ARE BLUE STRAGGLERS MIXED DURING COLLISIONS

The smoothed particle hydrodynamics (SPH) simulations of Bent & Hills led to the assumption that blue stragglers created by collisions are i nitially fully mixed. The theoretical models of stars created under th is assumption fit the observed blue straggler sequences well. Recently , however, Lombardi, Rasio, & Shapiro performed SPK calculations of st ellar collisions using a more realistic polytropic index for the struc ture of stars near the main-sequence turnoff. Their results show that the collision products are not mixed and, in fact, have composition pr ofiles similar to those of the original stars. We have calculated evol utionary tracks for both the ''unmixed'' products and fully mixed prod ucts 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 a s bright as the observed stars, and they only live on the main sequenc e for a very short time. To account for the observed blue stragglers i n NGC 6397, the collision products must either be more than twice the turnoff mass or mixed by some process subsequent to the initial collis ion and merger.