Master defense by Marcela Grcic

Title: The eccentric evolution of circumbinary gas disks

Gas disks around binaries are ubiquitous in the universe, naturally arising during the formation of binary stars and playing an important role in the mergers of black holes. The interaction of these circumbinary disks with the binary still holds many open questions that are important for understanding how populations of binaries are sculpted and also the observable signatures of these systems. Recent numerical simulations of these systems have discovered a number of surprising features in the response of the gas disk to the binary’s gravitational potential, including the growth of eccentric modes in the gas disk. These eccentric features have important consequences for binary evolution and observability, yet the physical means for their generation is still poorly understood.

This thesis provides an overview of the differential equations governing eccentricity growth in a circumbinary gas disk.
Equations governing the eccentricity evolution of adiabatic and isothermal disks, 2d and 3d disks, circumprimary and circumbinary disks are presented. The eccentricity equation is solved for a locally isothermal circumbinary disk. The number of modes such a disk can support in terms of the disk thickness and the binary mass ratio is found. Properties of elementary and higher order modes are explained. The effect of the disk density distribution on the disk eccentricity is discussed.

Advisors: Daniel D'Orazio, Martin Pessah
Censor: Steen Hannestad (Aarhus University)