Material falling onto dwarf stars, neutron stars, and black holes, a process called accretion, powers some of the most energetic and fascinating astronomical sources. Because rotation is common in the Universe, most accretion systems form disks swirling around the central gravitating object, where material must transfer away its rotational inertia, or angular momentum, before continuing inwards. The matter flowing inwards also spends time in the disk converting energy into the outgoing radiation by which these objects can be observed. This research will apply new methods to understanding the process of angular momentum transport in disks around white dwarf stars, and may finally settle whether the currently dominant model is in fact correct.
Magneto-rotational instability (MRI) turbulence has become the dominant research framework for understanding angular momentum transport and the release of accretion power in accretion disks across all of astrophysics, from proto-planetary disks to active galactic nuclei. Although the ultimate goal is to model observed sources, little attention has been paid so far to dwarf nova outbursts in accretion disks orbiting white dwarfs, which offer the most interesting observational constraints on angular momentum transport. Using vertically-stratified, radiation magneto-hydrodynamic (MHD) shearing box simulations of MRI turbulence, this research will build on recent results showing intermittent thermal convection. Three new studies are planned: (1) ensure that the thermal convection is numerically resolved, and understand how it enhances turbulent transport, (2) incorporate non-ideal MHD in the largely neutral state to see if MRI turbulent transport can be sustained in the quiescent state; and (3) incorporate lessons learned about the vertical structure and angular momentum transport into existing alpha-based models of the hydrogen ionization disk instability. This direct confrontation between the MRI model and observations will firm up the theoretical foundation of the hydrogen ionization disk instability, and allow observations of dwarf novae and low mass X-ray binaries to constrain more effectively the properties of MRI turbulence.
One graduate student will develop their thesis from this project, several undergraduates are expected to be involved, and the research will inform public talks, including a new one being developed specifically on the science contained in this work.
