Be Stars in Misaligned Binary Systems
Abstract
Classical B-emission stars (Be stars) are rapidly rotating B-type stars that form a gaseous circumstellar disk. As Be star disks evolve on human timescales of months to years unlike other astrophysical objects, they are excellent testbeds for studying the evolution, and testing our understanding, of astrophysical disks. In this thesis, I utilize and combine three-dimensional (3D) computation techniques to study the dynamical evolution and observational changes of Be star disks in systems with a binary companion whose orbit is misaligned to the initial plane of the disk.
I first use a 3D smoothed particle hydrodynamics (SPH) code to simulate the growth and dissipation of Be star disks in equal-mass binary systems with a misaligned binary orbit, and find some disks undergo phenomena of Kozai-Lidov oscillations, or disk tearing. I then perform a separate study using the three-dimensional Monte Carlo radiative transfer code, HDUST, to examine the differences in disk temperature, ionization, and resulting observables, of a disk tilted out of the equatorial plane versus a typical non-tilted disk. I show that a tilted disk has a non-axisymmetric temperature structure, and that a tilted disk is best detected via a change in the polarization position angle. I then developed an interface to allow output of SPH to be used as input into HDUST, and use said interface to predict observables of a tearing disk. I show that the long-term trends in the tearing disk model are the best match yet to the trends observed in the Be star Pleione (28 Tau). In the final work I expand the suite of SPH simulations to lower mass ratios and different viscosity values to show that Kozai-Lidov oscillations and disk tearing can occur for a range of system parameters, and also present predicted observables of a disk while it goes through Kozai-Lidov oscillations. I also show that a torn disk, or a disk with Kozai-Lidov oscillations, can be detected through changes in an interferometric visibility curve. I conclude by discussing the impact of these results and how these computational advancements can be utilized and expanded upon in future research of astrophysical disks.