ON BLUE STRAGGLER FORMATION BY DIRECT COLLISIONS OF MAIN-SEQUENCE STARS

We report the results of new smoothed particle hydrodynamics calculati ons of parabolic collisions between main-sequence (MS) stars. The star s are assumed to be close to the MS turnoff point in a globular cluste r and are therefore modeled as n = 3, Gamma = 5/3 polytropes. We find that the high degree of central mass concentration in these stars has a profound effect on the hydrodynamics. In particular, very little hyd rodynamic mixing occurs between the dense, helium-rich inner cores and the outer envelopes. As a result, and in contrast to what has been as sumed in previous studies, blue stragglers formed by direct stellar co llisions are not necessarily expected to have anomalously high helium abundances in their envelopes or to have their cores replenished with fresh hydrogen fuel.