MRI-Turbulent-Disk · Turbulence in Accretion Disks – New Perspectives

Turbulence in Accretion Disks – New Perspectives

Astrophysical disks rotating around young stellar objects, black holes and binary stars, attract great interest because one of the fundamental phenomena in the Universe – mass accretion and radiation – occur via such disks. Disks are involved in a range of spectacular phenomena, from planet formation to X-ray binaries and collimated jets from protostars and black holes. Knowledge of the basic physics of mass accretion onto the central object is essential for understanding its evolution and observational characteristics. Disks evolve by accreting matter to the central object as a result of losing angular momentum. The gravitational energy released during accretion is converted into radiation, which can be observed and give clues on the accretion rate – one of the central quantities for deducing the structure and evolution of disk. However, gas viscosity is too low to yield sufficiently rapid transport of angular momentum and hence consistent with observations inward accretion rates of disk matter. So, identifying mechanisms for more efficient redistribution of angular momentum has been a puzzle for decades. It is widely accepted today that such an enhanced transport is achieved by turbulence due to the magnetorotational instability (MRI), which arises from the combination of disk’s differential rotation and magnetic field and grows fast, on orbital time. The nonlinear development of MRI breaks down into turbulence which transports angular momentum outwards. Theoretically estimated accretion rates due to this turbulent transport are orders of magnitude higher than that due to viscosity and comparable to observed values. The main goal of frontier research in MRI in disks is to understand its key aspects – saturation and sustenance of resulting turbulence – and to compare corresponding mass accretion rates with observations. Because of enormous complexity of turbulence, these are tackled via numerical simulations. A great progress in this field over the last decade, thanks to powerful computers, demonstrated that various physical factors: strength and configuration of disk’s magnetic field, numerical resolution, stratification, non-ideal effects (viscosity, Ohmic resistivity), etc. play a crucial role in the dynamics of MRI-turbulence and its transport capability. However, MRI-turbulence theory has not yet grown sufficiently to have strong predictive power. In particular, accretion rates from theoretical studies still appear to depend on these factors and hence often do not agree with observations. Many key unresolved issues still remain from which the most important ones of the frontier research are: 1. Convergence problem of MRI-turbulence, i.e., decrease in the turbulent transport with increasing numerical resolution 2. Dependence of MRI-turbulent transport on viscous and resistive dissipation 3. Magnetic dynamo and its role in the sustenance of MRI-turbulence and transport Our main goal was to address these issues from a new perspective of analysis of the dynamical processes in Fourier space.

Data: CORDIS, © European Union