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Sodium production in asymptotic giant branch stars
Received 25 May 1999 / Accepted 23 July 1999
A new scenario is presented for the production of in asymptotic giant branch (AGB) stars. The scenario takes advantage of the periodic third dredge-up episodes characterizing those stars, which mix primary from their intershell layers to their surface. Two successive interpulse/pulse/dredge-up sequences are then required to produce . During the first sequence carbon and oxygen are converted into by the hydrogen burning shell, and subsequently transformed into by the helium burning shell. During the second sequence, is converted into by the hydrogen burning shell, which is brought to the surface by the subsequent dredge-up episode. The 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 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 0.4 dex in solar metallicity stars, and to 0.8 dex at a metallicity five times less than solar.
An analytical study of Na production further reveals that the surface abundance asymptotically evolves to a `line of primary sodium enrichment' (LOPSE) in the [C+N+O]- diagram. That LOPSE represents the abundance evolution predicted in zero metallicity AGB stars experiencing third dredge-up episodes. An analytical relation for the surface abundance evolution as a function of the surface C+N+O abundance is provided.
The predicted surface enhancements can exceed 0.5 dex depending on the level of surface enrichment, and increases with decreasing stellar metallicity. The quantitative prediction of surface abundances, however, is presently subject to a high level of uncertainty, partly due to the still poor quantitative prediction of the structural evolution of AGB stars (dredge-up episodes in particular), and partly due to the uncertainties still affecting some nuclear reaction rates (such as destruction by proton capture).
The case of massive AGB stars in which hot bottom burning occurs is also discussed. The production of secondary sodium in those stars is a natural consequence of burning in their envelope, if the temperature at the base of the envelope exceeds 70 million K. It requires, however, many interpulses to be significant. The production of primary sodium from the dredge-up of primary and its subsequent burning in the envelope, on the other hand, is estimated not to be very efficient, expect maybe in low-metallicity stars.
An eventual detection of high Na overabundances in carbon stars or related objects would support the scenario of primary sodium production in AGB stars. Such an observational evidence may have been found in at least one post-AGB star. Further observations of those objects are called for. Observations of in planetary nebulae are also encouraged. Finally, the production of primary by AGB stars, if confirmed observationally, may have played a non-negligible role in the chemical evolution of our Galaxy.
Key words: stars: AGB and post-AGB stars: carbon stars: evolution stars: interiors stars: abundances
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Online publication: September 24, 1999