The evolution of AmFm stars, abundance anomalies, and turbulent transport

Stellar evolution models of stars of 1.45-3.0 M. have been calculated, incl uding the atomic diffusion of metals and radiative accelerations for all sp ecies in the OPAL opacities. As the abundances change, the opacities and ra diative accelerations are continuously recalculated during evolution. These models develop iron-peak convection zones centered at a temperature of app roximately 200,000 K. If one then assumes that there is sufficient overshoo t to homogenize the surface regions between the hydrogen, helium, and iron- peak convection zones, it is shown here that the surface abundance variatio ns that are produced, without any arbitrary parameter, closely resemble the abundance anomalies of AmFm stars, except that they are larger by a factor of about 3. Detailed evolutionary model calculations have been carried out , varying the turbulence in the outer stellar regions in order to improve t he agreement with the observed anomalies in AmFm stars. The outer mass mixe d by turbulence has been varied, as well as the density dependence of the t urbulent diffusion coefficient. It is shown that the anomalies depend on on ly one parameter characterizing turbulence, namely, the depth of the zone m ixed by turbulence. The calculated surface abundances are compared to obser vations of a number of recently observed AmFm stars. For Sirius A, 16 abund ances (including four upper limits) are available for comparison. Of these, 12 are well reproduced by the model, while three are not so well reproduce d, and one is a very uncertain observation. In cluster AmFm stars, the age and initial abundances are known. There is then less arbitrariness in the c alculations, but fewer chemical species have been observed than in Sirius. The available observations (Hyades, Pleiades, and Praesepe stars are compar ed) agree reasonably well with the calculated models for the five stars tha t are compared. The zone mixed by turbulence is deeper than the iron convec tion zone, reducing the abundance anomalies to values that are too small fo r iron-peak convection zones to develop in many of the models. The origin o f the mixing process then remains uncertain. There is considerable scatter in the observations between different observers, so it is premature to conc lude that hydrodynamical processes other than turbulence are needed to expl ain the observations. We do not rule out the possibility that this might be the case, but the observations do not appear to us to be good enough to es tablish it.