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“ Analysis and interpretation of stellar spectra and nucleosynthesis processes in evolved stars ”

“ Analysis and interpretation of stellar spectra and nucleosynthesis processes in evolved stars ”. D. A. García-Hernández (IAC Support Astronomer). Instituto de Astrofísica de Canarias, Seminar presentation of IAC postdocs, December 13 2007. Main lines of research.

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“ Analysis and interpretation of stellar spectra and nucleosynthesis processes in evolved stars ”

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  1. “Analysis and interpretation of stellar spectra andnucleosynthesis processes in evolved stars” D. A. García-Hernández (IAC Support Astronomer) Instituto de Astrofísica de Canarias, Seminar presentation of IAC postdocs, December 13 2007

  2. Main lines of research • Chemical abundances of AGB stars and the role of AGB stars in the Early Solar System composition • Physical study of transition objects between the AGB phase and the Planetary Nebula stage and the spectral analysis (e.g. by using ISO and Spitzer) of their circumstellar dust shells • CNO isotopic abundances of hydrogen-deficient carbon stars (R Coronae Borealis and HdC stars)

  3. Stellar evolution: AGB stars Asymptotic Giant Branch: late stage of evolution of low- to intermediate-mass stars (1  M  8 M) TP phase: strong mass loss enriches the ISM with radionuclides and circumstellar dust grains!

  4. AGB internal structure

  5. AGB stellar nucleosynthesis • Thermal Pulsing phase 12C production, s-element production (Rb, Zr, Sr, Nd, Ba, Tc, etc.) • 3rd dredge-upvery efficient in AGB stars; C/O ratio increases • Stars eventually turn C-rich and s-process rich following the M-, MS-, S-, SC-, C-type sequence unless… • Hot Bottom Burning (if M > 45M) • WhenTbce 2.107 K 12C  13C, 14N (CN-cycle) andHBB prevents the carbon enrichment (stars remain O-rich) • 26Al, 7Liproduction,low 12C/13C& high 17O/16O ratios (Mazzitelli et al. 99; Karakas & Lattanzio 03 )

  6. The s-process in AGB stars • Free neutrons to form heavier elements (s-elements such as Rb, Zr, Sr, etc.) can be released by13C(,n) 16O or by22Ne (,n)25Mg reactions (Busso et al. 99) • 13C operates during the interpulse period. It is more efficient in 13 M AGB stars.All previous observations of AGB stars are consistent with 13C! • 22Ne is expected to be efficient in the convective thermal pulse at higher T and Nn. It should become strongly activated in more massive AGB stars (M>45 M).This prediction has never been confirmed by the observations!

  7. The 22Ne neutron source • The operation of the 22Ne neutron source favors the production of the stable isotope 87Rb (also of 60Fe, 41Ca, 96Zr, 25Mg, 26Mg, etc.) because of the operation of a branching in the s-process path at 85Kr (Beer & Macklin 89) [Rb/Zr] is a powerful neutron density (and mass) indicator in AGBs! 87Rb is a radioactive isotope (half-life time of ~48.8 Gyr ) and it is frequently used to date moon rocks and meteorites

  8. Massive Galactic O-rich AGBs • Where are they in our Galaxy? • Theoretical models predictsRb/Zr inmassive (M>45 M) O-rich AGBstars • Galactic candidates:OH/IRstars (L, C/O<1, Long Period Variables). Expected to be massive O-rich stars in the final stages of their AGB evolution • Optical observations very difficult due to strong mass-loss (~ 104  106 M/yr) their chemical composition (Rb, Zr, etc.) is unknown! Very red stars! Extremely variable stars!

  9. Discovery of Rb-rich AGB stars • We observed ~100 OH/IR stars in the optical range during 4 observational campaings in La Palma (Spain) and La Silla (Chile) • We obtained good high-resolution optical echelle spectra for half of the sample • The other~50 stars were completely invisible! • We found that these stars are Rb-rich but Zr-poor  22Neconfirmation as predicted theoretically 40 years ago! • We confirmed for the first time that the [Rb/Zr] ratio can be used as a mass indicator in AGB stars!

