SPIRAL PATTERNS AND SHOCKS IN LOW-COMPRESSIBILITY ACCRETION DISKS AROUND COLLAPSED OBJECTS: TWO-DIMENSIONAL SPH MODELING

摘要

In recent years contrasting results have been found regarding the onset of spiral structures and shock fronts in accretion disks around compact objects. Indeed, according to some authors, spiral structures and shock fronts do not develop if an adiabatic index γ > 1.16 is adopted. On the contrary, other authors obtain well-developed spiral patterns and shocks adopting γ = 1.2. In this paper, by using a smoothed particle hydrodynamics (SPH) code, we show that clear spiral patterns and strong radial shocks come out even in very low compressibility (γ = 1.3 > 1.16) accretion disk models in close binaries if the primary is a massive black hole (MBH) with a mass 30-60 times larger than the secondary, whatever the geometrical and dynamical conditions at the inner Lagrangian point, L1, may be, independent of sonic or subsonic injection flow boundary conditions. Indeed, the rationale of this work is that in close binary systems in which the primary is a MBH and the secondary is a low-mass star, we have enough initial energy and angular momentum at the inner Lagrangian point, L1, and a wide and deep enough primary potential well to favor the development of well-defined spiral structures, and eventually spiral shocks, independent of the gas compressibility. According to our results the presence of a MBH triggers the development of spiral structures and spiral shock fronts in the accretion disk both at its outer edge and in the disk bulk, because of the high particle concentration and the strong collisions induced by the strongly accelerated stream particles with the high initial angular momentum at L1.