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. AGB stellar nucleosynthesis Main processes during the Thermal Pulsing phase  12 C, s-element production (Rb, Zr, Ba, Tc, Nd, etc.) (3 rd dredge-up) 3 rd dredge-up increases C/O ratio forming M-, MS-, S-, SC-, C-type stars Hot Bottom Burning (M>4 M  ) When T bce  2.10 7 K  12 C  13 C, 14 N (CN-cycle) and HBB prevents the carbon star formation 7 Li production and low 12 C/ 13 C ratios (Sackman & Boothroyd 1992; Mazzitelli et al. 1999 ) D.A. García-Hernández 1

3. 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  M bol   6 ( M~ 4  8 M  ) log  (Li)  ( ~ 1  4 dex) C/O < 1 ( ~ 0.5 ) 12 C/ 13 C  ( < 10 )  (s-process)  ( [s/Fe] > 0.5 dex) D.A. García-Hernández 2

4. Previous works (Milky Way) Li-rich AGBs not so luminous (  6  M bol   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 3

5. Massive Galactic O-rich AGBs Long Period Variables (P ~ 300  1000 days) Large amplitude variability (8  10 mag in V) Late-type stars (>M5) OH maser emission emitters (V exp (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 Selection of the sample (102 OH/IR stars): 4

6. 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 I 6708 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 5

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

8. Overview 25 stars detected in the Li I 6708 Å line 32 stars non-detected in the Li I 6708 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 V doppler : 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) D.A. García-Hernández Li I 6708 Å region ZrO 6474 Å region 7

9. Progenitor masses Period and V exp (OH) as distance-independent mass indicators (e.g. Chen 2001; Jiménez-Esteban 2004) Different sources (masses) depending on P and V exp (OH) D.A. García-Hernández IRAS Galaxy 8 V exp (OH) (km s -1 )ObservedLi not detectedLi detectedToo red < 61312 (92%)0 (0%)1 (8%) 6  12 256 (24%)11 (44%)8 (32%) > 125310 (19%)8 (15%)35 (66%) Period (days)ObservedLi not detectedLi detectedToo red < 4001210 (84%)1 (8%) 400  700 264 (15%)12 (46%)10 (39%) > 700181 (6%)2 (11%)15 (83%)

10. 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 9

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

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

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

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

15. 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 D.A. García-Hernández [Zr/Fe] 3000 K [Zr/Fe] < 0.25  0.5 dex for T eff < 3000 K 14

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

17. Theory vs. observations - Stars with P<400 days and V exp (OH)<6 km s -1 are non- HBB stars (3 M  < M < 4 M  )  non Li-rich - Stars with higher P and V exp (OH)  are HBB stars (M > 4 M  )  Li-rich but 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 16

18. 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-up in 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 17

19. 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 (~10 4 years) D.A. García-Hernández 18

20. Conclusions -As a consequence of the different metallicity, massive galactic O-rich AGB stars are not s-process enriched in strong contrast 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 19

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. V exp (OH) D.A. García-Hernández

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

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 consistent with the strong IR excess observed by IRAS D.A. García-Hernández

27. Timescale of the Li production D.A. García-Hernández 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 (~10 4 years)!

28. Li production at low metallicity D.A. García-Hernández 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