Sodium production in asymptotic giant branch stars

Abstract

[condensed] A new scenario is presented for the production of Na23 in asymptotic giant branch (AGB) stars. The scenario takes advantage of the periodic third dredge-up episodes characterizing those stars, which mix primary C12 from their intershell layers to their surface. Two successive interpulse/pulse/dredge-up sequences are then required to produce Na23. During the first sequence carbon and oxygen are converted into N14 by the hydrogen burning shell, and subsequently transformed into Ne22 by the helium burning shell. During the second sequence, Ne22 is converted into Na23 by the hydrogen burning shell, which is brought to the surface by the subsequent dredge-up episode. The Na23 produced by this scenario is thus primary. The efficiency of this scenario is analyzed through standard evolutionary AGB model predictions combined with synthetic calculations for the surface chemical evolution. It is shown that primary Na23 can efficiently be produced as soon as the surface C+N+O abundance enhancement reaches a certain level depending on the stellar metallicity. The required surface C+N+O abundance enhancement amounts to about 0.4 dex in solar metallicity stars, and to about 0.8 dex at a metallicity five times less than solar. ...