3D simulations of the SS Cyg accretion disc in the quiescent phase

We present the results of 3D SPH simulations ($$\gamma=1.01;\gamma=1.1;\gamma=1.2$$) of polytropic accretion disc formation and evolution for the SS Cyg binary system in the quiescent phase, with M₁ = 1.3 M_⊙ (primary) and M₂ = 0.8 M_⊙ (secondary), and a mass ratio M₁/M₂ = 1.625, and compare them with results obtained by Molteni, Belvedere & Lanzafame (Paper I), with M₁/M₂ = 1. Disc formation seems to be inhibited for $$\gamma\ge1.1$$, a value still lower than that found in Paper I. This confirms that 2D simulations are meaningless, as they lack z-resolution. For γ = 1.01, our simulation does agree with the standard thin-disc theory, suggesting that the thin-disc approximation is valid, independent of the mass ratio. A systematic deviation (up to 20 per cent) from the Keplerian rotation law is found for the outer half of the disc, which may be attributed to collisions between particles, as a consequence of the impact of the accreting gas stream. Another interesting result is the presence of a cold asymmetric ‘over-disc stream’ which gets into the central region above the disc plane after colliding with the disc edge. Asymmetries in the disc shape are also discussed.