Hydrodynamic simulations of outburst events in accretion discs in close binaries

Abstract

In this work, the role of both inflow kinematic at the inner Lagrangian point L1 as initial boundary condition, and gas compressibility, and physical turbulent viscosity, on accretion disc's dynamics and structure in a Close Binary (CB), are investigated via simulations of 3D SPH stationary accretion disc models. Physical viscosity hampers the gas dynamics (rarefaction or compression), supporting the accretion disc development inside the primary gravitational potential well in a CB system, even for low compressibility modelling. Currently, the turbulent Shakura-Sunyaev parameter α ≃ 0.1 is widely adopted for these structures. Such investigation is here carried out in both high and low compressibility regimes, with the aim of evaluating, in a compressibility-viscosity graph, the most suitable domains where physical conditions allow a well-bound disc development as a function of mass transfer kinematic conditions. A selection of the adopted α physical turbulent viscosity parameter (α ≤ 1 according to the well-known Shakura-Sunjaev formulation) has been considered, whilst the adopted polytropic index γ is chosen in the range between 1 ÷ 5/3. Results show that domains exist where physical turbulent viscosity supports the accretion disc formation. In such domains, the lower the gas compressibility, the higher the physical viscosity requested. A role played by the injection kinematics at the inner Lagrangian point L1 is also found. Conclusions as far as dwarf novae outbursts are concerned, induced by mass transfer rate variations, are also reported. Considerations as far as periodicities in an accretion disc are also reported.