Does the oxygen-sodium anticorrelation in globular clusters require a lowering of the Na(p,Ne reaction rate?

INAF - Observatory of Rome, via Frascati 33, 00040 MontePorzio Catone (RM) – Italy e-mail: [ventura;dantona]@oa-roma.inaf.it

Received: 24 April 2006
Accepted: 3 July 2006

Abstract

Context.The chemical content of the ejecta of Asymptotic Giant Branch stars (AGBs) is important to understand whether intermediate mass stars played a role in the “self-enrichment scenario”, to explain the chemical anomalies observed among Globular Clusters stars. One difficulty of this model is that it is not fully consistent with the observed oxygen-sodium anticorrelation.

Aims.We look for the combinations of the input-physics parameters by which the yields of massive AGBs are sodium-rich and oxygen-poor.

Methods.Many evolutions from the early evolutionary phases until the mass of the envelope drops below ~ are calculated for a   model for various assumptions concerning the extra mixing from the bottom of the envelope and the relevant cross sections involving sodium.

Results.A modest amount of extra mixing from the bottom of the surface convective zone leads to ejecta that are moderately sodium rich and oxygen depleted. A different but appealing modelization allows us to reproduce the observations of stars with a lower oxygen content: extra mixing is not included, but the cross sections of the reaction ²³Na²⁰Ne must be a factor of ~4 lower than the recommended values. If the initial neon content of the star is not solar-scaled but enhanced as the other α elements by a factor ~2, the afore mentioned cross section must be lessened by only a factor ~2.