Speaker: Gordon Ogilvie (University of Cambridge)
Title: Celestial fluid mechanics: new perspectives on the hydrodynamics of astrophysical discs
Abstract:
Astrophysical discs often deviate from the circular, planar forms assumed in classical accretion-disc theory.  If the disc's angular momentum is misaligned with that of an orbital companion or a spinning central object, it will become warped.  Eccentric discs composed of elliptical orbits feature increasingly in observations and numerical simulations of various astrophysical processes, including tidal disruption events and circumbinary discs.  In this talk I will discuss some physical consequences of these situations and introduce some novel theoretical methods for studying them.  Generalizations of the shearing-box model, incorporating the oscillatory geometry experienced by orbiting fluid elements, reveal hydrodynamic turbulence that results from the destabilization of internal waves.  The nonlinear secular hydrodynamics of warped and eccentric discs can be discussed in a Hamiltonian framework that connects with methods of celestial mechanics.  The gas dynamics of a warped Keplerian disc is an especially rich problem involving linear and nonlinear resonances of coupled oscillators, as well as instabilities on small and large scales.  An affine model of astrophysical discs augments the computationally convenient equations of 2D hydrodynamics with the additional degrees of freedom needed to describe several of these situations, in which the discs are far from hydrostatic equilibrium.  I will discuss the progress that has been made recently as well as some of the important outstanding questions.