  10. Rb-rich AGB stars • OH/IR stars strong mass loss  source of dust • Li-rich HBB  26Al, 13C and 17O producers • Rb-rich but Zr-poor  22Ne important source of 87Rb, 60Fe, 41Ca (but also of96Zr, 25Mg, 26Mg, etc.) • The more extreme stars are not predicted by the current models which do not consider the higher mass stars neither the strong mass loss! • These stars, if present at the ESS, are more important at the early stages a highly variable Rb/Sr ratio and 87Rb/87Sr ages should be taken with caution • (García-Hernández et al. 06, 07)

  11. ESS radionuclides inventory • SN scenario may explain 60Fe but not other radionuclides such as 26Al, 41Ca, 107Pd • Low-mass AGBsreproduce the other radionuclidesbut do not explain 60Fe. 22Ne is needed! (Wasserburg et al.06) • Both models do not completely explain many of the radionuclide ESS concentrations. In particular, they cannot explain the 87Rb anomalies detected in CAIs • We cannot discard SN and low-mass AGBs in the ESS, but massive AGBs probably also played an important role, as evidenced by important Rb/Sr variations in CAIs (Podosek et al. 91; McKeegan and Davis 03) !

  12. CAIs evidence • 87Rb anomalies are present in CAIs (as deduced from 87Sr/86Sr variations)(e.g. Podosek et al. 91) • 41Ca and 26Al (also 25Mg, 26Mg) also present • CAIs display important 60Fe concentrations and it has been found that 60Fe excesses are correlated with 96Zr 22Ne!(Quitté et al. 07) • It is a mere coincidence that CAIs show all the chemical anomalies expected in massive AGB stars? New AGB stellar nucleosynthesis models explain these anomalies, giving a self-consistent solution to the ESS radionuclides (Trigo-Rodríguez et al. 08, submitted to MAPS)

  13. CS dust shells AGB-PN • The dust sequence from AGB to PNe as seen by ISO - A massive O-rich star - A very low-mass O-rich star • Spitzer/IRS survey of heavily obscured PN precursors - Characterization of the IR spectral properties - Study of the total obscuration phase

  14. O-rich transition sources O-rich PN precursors where the transition from amorphous to crystalline dust structure is taking place The crystalline silicate features are detected in emission inside the amorphous silicate absorptions at 9.7 & 18 m

  15. Young IR PNe IR PNe as indicated by the detection of nebular emission lines (e.g. [Ar II], [Ar III], [Ne II], [S III]) O-rich PNe showing amorphous and crystalline silicates  probably massive PNe

  16. R CrB and HdC stars • Hydrogen-deficient (~105) luminous stars • Their origins have remained a puzzle for decades • Two scenarios have survived theoretical and observational scrutiny: - DD scenario (merger of a He and C-O WD) - FF scenario (final pAGB He flash in a CSPN) • CNO isotopic abundances are a powerful tool for discriminating between the DD and FF scenarios

  17. CNO isotopic abundances Gemini/PHOENIX near-IR (R~50,000) spectra of RCB & HdC stars 12C/13C, 14N/15N 16O/17O/18O ratios can be derived from CO & CN transitions in the K-band (2.3m)

  18. 18O-rich stars • HdC stars and some RCBs are strongly enriched in 18O (16O/18O~0.24) but C and N are in the form of 12C and 14N, respectively • This suggests that HdC and RCB stars are related objects and that they probably formed from a WD merger. FF scenario cannot produce 18O-rich stars • Calculations of the nucleosynthesis achieved during the merger in the DD scenario need to be developed! (merger process in a few days with acretion rates of 150 Myr-1, Clayton et al. 07)

  19. A very red massive O-rich AGB Visual Red Infrared

  20. Extremely variable stars Visually Bright! Not found!

  21. Optical echelle spectra “Blue example” “Red example”

  22. Teff=3000 K [Rb/Fe]=+0.1 [Rb/Fe]=+0.9 [Rb/Fe]=+1.6 [Rb/Fe]=+2.3 A wide variety ofRb abundances areneeded to fit the observations! First detection of strong Rubidium overabundances in massive AGB stars!

  23. Rb abundances vs. Vexp(OH) Vexp(OH) can be taken as a distance-independent mass indicator in OH/IR stars (e.g. Jiménez-Esteban et al 05) The more extreme stars are not predicted by the current models which do not consider the higher mass stars neither the strong mass loss! [Rb/Zr] >[Rb/Fe]  0.5 in these stars

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