Nucleosynthesis in AGB stars
from observations
C. Abia
Russbach School, March 2010
Outline:
1. Introduction
2. Observational aspects of AGB stars
3. AGBs & nucleosynthesis:
- s-elements
- Fluorine
- Lithium
- ...
4. Summary
TP
-A
G
B
Evolution of low & intermediate mass stars
1- 8 M
EAG
B
-
He central
burning
H central
burning
RGB
Pre
Main Sequence
- Cool, Teff < 3500 K
- Luminuous, L > 100 L
- Large, R > 100 R
How they look ?
CW Leo
Thermal infrared (10 µm)
Visible (0.6 µm)
Asymptotic Giant Branch stars
H-rich
Conv. Env.
He
CO
H shell burning
~ 200 R
He shell burning
AGB chemical & spectral evolution
M MS S SC N (carbon stars)
C/O ~ 0.5..................... 1... > 1
12C
12C
Busso et al. (1999)
Variation of the atmosphere’s chemist with the C/O ratio
C/O = 0.50
C/O = 0.98
C/O = 1.2
Effect on the spectrum
C/O > 1
CN
C/O < 1
Li I
Ca I
Typical spectra of C-rich AGB stars
Barnbaum et al. (1996)
The C/O ratio determines types of condensates
C/O < 1, oxides & silicates
C/O>1, carbides, carbon, etc
SiC grains
Chemical analysis:
Atmosphere models:
- cool: molecular & dust opacities
- extended/thin atmospheres: sphericity, N-LTE effects
- variable stars: shock waves, dynamic atmosphere (Doppler shifts,
asymmetries, broadening...)
Uncertain stellar parameters: Teff, gravity, turbulence…
Crowded spectra: Accurate atomic & molecular spectroscopic data
is needed
U[X/H] ~ ± 0.3 - 0.5 dex (no systematic)
relative analysis, [X/Y], preferred !!
s-elements: evidence 99Tc (τ1/2~ 2 x 105 yr)
by Merrill (1952)
Tc in AGB stars in the galactic bulge, Uttenthaler et al. (2007)
Observations vs. theory: The neutron sources
Iben (1974), Truran & Iben (1977))
Straniero et al. (1995), Gallino et al. (1998)
13C(α,n)16O
22Ne(α,n)25Mg
Type: primary
Type: secondary &
primary
When: interpulse, T6 > 90
When: TP, T6 > 300
Where: intershell zone in
1- 8 M, < 3 M
Where: bottom of TP
in > 4 M
Nn: 106-107 cm-3
Nn: up to 1011 cm-3
more available n per seed nuclei (Fe), [s/Fe] ∝ 1/Z
22Ne(α,n)25Mg
branchings in the s-process path are opened,
excesses 25,26Mg...
13C(α,n)16O
Ratios between heavy-s (hs: Ba,La,Ce…) and light-s (ls: Sr,Y,Zr..)
elements will depend on the initial stellar metallicity
Travaglio et al. (2004)
The [hs/ls] ratio is an intrinsic index of the s-process
[hs/ls] vs. [Fe/H] in galactic s-enhanced giants (extrinsic AGBs)
A spread in the amount
of 13C burnt seems to exist ?
Busso et al. (2001) & Gallino et al. (1998)
[hs/ls] vs. [Fe/H] (intrinsic AGBs)
ST*2
ST
Extragalactic AGB C-stars:
SMC & LMC, Sagittarius,
& Carina
2 M, phys & chem. fully
coupled after 10th TP
Cristallo et al. (2009)
ST/12
Galactic AGB C-stars
The s-process branchings reveal the astrophysical scenarios
85Rb,
σ = 240 mb (30 keV)
87Rb,
σ = 15 mb (30 keV)
Nn < 108 cm-3, radiative 13C(α,n)
Nn>> 108 cm-3, convective 22Ne(α,n)
low [Rb/Sr,Y,Zr]
high [Rb/Sr,Y,Zr]
1.5 M C/O > 1, post-process, Gallino et al. (1998)
Extragalactic AGB C-stars:
SMC, Sagittarius & Carina
Galactic
AGB
AGB C-stars are of low mass, < 3 M
5 M
Some galactic, SMC & LMC O-rich (massive) AGB stars show
huge Rb enhancements with almost no s-elements !
