 He, C, N and O) involved in the main energy-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 postprocessing nucleosynthesis code

25 Output from evolution code

26 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 isotopes (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!

27 74 species nuclear network

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

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

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

31 Basic Stellar Evolution Prior to reaching the AGB, the stars evolve through core H and He-burning Main sequence: H to Helium τ ~ yrs for 1 ~ 10 8 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

32 Core H-burning and beyond: 1Msun Movies from John Lattanzio s website:

33 Core H-burning and beyond: 5Msun

34 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 At this point (owing to L = 4πσR 2 T eff 4 ) T eff 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)

35 Core H-burning and beyond: 1Msun

36 Core H-burning and beyond: 1Msun

37 Core H-burning and beyond: 5Msun

38 The first dredge-up: 1Msun

39 The first dredge-up: 5Msun

40 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 10 8 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 flash

41 Core He-flash

42 Core He-flash

43 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 T 40, shorter lifetimes! Typical He-burning lifetimes are ~100 million years for low-mass stars (~1Msun), compared to for H-burning Whereas core He-burning lasts about 20 million years for the 5Msun, compared to 80 million years for H-burning

44 Structure during second dredge-up Results for a 5 Msun, Z = 0.02 model:

45 The second dredge-up: 5Msun

46 Summary of 1 st 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

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