Smoothed particle hydrodynamic simulations of stellar collisions.

摘要

We report the results of smoothed particle hydrodynamics (SPH) calculations of parabolic collisions between two main-sequence stars. Such collisions are directly relevant to the formation of so-called blue straggler stars. We focus on the hydrodynamic mixing of helium and hydrogen, which plays a crucial role in establishing the color, luminosity and lifetime of collisional blue stragglers. In all cases we find negligible mixing of helium into the outer envelope of the merger remnant. The amount of hydrogen carried into the core of the remnant depends strongly on the entropy profiles of the colliding stars. For example, when the parent stars are close to turnoff, very little hydrogen is present at the center of the merger remnant, and the main-sequence lifetime of the resulting blue straggler could be short. For stars with nearly equal masses (and hence entropy profiles), the composition profile of the remnant closely resembles that of the parents. We conclude that blue stragglers formed by direct stellar collisions are not thoroughly mixed, unless convection, semiconvection or rotationally-induced mixing becomes significant during the subsequent stellar evolution.; We also perform a series of systematic tests to evaluate quantitatively the effects of spurious transport in SPH calculations. Our tests investigate (i) particle diffusion, (ii) shock heating, (iii) numerical viscosity, and (iv) the spurious transport of angular momentum due to artificial viscosity in differentially rotating, self-gravitating configurations. The results are useful for quantifying the accuracy of the SPH scheme, especially for problems where shear flows or shocks are present, as well as for problems where true mixing is relevant. We examine the different forms of artificial viscosity which have been proposed by Monaghan, by Hernquist & Katz, and by Balsara. We find that both the Hernquist & Katz and Balsara forms introduce relatively small amounts of numerical viscosity. Furthermore, both Monaghan's and Balsara's artificial viscosity do well at treating shocks and at limiting the amount of spurious mixing. For these reasons, we endorse the Balsara artificial viscosity for use in a broad range of applications. We also discuss how the artificial viscosity parameters can be adjusted to achieve optimal accuracy.