Low compressibility accretion disc formation in close binaries: the role of physical viscosity
INAF - Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy e-mail: email@example.com
2 Dipartimento di Fisica e Astronomia dell'Universitá - Sezione Astrofisica, via S. Sofia 78, 95123 Catania, Italy
3 Dipartimento di Fisica e Tecnologie Relative, Universitá di Palermo, Viale delle Scienze, 90128 Palermo, Italy
Accepted: 9 March 2006
Aims.Physical viscosity naturally hampers gas dynamics (rarefaction or compression). Such a role should support accretion disc development inside the primary gravitation potential well in a close binary system, even for low compressibility modelling. Therefore, from the astrophysical point of view, highly viscous accretion discs could exist even in the low compressibility regime showing strong thermal differences to high compressibility ones
Methods.We performed simulations of stationary Smooth Particle Hydrodynamics (SPH) low compressibility accretion disc models for the same close binary system. Artificial viscosity operates in all models. The absence of physical viscosity and a supersonic high mass transfer characterize the first model. Physical viscosity and the same supersonic high mass transfer characterize the second model, whilst physical viscosity and a subsonic low mass transfer characterize the third model. The same binary system parameters, such as stellar masses and their separation, have been adopted, as well as the same polytropic index . Thus we investigated the role of physical viscosity in mass and angular momentum transport in the two viscid models and compare them to the inviscid model. An initial value of the parameter has been considered for the physically viscous models, according to the well-known Shakura and Sunjaev formulation, but simulations were carried out also for and in the case of a supersonic mass transfer. Physical viscosity is represented by the viscous force contribution expressed by the divergence of the symmetric viscous stress tensor in the Navier-Stokes equation, while the viscous energy contribution is given by a symmetric combination of the symmetric shear tensor times the particle velocity.
Results.The results show that physical viscosity supports and favours accretion disc formation despite the very low compressibility assumed. On the contrary, in the inviscid case no evident disc structure appears. In all models neither shock fronts nor extended clear spirals in the radial flow develop.
Key words: accretion, accretion discs / stars: binaries: close / stars: dwarf novae, white dwarfs / methods: numerical / methods: N-body simulations
© ESO, 2006