  An Eulerian TVD code and a Lagrangian SPH code are used to simulate the
off-axis collision of equal-mass main sequence stars in order to address the
question of whether stellar mergers can produce a remnant star where the
interior has been replenished with hydrogen due to significant mixing. Each
parent main sequence star is chosen to be found near the turnoff, with hydrogen
depleted in the core, and is modelled with a M=0.8 solar mass realistic stellar
model and as a n=3 polytrope. An ideal fluid description with adiabatic index
gamma=5/3 is used for all hydrodynamic calculations. We found good agreement
between the simulations for the polytropic case, with the remnant showing
strong, non-local mixing throughout. In the interior quarter of the mass, ~35%
is mixed in from larger radii and on average the remnant is ~50% fully mixed.
For the realistic model, we found less mixing, particularly in the interior and
in the SPH simulation. In the inner quarter, ~20% of the contained mass in the
TVD case, but only ~3% in the SPH one is mixed in from outside. The simulations
give consistent results for the overall profile of the merger remnant and the
amount of mass loss, but the differences in mixing suggests that the intrinsic
difference between grid and particle based schemes remains a possible artifact.
We conclude that both the TVD and SPH schemes can be used equally well for
problems that are best suited to their strengths and that care should be taken
in interpreting results about fluid mixing.
