Introduction to nucleosynthesis inasymptotic giant branch stars
Amanda Karakas1 and John Lattanzio21) Research School of Astronomy & Astrophysics
Mt. Stromlo Observatory2) School of Mathematical Sciences,
Monash University
Lecture Outline
1. Introduction to AGB stars, and evolution prior tothe AGB phase
2. Nucleosynthesis before the AGB phase3. Evolution and nucleosynthesis of AGB stars4. The slow-neutron capture process in AGB stars5. Low and zero-metallicity AGB evolution6. Super-AGB stars and post-AGB objects
Outline of this lecture
1. Introduction to AGB stars2. Observational constraints3. Brief overview of stellar modelling techniques4. 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. Olofsson2. 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 nuclearburning 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)
Asymptotic Giant Branch stars
Mass scale:Total mass = 3Msun,Core mass = 0.6MsunEnvelope mass = 2.4Msun
Radial scale:
If we scale the core to the size of amarble (few cms) then to reach theouter layers we have to travel ~500 metres!
H-exhausted core
H-rich envelope
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 = helium mass
fraction, and Z = 1 - X - Y = “metals”• In the Sun: X = 0.705, Y = 0.28, Z = 0.015• [X/Y] = log10 (X/Y)star - log10 (X/Y)sun ; in our Sun[Fe/H] = 0.0 by definition
Birth statistics
From Frank Timmes website
Stellar Lifetimes
3.2 x 1052.0 x 1040.8
1.2 x 1049.2 x 1031
1.2 x 1038.7 x 1022
102785
131115
7.56.725
Total stellarlifetime (Myr)
Main sequencelifetime (Myr)Initial mass (Msun)
Age of the galaxy ≈1.2 x 1010 years; Universe ≈1.37 x 1010 years
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 mainsequence fusing hydrogen in their cores
• Hence these stars have not contributed to thechemical 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 theasymptotic giant branch (AGB) phase
• Relative lifetimes are different! SN are short-livedand 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, 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 thecomposition of AGB stars comes from their stellarspectra
• Carbon stars have strong bands of carboncompounds (e.g. CN, C2, CH) and no metallic oxidebands, 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 Cbut C/O < 1)
• Review by Knapp & Wallerstein (1998)
Carbon-star spectra (from SDSS)
A-type:blue7,500 to 11,000K
G-type:white/yelllow5,000 to 6,000K
M3-late type:red< 3,500K
Carbon star:red< 3,500K
Carbon-star spectra (from SDSS)
A-type:blue7,500 to 11,000K
G-type:white/yelllow5,000 to 6,000K
M3-late type:red< 3,500K
Carbon star:red< 3,500K
AGB stars are long-period variables
The Mbol-log(P) diagram forLMC long-period variables
(Wood 1998)
Spectra of two variables. Upper isM-type and lower is C-type(Olivier & Wood 2003)
Presolar grains
Murchisonmeteorite
Silicon carbidegrain
Graphite grain
Silicon carbide (SiC) grainsNot SN
Nova grains?
SN Type IILow-mass AGB stars
From José et al. (2004)
Stellar modelling
• With these observational constraints in mind, we’llhave 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 ateach mass-mesh point within the star at each timestep
1 solar mass ZAMS model
16O
12C
14N
Then we evolve forward in time…
Modeller’s view of an Hertzsprung-Russell diagram: showthe change in effective temperatures and luminosity as afunction 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 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 islost
• Or, convergence difficulties cause the calculation tocease (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 afunction of time
• This tells us how hot each burning region is, howextended the convective zones (in mass), howmany mixing episodes…
• Then, in the nucleosynthesis code we re-solve forthe abundances in the star, as a function of interiormass 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!
74 species nuclear network
Output from nucleosynthesis codeComposition as function of mass at
a given time-step:
Output from nucleosynthesis code
Composition at the surface,as as function of time
Composition as function of mass ata given time-step:
Output from nucleosynthesis codeBy integrating the surface abundances over
the star’s lifetime, we get yields:
Composition at the surface,as as function of time
Composition as function of mass ata given time-step:
Basic Stellar Evolution
Main sequence: H to Helium τ ~ 1010 yrs for 1 ~ 108 yrs for 5Red Giant Branch: core contracts outer layers expand
E-AGB phase: a7er core He-burning star becomes a red giant for the second time
Prior to reaching the AGB, the stars evolve through core H and He-burning
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 envelopeexpands and the core begins to contract
• A hydrogen-shell burning is established in a shellaround the contracting He-core
• This provides most of the surface luminosity• At this point (owing to L = 4πσR2Teff
4) Teff dropsowing to increasing L and R
• The envelope becomes convective, and movesinward into regions partially processed by previousH-burning (first dredge-up)
• Following a period of core He-burning, the starbecomes 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 corecontinues to contract and heat
• Once the temperature inside the core reachesabout 108 K, core He ignition takes place
• Low-mass stars need to contract substantiallybefore reaching this temperature, causing thecentral regions to become electron-degenerate
• Neutrino energy losses from the core cause thetemperature maximum to move outward
• Eventually, the triple alpha reactions are ignited atthe point of maximum temperature
• E.O.S only slightly dependent on T, leading to athermonuclear 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 forH-burning
• Whereas core He-burning lasts about 20 millionyears for the 5Msun, compared to 80 million yearsfor 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 passthrough the AGB phase
• This phase is brief, lasting less than 1% of the mainsequence 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
