Disks in close Binary Stars

Abstract

Disks in close binaries offer great opportunities for testing and refining astrophysical models and are studied in this thesis in the context of dwarf novae and planet formation. Dwarf novae are a subclass of cataclysmic variables (CVs), which are close binary systems with separations on the order of one solar radius, consisting of a white dwarf orbited by a mass transferring low-mass secondary star. The mass transfer forms a disk around the white dwarf, and it is this accretion disk that repeatedly undergoes dwarf nova outbursts. In SU UMa stars, a subclass of dwarf novae named after the prototype SU Ursae Majoris, some outbursts evolve into superoutbursts that exhibit brightness variations called super- humps, which typically have a slightly longer periods than the binary. The superhumps can be explained by an eccentric disk with a slow prograde precession. The extremely fast timescales on which these cycles evolve, with outbursts lasting a few days occurring every other week, make these systems ideal testbeds for probing disk models and studying binary disk interactions. On a different scale, planets have been discovered around primary stars in main sequence binary systems with separations of less than 40 au. At such close distances, the disks around the primary are dynamically perturbed and are smaller, have reduced masses, and have shorter lifetimes compared to disks around single stars. Despite this, a few dozen planets are known in close binary systems, suggesting that the planet formation process is robust and fast if they were formed at the positions currently observed. Current models predict that planetesimal growth, a critical step in the planet formation process, can only succeed if the disk remains dynamically calm despite the perturbations of the secondary. Because these systems are too small to resolve in observations and too complex to study analytically, they can currently only be studied using numerical simulations. In this thesis, I developed new two-dimensional numerical hydrodynamical models to simulate disks in close binaries. I then used these models to study both, SU UMa outburst cycles, and the environment around close binaries in which planets might form.