(García-Hernandez et al. 2006-09, & Plez et al. 1993)
What is the role of the 22Ne(α,n)25Mg source?
s-process models in M > 4 M AGB stars are required !!
Fluorine
Renda et al. (2004)
- Neutrino spallation in SN II ?
- Wolf-Rayet stars ?
- AGB stars
O-rich
C-rich
Jorissen et al. (1992)‫‏‬

M MS S SC C(N)
- 19F abundances up to 30-50 solar
- [F/O] vs. C/O correlation
production in TP-AGB stars
19F
nuclear chain in He-burning conditions in TP-AGB stars:
α, p, n, 15N, 18O are needed: n from
13C(α,n)16O
19F
has primary & secondary origins (13C): at low metallicity
primary source dominates large [F/Fe] are expected
Simultaneous 19F & s-element production (?)
...After up-dated s-element abundances in AGB C-stars (Abia et al. 2002)
z
{
C-rich AGBs
O-rich AGBs
1.5, 2 M, Z=0.006 TP-AGB
FRANEC code
- Spec. synthesis with the state-of-the-art C-rich atmosphere models
- Up-dated atomic and molecular (C2, CN, CO, CH) line lists
- HF R9 line as the main F indicator
TX Psc
Teff= 3100 K, [Fe/H]= -0.4,
C/O = 1.12, 12C/13C = 42
16O/17O =1240, 16O/18O =1450
[F/Fe] = 0.5 ± 0.1
[F/O] = 0.2
[F/Fe] ~0.7 dex lower than JSL!!
Intrinsic AGB stars
z
z
z
C(N)-stars
SC-stars
M,MS,S stars
1.5, 2, 3 M TP-AGB
Z = 0.006
z
FRANEC code
JSL
Lithium
One of the corner-stones in modern Cosmology
~ 20% of the observed Li produced during the BBN
Li monitors physics during all the stellar evolution phases
depleted by (p,α) reactions T> 2.5 x 106 K
Convection, mixing, mass-loss etc.
Li in main-sequence stars vs. [Fe/H]
BBN
Z << Zsun
Z ~ Zsun
... > 50% of the observed Li has been produced during the
Galactic evolution
Where?
Spallation reactions in ISM, p,α
CNO, ∼10%
Gravitational supernovae, ν-process ?
Flares in nova outbursts ?
AGB stars, ∼ 40-50%
ε(Li) ~ -0.5
ε(Li) ~ 5.0
230 C-stars
+
=
Cameron & Fowler’s (1971) mechanism...
7Be(e-,ν)7Li
Convective envelope
m (r)
τmixing < τ1/2 (7Be + e-)
TBCE> 3 x 107 K
_____________________________
H burning shell
He shell
CO core
...works for M > 4 M
Luminosity & Li evolution in massive AGB stars
® Ventura et al.
AGB time
Consequences:
Li-rich AGB stars should be O-rich, C/O < 1
Should show low 12C/13C (<10), and N-rich
Should be luminuous, Mbol < -5.5
OK.....however:
Massive AGBs do not contribute (much) to the galactic Li
Some Li-rich AGBs have low luminosity, Mbol > -5
in addition...
Li-rich C-stars exits.... –5.0 < Mbol < -3.5
Many of them also show low 12C/13C ratios
...these features are not explained by standard
AGB stellar models !!
Li abundances vs. 12C/13C in AGB C-stars
Expected in AGB stars
Non-standard mixing/burning mechanism(s)
is (are) required in the AGB phase !
The same conclusion is reached from isotopic
anomalies (O,N,Mg,Al...) found in different
types of dust grains probably formed in the
circumstellar envelopes of AGB
Why to care about AGBs ?
1. Final phase of the evolution of stars with M < 8 M
the Majority !!
B PNe B WDs B Novae/Thermonuclear SNe
2. 75% to the total mass return from Ø to the ISM
3. Elements Beyond the Fe peak (A > 85)
„ slow neutron captures (s-process)
4. Half of all the observed 12C, (?) at least 30% !!
5. 7Li (∼ 50%)
6. Most extrasolar grains recovered in meteorites
B Pieces of AGB stars in terrestial laboratories !!
7. Contamination of the protosolar nebula right before
its collapse by a local source B AGB or SN ??
26Al
36Cl 41Ca 60Fe 107Pd
(radiactivities)
Li abundances & evolutionary status ?
black symbols Li-rich stars
The Li enhancement seems to occur preferabily during the e-AGB