ON THE NATURE OF THE AM PHENOMENON OR ON A STABILIZATION AND THE TIDAL MIXING IN BINARIES .1. ORBITAL PERIODS AND ROTATION

The paper casts a questioning eye on the unique role of the diffusive particle transport mechanism in explaining the Am phenomenon and argue s that the so-called tidal effects might be of great importance in con trolling diffusion processes. A short period cutoff at approximate to 1.2(d) as well as a 180(d) - 800(d) gap were found in the orbital peri od 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 diffu sion, tidal mixing and stabilization processes. Because the tidal mixi ng might surpass diffusion in the binaries with the orbital periods P- orb less than several hundred days and might thus sustain the He conve ction zone, which would otherwise disappear, no Am stars should lie be low this boundary. The fact that they are nevertheless seen there impl ies the existence of some stabilization mechanism (as, e.g., that rece ntly proposed by Tassoul & Tassoul 1992) for the binaries with orbital periods less than 180(d). Further evidence is given to the fact that the OPD for the Am and the normal binaries with an A4-F1 primary are c omplementary to each other, from which it stems that Am stars are clos e to the main sequence. There are, however, indications that they have slightly larger radii (2.1-3 R.) than expected for their spectral typ e. The generally accepted rotational velocity cutoff at approximate to 100 km s(-1) is shown to be of little value when applied on Am binari es as here it is not a single quantity but, in fact, a function of P-o rb 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 < 100 km s(-1), or the lack of normal stars for P-orb > 2. 5(d). We have exploited this empirical cutoff function to calibrate th e corresponding turbulent diffusion coefficient associated with tidal mixing, having found out that the computed form of the lines of consta nt turbulence fits qualitatively the empirical shape of the curves of constant metallicity. As for larger orbital periods (20(d) < P-orb < 2 00(d)) these are characterized by the more-less constant boundary of r otational velocities of about approximate to 75 km s(-1). In the case of synchronized Am binaries here the upper constraint for rotational v elocities is tied with the short orbital period cutoff, and thus, prob ably, with characteristics for primaries with the filled Roche lobe. F inally, high metallic Am stars seem to possess larger orbital periods. The jump in metallicity for v sin i > 55 km s(-1) found by Burkhart ( 1979) would then be nothing but a manifestation of insufficiently popu lated corresponding area of larger P-orb.