Nucleosynthesis in low and intermediate mass stars

Nucleosynthesis in low and 

intermediate mass stars 

Amanda Karakas 

Mt Stromlo Observatory 

The Australian National University 

Canberra, Australia

Introduction 

• Stars with masses ~0.8 to 8M sun will become asymptotic giant 

branch (AGB) stars near the end of their life 

• It is estimated that ~50% of the gas and dust in the Galaxy 

comes from red giants including AGB stars 

• They are important factories for producing many elements 

(e.g. carbon, nitrogen, 19 F, elements heavier than Fe) 

• Models of AGB stars can help explain the unusual composition 

of very metal-poor stars, and of pre-solar grains 

• Do massive AGB stars play a role in the globular cluster 

abundance anomalies? 

• In this talk I will focus on a couple of examples that test our 

knowledge of AGB stars and stellar nucleosynthesis

Basic Stellar Evolution 

Z = 0.02 or [Fe/H] = 0.0 , metallicity refers to both Z, 

total metals, or [Fe/H] = log 10 (Fe/H) star - log 10 (Fe/H) sun 

Main sequence: 

H to Helium 

τ ~ 10 10 yrs for 1 

~ 10 8 yrs for 5 

Red Giant Branch: 

core contracts 

outer layers expand 

E-AGB phase: 

a-er core He-burning 

star becomes a red giant 

for the second time

Getting processed matter out 

• For massive stars, explosions end the star’s life and 

releases matter into the interstellar medium 

• For low-mass stars, there are no explosions 

• Instead, mass loss slowly removes the envelope 

• Mixing between the core and the envelope takes 

place to change the surface composition 

• There are standard mixing events: 

– First dredge-up: hydrogen-fusion processed material 

– Second dredge-up: H-burning 

– Third dredge-up: Products of He-fusion 

AGB{ – Hot bottom burning: H-burning

Where mixing takes place 

HBB, TDU 

SDU 

FDU

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

Example: 3Msun, Z = 0.02

Zoom in on first few thermal pulses 

Time is scaled: (t-4.23 x 10 8 /1x 10 5 ) 

White = radiated luminosity 

Green = he-burning luminosity, and Red = H-burning luminosity

AGB lifecycle in more detail 

Deep convective 

envelope 

H-burning shell 

He-rich intershell 

He-burning shell 

C-O core



envelope 




C-O core



envelope 




C-O core



envelope 




C-O core



envelope 




C-O core



envelope 




C-O core

The AGB Evolution Cycle 

1. On phase: He-shell burns brightly, producing up to 100 

million L sun , drives a convection zone in the He-rich intershell 

and lasts for ~ 100 years 

2. Power-down: He-shell dies down, energy released by flash 

drives expansion which extinguishes the H-shell 

3. Third dredge-up: convective envelope moves inward into 

regions mixed by flash-driven convection. Mixes partially Heburnt 

material to surface. 

4. Interpulse: star contracts and H-shell is re-ignited, provides 

most of the surface luminosity for the next 10 4 to 10 5 years 

Pulse (He-burning) TDU (mixing) Interpulse 

Few ~10 2 yrs ~10 2 years ~10 4-5 yrs

Nucleosynthesis: He-burning 

• Main energy-generating reactions: 

– 3α process: 3 4 He 12 C 

– 12 C(α,γ) 16 O – relatively unimportant during 

thermal pulses 

• Other reactions: 

– 14 N captures 2 α particles to make 22 Ne 

– 22 Ne can capture an α particle to produce 

25,26 

Mg. Only occurs when T > 300 million K 

– 19 F can be produced through complex series of 

reactions involving both H, He-burning and 

neutron-capture nucleosynthesis

Making Carbon Stars! 

• Thermal pulses and dredge-up can occur many 

times during the AGB 

• Dredge up mixes 12 C from the He-shell to the 

surface, increasing the C/O ratio to > 1 

• Can explain the transition from M-type star (with 

C/O 1) 

• Along with carbon, the third dredge-up mixes 

elements created by the slow-neutron capture 

process (s-process) into the envelope 

• Fluorine is produced in the intershell and mixed 

to the surface by the TDU (Jorissen et al. 1992)

This is how we make carbon stars!

And it’s easier at lower metallicity 

M = 3, Z = 0.004, [Fe/H] ~ - 0.7

Hot bottom burning 

• In massive (M > 3Msun) AGB stars the base of the convective 

envelope can dip into the H-shell 

• Typical temperatures between ~50 to 100 million K 

• Burning region is thin in mass (10 -4 Msun) but efficient mixing 

means that entire envelope is exposed to hot region at least 

1000 times per interpulse! 

