Mass transfer from a giant star to a main-sequence companion and its contribution to long-orbital-period blue stragglers

Binary population synthesis shows that mass transfer from a giant star to a main-sequence (MS) companion may account for some observed long-orbital-period blue stragglers. However, little attention is paid to this blue straggler formation scenario as dynamical instability often happens when the mass donor is a giant star. In this paper, we have studied the critical mass ratio, q_c, for dynamically stable mass transfer from a giant star to a MS companion using detailed evolution calculations. The results show that a more evolved star is generally less stable for Roche lobe overflow. Meanwhile, q_c almost linearly increases with the amount of the mass and angular momentum lost during mass transfer, but has little dependance on stellar wind. To conveniently use the result, we give a fit of q_c as a function of the stellar radius at the onset of Roche lobe overflow and of the mass-transfer efficiency during the Roche lobe overflow.

To examine the formation of blue stragglers from mass transfer between giants and MS stars, we have performed Monte Carlo simulations with various q_c. The simulations show that some binaries with the mass donor on the first giant branch may contribute to blue stragglers with q_c obtained in this paper but will not from previous q_c. Meanwhile, from our q_c, blue stragglers from the mass transfer between an asymptotic giant branch star and a MS companion may be more numerous and have a wider range of orbital periods than those from the other q_c.