Are Blue Stragglers Mixed during Collisions?

The smoothed particle hydrodynamics (SPH) simulations of Benz & Hills led to the assumption that blue stragglers created by collisions are initially fully mixed. The theoretical models of stars created under this assumption fit the observed blue straggler sequences well. Recently, however, Lombardi, Rasio, & Shapiro performed SPH calculations of stellar collisions using a more realistic polytropic index for the structure of stars near the main-sequence turnoff. Their results show that the collision products are not mixed and, in fact, have composition profiles similar to those of the original stars. We have calculated evolutionary tracks for both the “unmixed” products and fully mixed products and have compared the results to the very bright blue stragglers in the center of NGC 6397. The unmixed stars are not as blue and not as bright as the observed stars, and they only live on the main sequence for a very short time. To account for the observed blue stragglers in NGC 6397, the collision products must either be more than twice the turnoff mass or mixed by some process subsequent to the initial collision and merger.