• Envelope burning was originally proposed to explain 

existence of luminous O-rich AGB stars in the LMC (Wood, 

Bessel & Fox 1983) 

• Many of these stars also rich in lithium and s-process 

elements (Smith & Lambert 1989) 

• Dredge-up can still occurs but CNO cycling at the base of the 

envelope prevents the formation of a C-rich atmosphere

Dredge-up still occurs

But the base of the envelope is hot! 

6.5M sun , Z =Z solar : Peak temperature ~ 90 x 10 6 K

Surface abundance evolution 

The 13 C content increases, due to the processing of 12 C into 12 C. In extreme cases, 

when the entire envelope can be processed many time between pulses, HBB can 

produce the equilibrium ratio of 12 C/ 13 C of about 3.5. A consequence of this burning 

is the copious production of primary 14 N.

Production of sodium 

Surface abundance evolution during the AGB phase 

sodium production 

Production of 25,26 Mg

Production of Li and destruction of F 

F production and destruction! 

Short-lived Li production phase 

as a result of HBB

Model uncertainties 

• Mass loss: model calculations use simple parameterized 

formulae which are supposed to be an average of what is 

observed. What about mass loss for very low Z models?! 

• 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 (e.g. rotation, overshoot)? Multidimensional 

modelling is required! 

• Reaction rates: improvements have been made but large 

uncertainties remain for many important reactions e.g. 

22 

Ne(a,n) 25 Mg 

• Low-temperature molecular opacities: AGB stars have cool 

enough outer layers to demand a proper treatment

The slow-neutron capture process 

The s process is responsible for the 

production of about half the 

abundances of elements heavier 

than iron in the Galaxy. 

s-process peaks 

During the s process: 

N n ~ 10 7 n/cm 3 

t(n,g) >> t b

Chart of the Nuclides 

The further a nucleus is from the valley of nuclear stability, the more 

unstable it is to β ± decay i.e. the shorter is its half-life 

From Frank Timmes website

The s-process path 

p-process nuclei - proton rich! 

S-only isotope 

The unstable Tc is observed in stars! 

r-process isotope 

s-process branching point

Time evolution of their structure. 

Where in AGB stars? 

4 

He, 12 C, 22 Ne, s-process elements: Zr, Ba, ...

Time evolution of their structure. 

Where in AGB stars? 

4 

He, 12 C, 22 Ne, s-process elements: Zr, Ba, ... 

At the 

stellar 

surface: 

C>O, s- 

process 

enhance 

ments

Theoretical models: the neutron sources 

proton 

diffusion 

13 

C(α,n) 16 O 

Interpulse phase (t ~ 10 4 years)


proton 

diffusion 

13 

C(α,n) 16 O 

22 

Ne(α,n) 25 Mg 



Low mass AGBs 

Intermediate mass AGBs 

Lower temperature ~4.5 M Higher temperature 

Larger intershell mass 

Smaller intershell mass 

proton 

diffusion 

13 

C(α,n) 16 O 

22 

Ne(α,n) 25 Mg 


Inclusion of a 13 C pocket 

• Standard low-mass AGB models do not produce 

enough s-process elements to match observations 

of AGB stars 

• That is because there is not enough 13 C in the Heintershell 

to activate the 13 C(α,n) 16 O reaction 

• So we need to artificially add in some 13 C 

• This is not great, but the best we can do in 1D 

• The mixing mechanism and the extent in mass of 

the pocket are unknown 

• see Herwig (2005, ARAA) for more discussions 

about 13 C pockets and 3D efforts being made to 

understand how they form

Theoretical models 

Typical neutron 

density profile in 

time: 

Low mass 

Intermediate mass 

Neutron source 

13 

C(a,n) 16 O 

22 

Ne(a,n) 25 Mg 

Maximum 

neutron density 

Timescale 

10 8 n/cm 3 

10,000 yr 

10 13 n/cm 3 

10 yr 

Neutron exposure 

0.3 mbarn -1 0.02 mbarn -1 

(at solar metallicity)

Neutron density indicators 

At high neutron densities, two branching points open that allow 

rubidium to be produced. At N n =5 x 10 8 n/cm 3 ~80% of the flux goes 

through 85 Kr, and the branching at 86 Rb opens to make 87 Rb 

1. 

85 

Rb has a high σ = 240 mb (30 keV) 

2. 87 Rb is magic, has a low σ = 15 mb (30 keV) 

Rb in AGB stars in an indicator of the neutron density! 

86 

Kr, 87 Rb, and 88 Sr are 

all magic, with low neutron 

capture cross sections 

In low-mass stars: 88 Sr produced 

In massive AGB: 87 Rb

Rubidium enhancements in AGB stars 

Max. production factor ~ 100! 

[Rb/Fe] 

Increasing stellar mass 

From D. A. Garcia-Hernandez et al. (2006, Science)

Comparison to observations: Rb 

Carbon stars from Abia et al. 

