Three-dimensional numerical simulation of gaseous flow structure in semidetached binaries

Abstract

The results of numerical simulation of mass transfer in semidetached non‐magnetic binaries are presented. We investigate the morphology of gaseous flows on the basis of three‐dimensional gas‐dynamical calculations of interacting binaries of different types (cataclysmic variables and low‐mass X‐ray binaries). We find that taking into account a circumbinary envelope leads to significant changes in the stream–disc morphology. In particular, the obtained steady‐state self‐consistent solutions show an absence of impact between the gas stream from the inner Lagrangian point L1 and the forming accretion disc. The stream deviates under the action of the gas of the circumbinary envelope, and does not cause the shock perturbation of the disc boundary (traditional hotspot). At the same time, the gas of the circumbinary envelope interacts with the stream and causes the formation of an extended shock wave, located on the stream edge. We discuss the implication of this model without hotspot (but with a shock wave located outside the disc) for interpretation of the observations. The comparison of synthetic light curves with observations proves the validity of the discussed gas‐dynamical model without hotspot. We have also considered the influence of the circumbinary envelope on the mass transfer rate in semidetached binaries. The obtained features of flow structure in the vicinity of L1 show that the gas of the circumbinary envelope plays an important role in the flow dynamics, and that it leads to significant (in order of magnitude) increase of the mass transfer rate. The most important contribution to this increase is from the stripping of the mass‐losing star atmosphere by interstellar gas flows. The parameters of the formed accretion disc are also given in the paper. We discuss the details of the obtained gaseous flow structure for different boundary conditions on the surface of mass‐losing star, and show that the main features of this structure in semidetached binaries are the same for different cases. The comparison of gaseous flow structure obtained in two‐ and three‐dimensional approaches is presented. We discuss the common features of the flow structures and the possible reasons for revealed differences.

Structure of the pre-outburst accretion disk in SS Cygni

“…The markers in the upper panel denote the following features of the flow pattern: two lines with triangles correspond to the arms of the spiral tidal shock; the line with the hollow diamonds denotes the "hot line", a shock that occurs during the interaction of the circumdisk halo with the stream from the L 1 point; and the line with the filled diamonds lies in the region along the stream from the L 1 point. In the case of large disks and more intensive circumdisk halos, the "hot line" begins in regions edging the stream nearer to the L 1 point (Bisikalo et al 1998). The line with the filled diamonds schematically denotes the beginning of such a "hot line".…”

Why Canonical Disks Cannot Produce Advection-dominated Flows

“…In accretion occurring in active galactic nuclei, these sub-Keplerian flows could easily come from the star cluster, which is known to have a very slow average rotation (Ho 1999). In a low-mass binary system this sub-Kepler flow could come from a wind induced by the high-energy radiation impinging on the normal star surface, or from gas recirculating in the binary potential well (Bisikalo et al 1998). We are also testing the hypothesis that a physically motivated change in Z of the viscosity prescription can lead to differentially rotating flows.…”

On the role of duplicity in the Be phenomenon

“…The equation of state of a perfect gas P = (γ − 1)ρε was adopted, γ being the adiabatic index. In calculations, we adopted the value of γ very close to 1 to mimic the energy losses due to radiative cooling (see, e.g., Sawada et al 1986;Bisikalo et al 1998). The Roche potential in cylindrical coordinates has the following form:…”

“…Note that an arbitrary value of the boundary density ρ 0 can be chosen since the system of equations can be scaled with respect to ρ and P. To derive the true values of density in a specific system with a known mass loss rate, the calculated densities must simply be changed in accordance with the scale determined from the ratio of the true and model mass-loss rate. The boundary conditions were derived by solving the Riemann problem between the gas parameters (ρ 0 , u 0 , P 0 ) in L rot 1 point and the parameters in the computation gridpoint closest to it (see, e.g., Sawada et al 1986;Sawada & Matsuda 1992, or Bisikalo et al 1998. A full absorption of matter was assumed for the rest of the Bestar surface and for the outer boundary of the computational domain.…”