A Three-Dimensional Smoothed Particle Hydrodynamics Simulation of the Active Phase of Ss-Cygni Type Discs and its Implications for the Mass Transfer Burst Model
We perform a smoothed particle hydrodynamics (SPH) three-dimensional simulation of the outburst phase of the accretion disc of a typical SS Cyg-like dwarf nova in the framework of the mass transfer burst model (MTBM), where we assume that the active phase is triggered by a sudden increase in the accretion rate due to some instability in the secondary's atmosphere.The evolution of the accretion disc is followed for a single orbital period, starting from the initial quiescent disc configuration obtained by us in a previous paper. This is a suitable integration time for determining the geometrical and physical properties of the disc in the impulsive phase and is comparable with observed outburst time- scales. We find that the accretion disc in the initial quiescent phase behaves like a tenuous diffused medium when subjected to a massive inflow of accreting stream particles. With an active-phase injection rate of Mdotin 1.60 × 1018 g s-1 and an accretion rate of Mdotacc ≈ 1.30 × 1017 g s-1, the mass of the disc increases by a factor of 5 during an orbital period. In the course of the active phase, the average radius of the disc decreases by a factor of 2, which is inconsistent with observations. This result resembles that obtained in a bidimensional simulation by Ichikawa & Osaki, and suggests that the MTBM cannot account for the active phase of dwarf novae. We are, however, perfectly aware that the mechanism which triggers the outburst phase is far from being understood. A 3D numerical simulation in the framework of the intrinsic disc instability model appears to be necessary; 2D results by Ichikawa & Osaki are in favour of the latter hypothesis. A decrease in the average radius of the disc implies a remarkable global thickening of the disc that starts in the outer regions. This effect cannot, of course, be seen in 2D simulations. Apart from its implications for the MTBM, our numerical experiment provides useful information on the dynamics of accretion discs whenever a sudden increase in the accretion rate occurs, and particularly useful information on the interaction between the accreting stream and the disc itself. All the results found in this experiment have internal consistency and may increase our understanding of the structure and dynamics of discs in cataclysmic variables. A hotspot of collisional origin is clearly identified in the outer edge of the disc, and the presence of an `over-disc stream', previously noted by us, is confirmed.