We have updated both micro and macro-physical inputs in ATON 2.0 code for general stellar evolution. In particular, we treated convective mixing inside and outside formally convective regions as a diffusive process.
Since the diffusion coefficients cannot yet be evaluated according to first principles, we assumed an FST local model to get the turbulent velocities (projected at the boundaries) inside formally convective regions, and exponential decay of the turbulent velocity outside. Elements of "non-locality" are introduced when considering the diffusive scale lengths tied to radial distances from convective boundaries. Full coupling between nuclear chemical evolution and turbulent mixing is assumed.
One relevant result is that the CNO equilibria in the convective cores of large mass main sequence stars do require such coupling in the models computation. Instantaneous or diffusive mixing decoupled from nuclear evolution leads to discrepances in the evaluation of evolutionary times.
As for overshooting, we chose a conservative value for the free parameter of diffusion such that it should have a negligible effect on the early convective core of the Sun, and mimick the effect of an instantaneous overshooting of for intermediate to large mass stars. This tuning is obviously preliminary, and requires further tests and comparisons not only in main sequence but also in other evolutionary phases. Next papers on the subject will follow, with application of the present diffusive scheme mainly to horizontal branch and to thermally pulsating stars.
We also compared our results with the more recent ones in the literature, finding satisfactory agreement so far as the different chemical compositions and mixing schemes could allow.
© European Southern Observatory (ESO) 1998
Online publication: June 2, 1998