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