University of Cambridge Department of Applied Maths and Theoretical Physics
Accretion disks are both ubiquitous in astrophysics, and essential to many of the phenomenon observed in our universe. Appearing around young stars, compact objects like black holes, and even Saturn, astrophysical disks provide a host of interesting problems for theoreticians to consider. In my PhD I have focused in particular on the magnetohydrodynamic aspects of accretion disk oscillations and instabilities.
Learn more about my research here:
`High-frequency quasi-periodic oscillations' (HFQPOs) observed in the emission from black hole binary systems present an important but unresolved problem in astrophysics. One explanation involves the amplification of `diskoseismic' trapped inertial waves, confined to the inner regions of a black hole accretion disk by relativistic effects.
The un-magnetized, hydrodynamic theory of r-modes fits well with observations. However, the waves' properties are altered by magnetic fields, which provide restoring forces through magnetic tension and pressure. In pursuing my PhD, I have examined the effects of magnetic fields with different geometries on trapped inertial waves. While the tension force from a vertically aligned magnetic field can modify the r-mode trapping cavity, I have found that a `toroidal' magnetic field aligned with the circulation of the flow reduces this effect by altering the acoustic properties of the oscillations.
Trapped inertial waves grow in amplitude when they interact with large-scale deformations (warps and eccentricities) in the accretion disc. I have been running non-linear, magnetohydrodynamic simulations to explore this excitation mechanism in relativistic disks with eccentric (non-circular) streamlines.
I am also interested in the evolution of eccentric disks with non-circular streamlines in more general (non-relativistic) contexts. One of my current projects is related to the excitation of oscillations on both large and small scales in eccentric, purely Newtonian disks.
The magnetorotational instability (MRI) provides a widely accepted explanation for turbulent accretion in astrophysical disks. Although studied extensively in local simulations, numerical capabilities have only recently permitted explorations of the dynamics of MRI turbulence on a truly global scale. I am interested in how the MRI might drive, damp or otherwise interact with other waves and instabilities in astrophysical accretion flows.