THE FORMATION OF BE STARS IN CLOSE BINARY-SYSTEMS - THE IMPORTANCE OFKICKS AND ANGULAR-MOMENTUM LOSS

With a simplified model for case B close binary-evolution, the product ion of Be stars and companions is studied in order to address the obse rvation that all Be stars with a neutron-star companion (similar to 30 ) are more massive than about 8 M(.) (spectral type earlier than B 2.5 ). In the model we assume that Be stars are formed in binary systems w here mass transfer spins up the accretor to high rotational velocity s o that it starts showing the Be-effect. Since mass transfer is thought to occur on a relatively short timescale, the Be stars will be accomp anied by a helium star (mass-transfer remnant), a white dwarf or a neu tron star. Kicks (of the order of 100 to 200 km s(-1)), as result of a non-symmetric supernova event, reduce the number of late type Be star s with a neutron-star companion only slightly and thus do not seem to satisfy the observational constraint. If during non-conservative binar y-evolution, mass lost from the system is removed with the specific an gular-momentum per unit mass at the second Lagrange point, systems wit h small initial mass ratios do not evolve into Be x-ray binaries. Cons equently, no Be stars with a neutron-star companion are formed with a spectral type later than B 2.5, in good a agreement with thp observati ons.