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Light Elements (Li and Be) in Globular Clusters

Light Elements (Li and Be) in Globular Clusters. L. Pasquini , ESO. Li in Turn-Off (or close to TO) Li in evolved stars A new result.. Be in TO stars. Li Observations in GC: a summary. Li in (close to) Turn-Off Stars.

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Light Elements (Li and Be) in Globular Clusters

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  1. Light Elements (Li and Be) in Globular Clusters L. Pasquini , ESO • Li in Turn-Off (or close to TO) • Li in evolved stars • A new result.. • Be in TO stars

  2. Li Observations in GC: a summary

  3. Li in (close to) Turn-Off Stars Early Studies: Is the Li Plateau Primordial ? At which abundance level ? Two different ‘views’ : NGC 6397 [Fe/H]=-2.0 : Li Constant, Plateau (MP94, PM96) 47 Tuc [Fe/H]=-0.7 : Li Compatible with Plateau (PM97) (same Li in 1 CN rich and 1 CN-poor star within uncertainties ) M92 [Fe/H]=-2.3?: Li variable, 1 star higher than Plateau (BDSK99) : Statistically significant? (Bonifacio 02) BUT: Is this really the ‘right’ question (in 2004)?

  4. Li in (close to) Turn-Off Stars Globular Clusters are known to have abundance anomalies! Is perhaps this the critical issue? T00, B02, find an impressive constancy and similarity to Plateau in the UVES spectra of NGC6397 stars with different [O/Fe] Ratio. Li/H=2.34  0.056 (Intrinsic scatter <0.035) Li/H = 2.32  0.014+-0.05 for ‘Plateau’ stars.

  5. Li in (close to) Turn-Off Stars M92 show as well anomalous abundances: K99 finds that M92 stars show Mg and Na anomalous abundances as If the gas was processed through Mg-Al and Ne-Na cycling. BUT Li is “too high” for being polluted… it should have been destroyed! Li vs. Teff for the M92 Stars (B02 analysis): Li/H=2.36  0.19

  6. Li in (close to) Turn-Off Stars FACTS SEEM SIMILAR BUT CONTRADDICTORY: Both metal poor clusters (NGC6397 and M92): Show abundance differences typical of “CNO” processing Show Li abundances either ‘constant’, or close to, compatible with ‘Plateau’. How is this possible ? Ventura et al.: previous AGB processing , Li AGB production and Cameron-Fowler process+ Mass Losses BUT: Fine Tuning Is required !

  7. Li in Evolved Stars Most works were devoted to the search of Li-rich giants. But Li-Rich giants seems to be rare in GC: Pilachowski et al. 2000, for instance, do not find any Li-Rich giant out of the 261 sampled in 4 clusters (20 mA limit). A systematic approach was followed by Castilho et al on NGC6397 (16 stars) and by Grundhal et al on NGC6752 (21 stars): they both follow sistematically the stars from the SGB to the RGB.

  8. Li in Evolved Stars The Grundahl et al. Data on NGC6752 results are quite impressive: while a mixed situation is present for stars below the RGB bump, stars which are more luminous only present upper limits. M71 has a similar behaviour (Ramirez et al. 02) M71 data from Ramirez et al 02 Mixing seems secured after the ‘bump’, as expected from theory. What about the spread for fainter NGC6752 stars?

  9. Li-Na anticorrelation at the TO! New results from the NGC6752 spectra ESO Large Programme (P.I. Gratton): 9 TO stars, high S/N ratio UVES spectra (Pasquini et al. 2005) Qualitative agreement with AGB predictions! quantitative TBD Differences NGC6397 ?? Same mechanism for all clusters ??

  10. Be in TO of NGC6397 2 NGC6397 TO stars of the Thevenin et al sample. V~16 at the TO. 10 hours/star at the VLT+UVES R=40000 , S/N 8.5 and 15 respectively. O Triplet Log Be/H = -12.35  0.2

  11. Be in TO of NGC6397 Oxygen is low and quite different between the stars: almost 0.6 Dex. Oxygen in NGC6397 is also found to vary in subgiants by Carretta et al. Nitrogen in the NGC6397 stars is high (N/H~ - 0.74) with respect to field stars.

  12. Be in TO of NGC6397 Be can be used as cosmochronometer, NGC6397 formed within ~0.5 Gyr from the onset of star formation in the Galaxy. Chemical evolution Model from Valle et al. 02 Age of 13.4 +- 0.8 from Gratton et al. 03

  13. Be in TO of NGC6397 Unlike Li, Be cannot be produced by the Fowler-Cameron mechanism. The presence of Be points out that the gas must have been ~200 My exposed to the GCR before forming the stars we observed now. Li and Be further exclude ‘internal mixing’ as cause of chemical anomalies. Since Be can only be destroyed but not produced during AGB phase, its presence seems incompatible with this hypothesis. AGB can still be retained only if processing occurs ‘immediately’ and the processed gas is ‘immediately’ released to the ISM. (tuning again?) We looked at some alternative formation scenario, but we did not find a satisfactory one which explains the whole abundance patterns and does not require fine tuning.

  14. Li and Be in Globular Clusters Light elements, such as Li and Be, contains unique information, essential to test predictions of Globular Clusters formation and mixing theory.

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