Sodium production in asymptotic giant branch stars

A new scenario is presented for the production of 23 Na in asymptotic giant branch (AGB) stars. The scenario takes advantage of the periodic third dredge-up episodes characteriz- ing those stars, which mix primary 12 C from their intershell layers to their surface. Two successive interpulse/pulse/dredge- up sequences are then required to produce 23 Na. During the first sequence carbon and oxygen are converted into 14 N by the hy- drogen burning shell, and subsequently transformed into 22 Ne by the helium burning shell. During the second sequence, 22 Ne is converted into 23 Na by the hydrogen burning shell, which is brought to the surface by the subsequent dredge-up episode. The 23 Na 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 23 Na can efficiently be produced as soon as the sur- face C+N+O abundance enhancement reaches a certain level de- pending on the stellar metallicity. The required surface C+N+O abundance enhancement amounts to 0.4 dex in solar metal- licity stars, and to 0.8 dex at a metallicity five times less than solar.