(2001): low mass AGBs 

OH stars from Garcia-Hernandez et 

al. (2006): intermediate mass AGBs 

Stellar model of 1.5, 5 M and 

Z (FRANEC + Torino) 

Stellar model of 3 M and Z 

(MONSSTAR + Monash).


















(MONSSTAR + Monash). 

Stellar model of 6.5 M and Z 

(MONSSTAR + Monash). 



We also produce Zr…

How do AGB stars make F? 

• The reaction chain: 18 O(p, α) 15 N(α, γ) 19 F(α, p) 22 Ne 

• Fluorine production takes place in the Heintershell 

region: He-rich, H poor 

• There are almost no protons, and little 15 N 

• These are created by other reactions including: 

– 13 C(α, n) 16 O - produces free neutrons 

– 14 N(n, p) 14 C - produces free protons 

– 18 F(α, p) 21 Ne - alternative proton production 

– 14 N(α, γ) 18 F(β + ) 18 O - main reaction to produce 18 O 

– 14 C(α, γ) 18 O - alternative reaction 

– 18 O(α, γ) 22 Ne - main 18 O destruction reaction 

– 15 N(p, α) 12 C - destroys 15 N

Fluorine production 

Results for a 3Msun model: 

Composition profile showing intershell 

region just a-er last TP. TDU will mix 

the 19 F created by the pulse into the 

envelope 

C/O 

Abundances from Jorissen et al. 

(1992) compared to model results 

⊗ - shows SC stars, with C/O = 1.0

The 18 F(a,p) 21 Ne reaction 

• Lee et al. (2008) recently performed experiments 

to obtain this reaction rate for the first time 

• Up until 2006, only theoretical rate was available 

• This experimental evaluation, when considering its 

associated uncertainties, presented significant 

differences compared to the theoretical rate at T ≈ 

300 million K 

• Using the upper limit of this rate, we find 

increases in the production of 19 F and 21 Ne 

• How does this come about? Through the extra 

protons released by this (a,p) reaction 

• Further experimental results needed to test the 

validity of the upper limit

[F/O] versus C/O ratio 

With upper limit 

Recommended rate 

Karakas et al. (2008) 

Increasing C/O ratio

Ne isotopic ratios 

Previous models 

Pre-solar grain data 

With 18 F(a,p) rate, we 

can move horizontally 

Karakas et al. (2008)

Fluorine in C-rich metal-poor stars 

• The fluorine abundance of the carbon-enhanced metal-poor 

star HE 1305+0132 is [F/Fe] = 2.90 (Schuler et al. 2007) 

• The iron abundance is [Fe/H] = -2.5, making HE 1305+0132 

the most Fe-deficient star, by more than an order of 

magnitude, for which the abundance of fluorine has been 

measured 

• Schuler et al. concluded that the atmosphere of HE 

1305+0132 was polluted via mass transfer by a primary 

companion during its AGB phase 

• Lugaro et al. (2008) estimate that AGB models of 2Msun, Z 

= 0.0001 can explain the composition of HE 1305 

• Allowing for 3 to 11% of the AGB matter to be accreted by a 

companion 

• We also predict that most CEMP stars should also be rich in 

fluorine, since massive rotating stars of low Z do not 

produce F

Fluorine in HE 1305 

Note that HE 1305 has an 

Fe abundance ~2.5 orders 

of magnitude smaller than 

the MS/S/C stars in this 

sample 

Most MS/S/C stars are disk 

stars with near solar Z 

Logarithmic abundances of 19 F versus 12 C. 

The abundances of HE 1305+0132 are marked 

by the magenta box with error bars. 

From Schuler et al. (2007).

Model results: F, C+N abundances 

The grey area is the region 

where the F, and C+N 

abundances from the AGB 

companion should be to 

match the observed 

abundances, a-er dilution 

The darker grey is the area 

covered by our models 

Models: [Fe/H] = -2.3, scaled 

solar initial composition 

From Lugaro et al. (2008, A&A Letters, in press)

Summary 

• All stars in the mass range 0.8 to ~8 Msun will 

pass through the AGB phase 

• AGB stars make an important contribution to 

the chemical evolution of galaxies 

• Important factories for C, N, F, and s-process 

elements 

• Model uncertainties (e.g. convection, mass loss, 

nuclear reaction rates) still problematic 

• Necessary to compare the models to the 

observations 

• Abundances from carbon enhanced metal-poor 

stars provide an interesting set of stars for 

comparison!

Fluorine in the intershell 

Fluorine intershell abundance reach a maximum (150 to 290 

times solar) for stars between 2 and 3.5Msun 

Lines from F v and F vi have been discovered in FUSE spectra 

of several PG1159 stars (Werner et al. 2005). 

Abundance ranging from solar to 250 times solar

Nucleosynthesis in low and intermediate mass stars

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