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“Why are massive O-rich AGB stars in our Galaxy not S-stars?”

“Why are massive O-rich AGB stars in our Galaxy not S-stars?”. D. A. García-Hernández (IDC-ESAC, Madrid, Spain) In collaboration with P. García-Lario (IDC-ESAC), B. Plez (GRAAL, France), A. Manchado (IAC, Spain), F. D’Antona (OAR, Italy), J. Lub & H. Habing (Sterrewacht Leiden, The Netherlands).

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“Why are massive O-rich AGB stars in our Galaxy not S-stars?”

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  1. “Why are massive O-rich AGB stars in our Galaxy not S-stars?” D. A. García-Hernández (IDC-ESAC, Madrid, Spain) In collaboration with P. García-Lario (IDC-ESAC), B. Plez (GRAAL, France), A. Manchado (IAC, Spain), F. D’Antona (OAR, Italy), J. Lub & H. Habing (Sterrewacht Leiden, The Netherlands) Gdansk, June 29 2005

  2. 1 AGB stellar nucleosynthesis • Main processes during the Thermal Pulsing phase 12C, s-elementproduction (Rb, Zr, Ba, Tc, Nd, etc.) (3rd dredge-up) • 3rd dredge-up increases C/O ratio forming M-, MS-, S-, SC-, C-type stars Hot Bottom Burning (M>4 M) • When Tbce 2.107 K  12C  13C, 14N (CN-cycle) and HBB prevents the carbon star formation • 7Li production and low 12C/13C ratios (Sackman & Boothroyd 1992; Mazzitelli et al. 1999 ) D.A. García-Hernández

  3. 2 Previous works (MCs) • HBB activation in massive AGB stars in the Magellanic Clouds(MCs) (e.g. Smith & Lambert 1989; Plez et al. 1993; Smith et al. 1995) • Characteristics:7  Mbol  6 ( M~ 4  8 M ) log (Li) ( ~ 1  4 dex) C/O < 1 ( ~ 0.5 ) 12C/ 13C  ( < 10 ) (s-process)  ( [s/Fe] > 0.5 dex) D.A. García-Hernández

  4. 3 Previous works (Milky Way) • Li-rich AGBs not so luminous (6  Mbol  3.5) •  S-, SC-, C-type stars with low mass (e.g. Abia & Isern 96, 97, 00; Abia & Wallerstein 98)  not yet well understood!! • HBB models predicts log (Li) in massive (M>4 M) O-rich AGB stars (e.g. Mazzitelli et al. 99) • Galactic candidates: OH/IR stars (L, C/O<1, Long Period Variables)  Optical observations very difficult due to strong mass-loss (~ 104  106 M/yr)  at present (Li) and (s-process) are unknown! D.A. García-Hernández

  5. 4 Massive Galactic O-rich AGBs Selection of the sample (102 OH/IR stars): • Long Period Variables (P ~ 300  1000 days) • Large amplitude variability (8  10 mag in V) • Late-type stars (>M5) • OH maser emission emitters (Vexp(OH) < 25 km s-1) • Comparison stars plus 9 C-rich stars (18 objects) • Members of the galactic disk population with strong IR excesses detected by IRAS D.A. García-Hernández

  6. 5 Observations and data reduction • Echelle spectra with UES (WHT, La Palma) and CASPEC (ESO 3.6m) in 1996-1997 at R ~ 50,000 (4 runs) • Spectral range: ~ 5000  9000 Å. We were mainly interested in the Li I6708 Å region • Exposures times of ~ 10 30 minutes with S/N>100 in the Li I region • Data reduction with the ECHELLE software package in IRAF D.A. García-Hernández

  7. 6 UES echelle spectra “Blue example” “Red example” D.A. García-Hernández

  8. 7 Overview • 25 stars detected in the Li I6708 Å line • 32 stars non-detected in the Li I6708 Å line • 45 stars too red at 6708 Å or without OPC • Extremely red spectra dominated by TiO bands • Absence of molecular bands of ZrO (YO, LaO, etc.) • From Vdoppler: Li I, Ca I, TiO (stellar) are formed deeper than K I, Rb I (very probably of circumstellar origin) • Some stars also show H emission (shock-waves) ZrO 6474 Å region Li I 6708 Å region D.A. García-Hernández

  9. 8 Progenitor masses • Period and Vexp(OH) as distance-independent mass indicators (e.g. Chen 2001; Jiménez-Esteban 2004) • Different sources (masses) depending on P and Vexp(OH) IRAS Galaxy D.A. García-Hernández

  10. 9 Chemical analysis • Classical model atmospheres (HE, LTE, etc.) for cool stars (MARCS) and the “TURBOSPECTRUM” spectral synthesis code (Plez et al. 1992) • TiO, ZrO are included and atomic lines from VALD-2 • The whole machinery was tested on the high resolution spectrum of the Sun and Arcturus • Spectral regions of interest (~60 Å): Li I 6708 Å ZrO 6474 Å K I 7699 Å; Rb I 7800 Å D.A. García-Hernández

