EVOLUTION OF STELLAR COLLISION PRODUCTS IN GLOBULAR-CLUSTERS .1. HEAD-ON COLLISIONS

We explore the evolution of collisionally merged stars in the blue str aggler region of the H-R diagram. The starting models for our stellar evolution calculations are the results of the smoothed particle hydrod ynamics (SPH) simulations of parabolic collisions between main-sequenc e stars performed by Lombardi, Rasio, & Shapiro. Since SPH and stellar evolution codes employ different and often contradictory approximatio ns, it is necessary to treat the evolution of these products carefully . The mixture and disparity of the relevant timescales (hydrodynamic, thermal relaxation, and nuclear burning) and of the important physical assumptions between the codes makes the combined analysis of the prob lem challenging, especially during the initial thermal relaxation of t he star. In particular, the treatment of convection is important and s emiconvection must be modeled in some detail. The products of seven he ad-on collisions are evolved through their initial thermal relaxation and then through the main-sequence phase to the base of the giant bran ch. Their evolutionary tracks are presented. In contrast to what was a ssumed in previous work, these collision products do not develop subst antial convective regions during their thermal relaxation and therefor e are not mixed significantly after the collision.