Dynamics of Accretion Disks around Magnetized Stars

Cornell University, Ithaca NY

Investigators

Abstract

Accreting magnetized stars, such as Classical T Tauri stars (CTTSs), cataclysmic variables, and X-ray pulsars may have dynamically important magnetic fields that can disrupt the accretion disk. Prior NSF-supported pioneering simulations have shown that accreting magnetized stars may be in stable or unstable regimes of accretion with different observational properties (ordered or stochastic light-curves). If the disk comes close to the star, it interacts through the boundary layer, where simulations have shown Kelvin-Helmholtz-type instabilities. Although these simulations and associated theoretical analyses show many interesting features which could be compared with observation, there is a need further to investigate the disk-magnetosphere/disk-star interaction, including the possibility of some type of Magneto-Rotational Instability (MRI). This new project includes simulations of the disk-magnetosphere interaction specifically tailored with 2.5D and 3D Godunov-type codes to model MRI-driven accretion in relatively thin disks. Associated theoretical analysis will give a deeper understanding of different types of instabilities at the disk-magnetosphere/disk-star boundaries. Because CTTSs show multiple variability features in rotation timescales, theirs are the photometric and spectral light-curves that will be compared with simulations. Spectrum modeling will use a 3D Monte Carlo radiative transfer code. This study will 1) investigate MRI-driven accretion onto a magnetized star; 2) investigate, theoretically and numerically, different instabilities at the disk-magnetosphere and disk-star boundaries; and 3) compare simulated light-curves and spectra with those observed in different CTTSs. The results will apply to different magnetized stars, including the CTTSs, cataclysmic variables, accreting brown dwarfs and accreting neutron stars. The research will provide a salutary example of comparing state-of-the-art 2.5D and 3D models of magnetized stars with the best available observations. The project will train undergraduate and graduate students and a postdoctoral research associate in this modern area of multidimensional magneto-hydrodynamics. Results will be included in lectures to undergraduate students, to school students, and to Cornell's public visitors. Animated visualizations have recently been developed and will be used in various talks. The team also supports an exhibit in the Tompkins County Science Museum.