MONTE-CARLO SIMULATIONS OF BINARY STELLAR EVOLUTION IN YOUNG OPEN CLUSTERS

We present a model for calculating the evolution of close binaries, in cluding mass exchange episodes, in an approximate way. Our model cover s all parts of parameter space (initial masses and orbital separations ) for binaries with primaries more massive than 2 M., and calculates t he evolution until both stars have become compact objects. The model i s described in detail and its approximations and uncertainties are dis cussed. The model is applied in Monte-Carlo simulations of a large pop ulation of stars, representing the initial population of young open cl usters. The colour-magnitude diagrams of these synthetic clusters repr oduce several observed features of real young open clusters that can b e attributed to the presence of binaries. In particular, blue straggle rs are produced as remnants of mass exchange in close binaries. We ide ntify five types of blue straggler. Four of these types are binaries w ith helium-star, white-dwarf, neutron-star, and stripped main-sequence -star companions; the fifth type are single, merged main-sequence star s. We derive the expected numbers and properties of each of these blue -straggler types as a function of cluster age. The numbers and propert ies of our synthetic blue stragglers (rapid rotation, small or absent radial-velocity variations) are consistent with the observed incidence and characteristics of blue stragglers in clusters younger than about 300 Myr. In contrast, our model appears to be unable to account for a ll the observed blue stragglers in intermediate-age clusters (between 300 and 1500 Myr). Other observed features that are reproduced by the inclusion of binaries are: the occurrence of a second main sequence, t he appearance of 'yellow straddler' giants between the giant branch an d the turn-off point, and the occurrence of 'blue interlopers' below t he main sequence.