presolar grains and agb stars maria lugaro sterrenkundig instituut university of utrecht
TRANSCRIPT
Presolar grains
and
AGB stars
Maria LugaroSterrenkundig Instituut
University of Utrecht
Outline of the talk
1. Intro to asymptotic giant branch (AGB) stars2. Information from presolar grains on AGB stars 3. Examples:
A. The s processB. Presolar grains from massive AGB stars?C. The “isotopic evolution” of the Galaxy
4. Summary and future opportunities
Courtesy ofRichard Powell
AGB stars
Theoretical evolutionary
track of a star of 2 M
All stars with masses 1 - 7 M go through the
AGB phase
Core H exhaustion
Core He burning starts
Core He exhaustion
1. Intro to AGB stars
Schematic out-of-scale picture of the structure of AGB stars.
1. Introduction to AGB stars
is activated most of the time
triggers convection in the He intershell
1. Introduction to AGB stars
4He, 12C, 22Ne, elements heavier than Fe produced by slow neutron captures (the s process): Zr, Ba, ...
At the stellar
surface: C>O, s-process enhancements
Time evolution of the structure of AGB stars.
Consider the main
ingredient to constructing theoretical AGB stars:
Consider presolar grains:
Silicon Carbide grains: 95% show the signature of AGB star origin
Oxide and Silicate grains: a large fraction of them are believed to be from AGB stars
The vast majority of presolar grains analyzed to date come from AGB stars!
2. Information from presolar grains on AGB stars
Light elements, e.g.: C, N,
O, Ne, Mg, Al
Intermediate-mass elements, e.g.: Si, Ca, Ti,
Cr, Fe, Ni
Heavy elements,
e.g.: Sr, Zr, Mo, Ba
Very precise isotopic ratios
2. Information from presolar grains on AGB stars
Light elements, e.g.: C, N, O, Ne, Al
Intermediate-mass elements, e.g.: Si, Ca, Ti,
Cr, Fe, Ni
Heavy elements,
e.g.: Sr, Zr, Mo, Ba
Nuclear reactions + mixing in AGB stars
Chemical evolution
of the Galaxy
Processes in binary systems
2. Information from presolar grains on AGB stars
3. Examples A. The s process
3. Examples A. The s process
proton diffusion
13Cn)16O
22Nen)25Mg
Where are the neutrons in the AGB intershell?
3. Examples A. The s process
Single star models showed that a large spread of 13C amounts at any given [Fe/H] was needed
to cover spectroscopic observations: Busso et al. (2001) use a spread of a factor of ~ 50.
Stellar population synthesis including the s process shows that a small spread is needed. Bonacic-Marinovic et al. (2006)
use a spread of 2 See poster.
3. Examples A. The s process
From analysis of more than one element in the same
presolar SiC grain, Barzyk et al. (2006) independently
find the same spread of 2 as population synthesis models.
Lugaro et al. (2003) used a spread of a factor of 24 to cover single presolar SiC
grain data.
Light elements, e.g.: C, N,
O, Ne, Mg, Al
Intermediate-mass elements, e.g.: Si, Ca, Ti,
Cr, Fe, Ni
Heavy elements,
e.g.: Sr, Zr, Mo, Ba
Nuclear reactions + mixing in AGB stars
Chemical evolution
of the Galaxy
Processes in binary systems
3. Examples B. Presolar grains from massive AGBs?
Presolar spinel grain OC2 is unique in that it shows large excesses in the heavy Mg
isotopes...
...and very low 18O/16O.
The origin of grain OC2 has been tentatively attributed to a massive AGB star ≈ 4 - 7 M
+117% of solar
+43% of solar
3.3 solar
solar/26
3. Examples B. Presolar grains from massive AGBs?
64
90
87
81
3. Examples B. Presolar grains from massive AGBs?
Lugaro et al. (2006) compare OC2 to detailed models of massive AGBs.
Proton captures occur at the base of the convective envelope: hot bottom
burning.
Within this solution we predict: a 17O(p,)14N rate close to its current upper limit (+25%)
and a 16O(p,)17F rate close to its current lower limit (-43%)
Light elements, e.g.: C, N,
O, Ne, Mg, Al
Intermediate-mass elements, e.g.: Si, Ca, Ti,
Cr, Fe, Ni
Heavy elements,
e.g.: Sr, Zr, Mo, Ba
Nuclear reactions + mixing in AGB stars
Chemical evolution
of the Galaxy
Processes in binary systems
3. Examples C. The “isotopic evolution” of the Galaxy
The Si composition of different SiC populations is determined by:
2. Neutron captures in the AGB parent star.
1. The initial composition of the parent star
produced by Galactic chemical evolution
effects, which are still very uncertaint.
3. Examples C. The “isotopic evolution” of the Galaxy
Zinner et al. (2006) combine SiC data and theoretical predictions for nucleosynthesis in AGB stars to obtain
information on the Galactic evolution of the Si isotopes.
3. Examples C. The “isotopic evolution” of the Galaxy
“At Z < 0.01 the 29Si/28Si ratio rises much faster than predicted by the model of Timmes & Clayton (1996). The grain data
suggest a low-metallicity source of 29Si and 30Si not cosidered in the present Galactic chemical evolution models.”
... or something wrong with the models???
Light elements, e.g.: C, N,
O, Ne, Mg, Al
Intermediate-mass elements, e.g.: Si, Ca, Ti,
Cr, Fe, Ni
Heavy elements,
e.g.: Sr, Zr, Mo, Ba
Nuclear reactions + mixing in AGB stars
Chemical evolution
of the Galaxy
Perform detailed computations of the “isotopic evolution” of the
Galaxy.
Test the modelling of AGB stars.
Test nuclear reaction rates.
There are not yet models of the
composition of AGB stars with a compact binary
companion.
Processes in binary systems
4. Summary and future opportunities