1996A&A...313..523B

The paper casts a questioning eye on the unique role of the diffusive particle transport mechanism in explaining the Am phenomenon and argues that the so-called tidal effects might be of great importance in controlling diffusion processes. A short period cutoff at ≃1.2d as well as a 180-800d gap were found in the orbital period distribution (OPD) of Am binaries. The existence of the former can be ascribed to the state of the primaries with the almost-filled Roche lobes. The latter could result from the combined effects of the diffusion, tidal mixing and stabilization processes. Because the tidal mixing might surpass diffusion in the binaries with the orbital periods P_orb less than several hundred days and might thus sustain the He convection zone, which would otherwise disappear, no Am stars should lie below this boundary. The fact that they are nevertheless seen there implies the existence of some stabilization mechanism (as, e.g., that recently proposed by Tassoul & Tassoul 1992) for the binaries with orbital periods less than 180d. Further evidence is given to the fact that the OPD for the Am and the normal binaries with an A4-F1 primary are complementary to each other, from which it stems that Am stars are close to the main sequence. There are, however, indications that they have slightly larger radii (2.1-3 R_☉) than expected for their spectral type. The generally accepted rotational velocity cutoff at ≃100km/s is shown to be of little value when applied on Am binaries as here it is not a single quantity but, in fact, a function of P_orb whose shape is strikingly similar to that of the curves of constant metallicity as ascertained from observations. This also leads to the well known overlap in rotational velocities of the normal and Am stars for 40<v<100km/s, or the lack of normal stars for P_orb>2.5d. We have exploited this empirical cutoff function to calibrate the corresponding turbulent diffusion coefficient associated with tidal mixing, having found out that the computed form of the lines of constant turbulence fits qualitatively the empirical shape of the curves of constant metallicity. As for larger orbital periods (20d<P_orb<200d) these are characterized by the more-less constant boundary of rotational velocities of about ≃75km/s. In the case of synchronized Am binaries here the upper constraint for rotational velocities is tied with the short orbital period cutoff, and thus, probably, with characteristics for primaries with the filled Roche lobe. Finally, high metallic Am stars seem to possess larger orbital periods. The jump in metallicity for v sin i>55km/s found by Burkhart (1979) would then be nothing but a manifestation of insufficiently populated corresponding area of larger P_orb.