Presolar grains and AGB stars

1. Presolar grains and AGB stars Maria Lugaro Sterrenkundig Instituut University of Utrecht

2. Outline of the talk • Intro to asymptotic giant branch (AGB) stars • Information from presolar grains on AGB stars • Examples: • The s process • Presolar grains from massive AGB stars? • The “isotopic evolution” of the Galaxy • Summary and future opportunities

3. 1. Intro to AGB stars Core H exhaustion Core He burning starts Core He exhaustion AGB stars Theoretical evolutionary track of a star of 2 M All stars with masses 1 - 7 M go through the AGB phase Courtesy of Richard Powell

4. 1. Introduction to AGB stars Schematic out-of-scale picture of the structure of AGB stars. DUST triggers convection in the He intershell is activated most of the time

5. 1. Introduction to AGB stars At the stellar surface: C>O, s-process enhancements Time evolution of the structure of AGB stars. 4He, 12C, 22Ne, elements heavier than Fe produced by slow neutron captures (the s process): Zr, Ba, ...

6. 2. Information from presolar grains on AGB stars Consider presolar grains: Consider the main ingredient to constructing theoretical AGB stars: 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!

7. 2. Information from presolar grains on AGB stars Light elements, e.g.: C, N, O, Ne, Mg, Al Very preciseisotopic ratios Heavy elements, e.g.: Sr, Zr, Mo, Ba Intermediate-mass elements, e.g.: Si, Ca, Ti, Cr, Fe, Ni

8. 2. Information from presolar grains on AGB stars 3. Examples A. The s process Light elements, e.g.: C, N, O, Ne, Al Nuclear reactions + mixing in AGB stars Processes in binary systems Chemical evolution of the Galaxy Heavy elements, e.g.: Sr, Zr, Mo, Ba Intermediate-mass elements, e.g.: Si, Ca, Ti, Cr, Fe, Ni

9. 3. Examples A.The s process Where are the neutrons in the AGB intershell? The main uncertainty! 13C(,n)16O proton diffusion 22Ne(,n)25Mg

10. 3. Examples A. The s process 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. 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.

11. 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.

12. 3. Examples B.Presolar grains from massive AGBs? Light elements, e.g.: C, N, O, Ne, Mg, Al Nuclear reactions + mixing in AGB stars Processes in binary systems Chemical evolution of the Galaxy Heavy elements, e.g.: Sr, Zr, Mo, Ba Intermediate-mass elements, e.g.: Si, Ca, Ti, Cr, Fe, Ni

13. 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. +43% of solar 3.3  solar +117% of solar solar/26 The origin of grain OC2 has been tentatively attributed to a massive AGB star ≈ 4 - 7 M

14. 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. 90 87 81 64 Within this solution we predict: a 17O(p,)14Nrate close to its current upper limit (+25%) and a 16O(p,)17F rate close to its current lower limit (-43%)

15. 3. Examples C. The “isotopic evolution” of the Galaxy Light elements, e.g.: C, N, O, Ne, Mg, Al Nuclear reactions + mixing in AGB stars Processes in binary systems Chemical evolution of the Galaxy Heavy elements, e.g.: Sr, Zr, Mo, Ba Intermediate-mass elements, e.g.: Si, Ca, Ti, Cr, Fe, Ni

16. 3. Examples C. The “isotopic evolution” of the Galaxy The Si composition of different SiC populations is determined by: 1. The initial composition of the parent star produced by Galactic chemical evolution effects, which are still very uncertaint. 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. 2.Neutron captures in the AGB parent star.

17. 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???

18. 4. Summary and future opportunities Light elements, e.g.: C, N, O, Ne, Mg, Al There are not yet models of the composition of AGB stars with a compact binary companion. Test the modelling of AGB stars. Test nuclear reaction rates. Nuclear reactions + mixing in AGB stars Processes in binary systems Chemical evolution of the Galaxy Heavy elements, e.g.: Sr, Zr, Mo, Ba Intermediate-mass elements, e.g.: Si, Ca, Ti, Cr, Fe, Ni Perform detailed computations of the “isotopic evolution” of the Galaxy.