Blue straggler formation via close binary mass transfer

Abstract

Several mechanisms of blue straggler (BS) formation via mass transfer in close binaries are studied in this work. Detailed close binary evolutionary models including both Cases A and B are calculated. We follow the evolution of a binary of 1.4 M⊙+ 0.9 M⊙ and compare the evolutionary behaviour of both cases. The results show that Case B is as important as Case A in forming BSs. Moreover, BSs formed via Case B are generally bluer and even more luminous than those produced by Case A, which provides a clue as to the origin of luminous BSs - such as those in M67. Based on the models calculated at 4.0 Gyr of age, a Monte Carlo simulation of the BS population in M67 is carried out using both Case A and Case B scenarios. Five BSs formed via mass transfer were predicted by our simulation and another three by dynamical merger processes. The simulation uses an artificially enlarged sample size for better statistics. The contributions from the different channels are estimated. We also investigate the influence of mass transfer efficiency β during Roche lobe overflow on close binary evolution. The result shows that a high value of β could reproduce the observed data better. The mechanism discussed in this work cannot predict all observed BSs in M67 because many factors are involved in such an old open cluster that affect the formation of BSs. However, we offer a better explanation of blue and luminous BSs in the binary evolution channel. © 2010 The Authors. Journal compilation © 2010 RAS.