By applying the concept of rotationally induced mixing as it has been developed for massive stars in our group during the last years without alteration to a TP-AGB model sequence, we obtain conditions which appear favorable for the development of the s-process, i.e. a 13C-rich layer which produces a considerable neutron flux later-on. Although our model develops only a very late and weak third dredge-up we believe that the mechanism which diffuses the protons into the carbon layer and 12C into the envelope must occur with a similar magnitude in all TP-AGB stars which develop a third dredge-up. The reason is that the huge specific angular momentum jump at the hydrogen/carbon interface - five orders of magnitude in our case - is independent of the depth of the third dredge-up.
The maximum 13C abundance and its distribution in our model is, at first, similar to that found due to diffusive convective overshooting by Herwig et al. (1997). However, in our case the rotational mixing spreads the 13C peak out before the neutrons are produced (cf. Figs. 5 and 6), which is not the case in the models of Herwig et al. (1997). At the present time we can not discriminate which of these scenarios would agree better with empirical constraints. However, we want to stress that both mechanisms of 13C production, rotation and overshooting, do not exclude each other, and that it is possible that they act simultaneously in AGB stars.
Finally, we want to emphasize that, although stars of less than lose 99% of their angular momentum due to a magnetic wind during their main sequence evolution, it can not be excluded that the proposed mechanism of 13C-production due to differential rotation also works for them. Certainly, the sun's core will spin-up and the envelope will further spin down during its post-main sequence evolution, which may result in a specific angular momentum jump of similar magnitude. The investigation of the mass and metallicity dependence of the production of 13C due to rotation is an exciting task for the near future.
© European Southern Observatory (ESO) 1999
Online publication: June 17, 1999