1994A&A...288..475P

We present a model for calculating the evolution of close binaries, including mass exchange episodes, in an approximate way. Our model covers all parts of parameter space (initial masses and orbital separations) for binaries with primaries more massive than 2M_☉, and calculates the evolution until both stars have become compact objects. The model is described in detail and its approximations and uncertainties are discussed. The model is applied in Monte-Carlo simulations of a large population of stars, representing the initial population of young open clusters. The colour-magnitude diagrams of these synthetic clusters reproduce several observed features of real young open clusters that can be attributed to the presence of binaries. In particular, blue stragglers are produced as remnants of mass exchange in close binaries. We identify five types of blue straggler. Four of these types are binaries with helium-star, white-dwarf, neutron-star, and stripped main-sequence-star companions; the fifth type are single, merged main-sequence stars. We derive the expected numbers and properties of each of these blue-straggler types as a function of cluster age. The numbers and properties 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 all 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, the appearance of "yellow straddler" giants between the giant branch and the turn-off point, and the occurrence of "blue interlopers" below the main sequence.