UC Santa Barbara

This thesis presents our results of global 3D magnetohydrodynamic (MHD) simulations of AM Canum Venaticorum (AM CVn) accretion disks. We discuss our numerical methods and tools in developing these simulations. We also discuss some of our attempts and challenges faced when including radiation, and the numerical lessons learned along the way.

We find that our 3D MHD simulations fail to develop eccentric disks needed to explain the phenomenon of superhumps, present in the observations of the AM CVn we modeled. To investigate this, we develop an eccentricity conservation equation and use it to understand eccentricity evolution in computationally cheaper 2D and 3D alpha disk simulations. We find that both high alpha values of 0.1 and low scale height of 2.5% are needed for the growth of eccentricity in our system. The high alpha spreads the disk to large radii where tidal coupling with spiral waves can grow eccentricity. A low scale height is needed because we find that vertical pressure forces damp eccentricity. We also find that the Maxwell stresses in the 3D MHD simulation act to damp eccentricity, whereas the viscosity in the alpha disk simulations acts to grow eccentricity, a key difference between the MHD and alpha disk simulations. These findings act as constraints for future 3D MHD simulations seeking to model these systems more accurately.