Evolution of Stellar Collision Products in Globular Clusters -- I. Head-on Collisions

We explore the evolution of collisionally merged stars in the blue straggler region of the HR diagram. The starting models for our stellar evolution calculations are the results of the smoothed particle hydrodynamics (SPH) simulations of parabolic collisions between main sequence stars performed by Lombardi, Rasio and Shapiro (1996). Since SPH and stellar evolution codes employ different and often contradictory approximations, 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 problem challenging, especially during the initial thermal relaxation of the star. In particular, the treatment of convection is important, and semiconvection must be modeled in some detail. The products of seven head-on collisions are evolved through their initial thermal relaxation, and then through the main sequence phase to the base of the giant branch. Their evolutionary tracks are presented. In contrast to the assumptions in previous work, these collision products do not develop substantial convective regions during their thermal relaxation, and therefore are not mixed significantly after the collision.