Mass Transfer in Binary Stars using Smoothed Particle Hydrodynamics

Abstract

<p> Despite numerous efforts to better understand binary star evolution, some aspects of it remain poorly constrained. In particular, the evolution of eccentric binaries has remained elusive mainly because the Roche lobe formalism derived for circular binaries does not apply, and other approximations must be used. Here, we report the results of our Smoothed Particle Hydrodynamics simulations of eccentric binaries using a novel way of modeling only the outermost layers of the stars with appropriate boundary conditions. We find that our boundary treatment conserves energy well and that it is well suited for the modeling of interacting binary stars. Using this new technique, along with properly relaxed model stars, we find clear trends in the mass transfer episodes. In particular, we show that these episodes can be described by Gaussians with a FWMH of ~ 0.12 P orb and that the peak rates occur after periastron, around an orbital phase of ~ 0.55, independent of the eccentricity and masses of the stars. The accreted material, on the other hand, is observed to form a rather sparse envelope around either or both stars. Although the fate of this envelope is not modeled in our simulations, we show that a constant fraction (~5%) of the material transferred is ejected from the systems. We discuss this result in terms of the poorly constrained non-conservative mass transfer scenario and argue that it can help calibrate it. Finally, we discuss the limitations of our technique and conditions under which it performs best. The results presented in this thesis represent an improvement upon previous hydrodynamical work and could be used in analytical and binary population synthesis studies to better constrain the evolution of eccentric binaries and the formation of exotic stellar populations. </p>