Modelling Collision Products of Triple-Star Mergers

Abstract

In dense stellar clusters, binary-single and binary-binary encounters can ultimately lead to collisions involving two or more stars. A comprehensive survey of multi-star collisions would need to explore an enormous amount of parameter space, but here we focus on a number of representative cases involving low-mass main-sequence stars. Using both Smoothed Particle Hydrodynamics (SPH) calculations and a much faster fluid sorting software package (MMAS), we study scenarios in which a newly formed product from an initial collision collides with a third parent star. By varying the order in which the parent stars collide, as well as the orbital parameters of the collision trajectories, we investigate how factors such as shock heating affect the chemical composition and structure profiles of the collision product. Our simulations and models indicate that the distribution of most chemical elements within the final product is not significantly affected by the order in which the stars collide, the direction of approach of the third parent star, or the periastron separations of the collisions. We find that the sizes of the products, and hence their collisional cross sections for subsequent encounters, are sensitive to the order and geometry of the collisions. For the cases that we consider, the radius of the product formed in the first (single-single star) collision ranges anywhere from roughly 2 to 30 times the sum of the radii of its parent stars. The final product formed in our triple-star collisions can easily be as large or larger than a typical red giant. We therefore expect the collisional cross section of a newly formed product to be greatly enhanced over that of a thermally relaxed star of the same mass.