ON THE LUMINOSITY FUNCTION, LIFETIMES, AND ORIGIN OF BLUE STRAGGLERS IN GLOBULAR-CLUSTERS

We compute theoretical evolutionary tracks of blue stragglers created by mergers. Two formation scenarios are considered: mergers of primord ial binaries, and stellar collisions. These two scenarios predict stri kingly different luminosity functions, which are potentially distingui shable observationally. Tabulated theoretical luminosity functions and lifetimes are presented for blue stragglers formed under a variety of input conditions. We compare our results with observations of the blu e straggler sequences in 47 Tucanae and M3. In the case of 47 Tuc, the luminosity function and the formation rate are compatible with the hy pothesis that the blue stragglers formed through the collision of sing le stars. Mergers of primordial binaries are only marginally consisten t with the data, and a significant enhancement of the collision cross section by binary-single-star encounters appears to be ruled out. In t he case of M3, we find that the innermost blue stragglers have a lumin osity function significantly different from that of the outer straggle rs, thus confirming earlier suggestions that there are two distinct po pulations of blue stragglers in this cluster. The inner stragglers are preferentially brighter and bluer, as would be expected if they were made by collisions, but there are so many of them that the collision r ate would need to be enhanced by interactions involving wide binaries. The luminosity function of the outer stragglers is almost identical t o the predictions of mergers from primordial binaries and is inconsist ent with the collision hypothesis.