  11. 10 Overall strategy • Initial range for Teff and log gfrom the VK photometry • Further constraints on the set of stellar parameters (M, Teff, C/O, log g, , z, (Zr), CNO, 12C/13C)using spectral synthesis Model vs. observations M=2 M C/O=0.5 log g=0.5 =3 km s-1 (z, CNO,12C/13C)  Teff, FWHM Li and Zr (s-elements) chemical abundances ( log (Li), log (Zr) ) 2 test   fixed parameters D.A. García-Hernández

  12. 11 Best fit in the Li I region Teff=3000 K, log (Li)=+1.3 are needed to fit the observations! Zoom D.A. García-Hernández

  13. 12 Best fit in the ZrO 6474 Å region [Zr/Fe]=+0.0 is needed to fit the observations! Comparison with a galactic S-star D.A. García-Hernández

  14. 13 IRAS 10436: a galactic S-star [Zr/Fe]=+1.0 is needed to fit the observations! D.A. García-Hernández

  15. 14 Li and Zr abundances • Li detected stars show log (Li)~ 1  3 dex • Li non-detected stars show log (Li) < 0.0 dex • Uncertainty of log (Li) ~ 0.4  0.6 dex (sensitivity to the atmosphere parameters) • All stars show upper limits to the Zr abundance consistent with no s-element overabundance [Zr/Fe] < 0.0  0.25 dex for Teff > 3000 K [Zr/Fe] < 0.25  0.5 dex for Teff < 3000 K D.A. García-Hernández

  16. 15 P and Vexp(OH)vs. HBB No clear correlation between log (Li) and P, Vexp(OH) But no Li-rich stars with P < 400 days and Vexp(OH) < 6 km s-1 Half of the stars with higher P and Vexp(OH) are Li-rich D.A. García-Hernández

  17. 16 Theory vs. observations - Stars with P<400 days and Vexp(OH)<6 km s-1are non-HBB stars (3 M < M < 4 M)  non Li-rich - Stars with higher P and Vexp(OH) are HBB stars (M > 4 M)  Li-richbut why only half of them are Li-rich? - Both type of stars experience strong mass loss and only a few thermal pulses (and less efficient because of the high metallicity)  no s-process enhancement - The obscured stars must also be HBB stars and they represent the more massive AGB stars in the Galaxy  This scenario is consistent with the strong IR excess detected by IRAS and the HBB and nucleosynthesis model predictions! D.A. García-Hernández

  18. 17 Galaxy vs. Magellanic Clouds • Massive O-rich AGB stars in the MCs are S-stars and ~80 % of them are also Li-rich  HBB stars • Why are these stars s-element enriched? Metallicity effect! - Theoretical models predict a higher efficiency of the dredge-upin low metallicity environments (e.g. Busso et al. 1988; 2001; Straniero et al. 1995; 2000; Lugaro et al. 2003; Herwig 2004) - Lower metallicity lower dust production (van Loon 00)  less efficient mass loss  longer AGB lifetime in the MCs compared to the Galaxy! D.A. García-Hernández

  19. 18 Conclusions - 25 stars detected in the Li I 6708 Å line, 32 stars non-detected and 45 stars too red (or no OPC) • The chemical analysis revealed that half of the stars with useful optical spectra are Li-enriched  HBB - All stars in the sample are considerably massive (M > 3 M) but only the more massive ones (M > 4 M) experience HBB. The lack of lithium in some HBB stars is a consequence of the timescale of the Li production phase (~104 years) D.A. García-Hernández

  20. 19 Conclusions • As a consequence of the different metallicity, massive galactic O-rich AGB stars are not s-process enriched in strongcontrast to Magellanic Cloud massive AGB stars  Observational evidence that the chemical evolution during the AGB is strongly modulated by the metallicity!! • Need of extending the analysis to other Galaxies in the Local Group with a wide variety of metallicities D.A. García-Hernández

  21. Li I 6708 Å region D.A. García-Hernández

  22. ZrO 6474 Å region D.A. García-Hernández

  23. IRAS vs. P and Vexp D.A. García-Hernández

  24. Galactic latitude vs. Vexp(OH) D.A. García-Hernández

  25. Zoom in the Li I region log (Li)=+1.3 is needed to fit the observations! D.A. García-Hernández

  26. Other possible hypotheses? • Are they more massive stars (M > 4 M)? - This is not consistent with the non-detection of Li in any of them! • Are they lower mass stars (M < 1.5 M)? - This is not consistent with the lack of s-process elements. Other low-mass stars of S- and C-type show strong s-process element enrichment - A early stage as AGB stars is also not consistentwith the strong IR excess observed by IRAS D.A. García-Hernández

  27. Timescale of the Li production HBB models(Mazzitelli et al. 1999) explain the lack of lithium in half of the massive O-rich AGB stars where the HBB is active! The Li-rich phase is of the order of the interpulse time (~104 years)! D.A. García-Hernández

  28. Li production at low metallicity HBB models(Mazzitelli et al. 1999) explain the higher detection rate of Li-rich stars in the MCs because they predict a lower mass limit of only 3.03.7 M (in the LMC) for the HBB activation and a faster lithium production D.A. García-Hernández

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