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Observational evidence

Observational evidence. The role of massive AGBs in the “self-enrichment” scenario. Stars in Globular clusters are not chemically homogeneous; star-to-star variations are present in their surface chemistries, tracing abundance patterns. Oxygen – Sodium anticorrelation in M3&M13.

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Observational evidence

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  1. Observational evidence The role of massive AGBs in the “self-enrichment” scenario Stars in Globular clusters are not chemically homogeneous; star-to-star variations are present in their surface chemistries, tracing abundance patterns

  2. Oxygen – Sodium anticorrelation in M3&M13 This trend is not observed in field stars! M3 “Standard” “Anomalous” M13

  3. Magnesium – Aluminum anticorrelation in M3&M13

  4. The Oxygen – Sodium anticorrelation in GCs stars M13 M3 NGC6752 M4 M5 NGC2808

  5. Chemical anomalies in GCs • In all GCs examined, along with a stellar component with “standard” chemistry, there is a group of stars showing anomalous surface abundances. • Sodium is anticorrelated with Oxygen, Fluorine and Magnesium, and is correlated with Aluminium. • The C+N+O sum is constant ! CNO cycling & advanced nucleosynthesis!

  6. Possible explanations a) The most straightforward interpretation is the “internal mixing” idea during the RGB phase... b) Some “self-enrichment” in the cluster …, by what kind of stars? Massive stars (M>10Msun) Intermediate mass stars (3<M/Msun<7) Fe/H is constant! Ventura et al 2001 ApJ 550 L65

  7. How is the AGB modelling dependent on the treatment of convection? FST MLT (a=2.1) MLT (a=1.7) Ventura & D’Antona 2005 A&A 431 279 M=5Msun

  8. Which effects on the surface chemistry ? The extent of HBB, shown here by oxygen depletion, is stronger in the FST case When the treatment of convection is efficient, the number of TPs is reduced, so that less 3rd dredge-up episodes are expected! In the MLT case an increase of the CNO sum is expected, contrary to FST

  9. Summary • All the massive AGBs produce great quantities of helium, due do the 2nd dredge-up. • The treatment of convection proves to be the most important ingredient determining the physical evolution during the TPs phase, much more relevant than the other uncertainties related to stellar evolution theory (mass loss, cross sections, overshooting..). • The FST model, compared to MLT, leads to shorter life-times, larger luminosities, a smaller number of TPs, and a more advanced nucleosynthesis at the bottom of the envelope. • The ejecta of the massive FST AGB models are sodium and aluminum rich, oxygen and magnesium depleted, and are thus at least in qualitative agreement with the demand of the self-enrichment scenario.

  10. Sodium-Oxygen anticorrelation Evolved stars only? 2nd generation formed from AGB winds 1st generation initial abundances D’Antona & Ventura 2007 MNRAS 379 1431 Ventura & D’Antona 2008 A&A in press

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