Abstract. Three-dimensional hydrodynamic calculations are performed in order to investigate mass transfer in a close binary system, in which one component undergoes mass loss through a wind. The mass ratio is assumed to be unity. The radius of the mass-losing star is taken to be about a quarter of the separation between the two stars. Calculations are performed for gases with a ratio of specific heats γ = 1.01 and 5/3. Mass loss is assumed to be thermally driven so that the other parameter is the sound speed of the gas on the mass-losing star. Here, we focus our attention on two features: flow patterns and mass accretion ratio, which we define as the ratio of the mass accretion rate onto the companion,Ṁ acc , to the mass loss rate from the mass-losing primary star,Ṁ loss . We characterize the flow by the mean normal velocity of the wind on the critical Roche surface of the mass-losing star, V R . When V R < 0.4 AΩ, where A and Ω are the separation between the two stars and the angular orbital frequency of the binary, respectively, we obtain Roche-lobe over-flow (RLOF), while for V R > 0.7 AΩ we observe wind accretion. We find very complex flow patterns in between these two extreme cases. We derive an empirical formula of the mass accretion ratio as 0.18 × 10 −0.75V R /AΩ in the low velocity regime and 0.05 (V R /AΩ) −4 in the high velocity regime.

Abstract. We simulate numerically the surface flow of a gas-supplying companion star in a semi-detached binary system. Calculations are carried out for a region including only the mass-losing star, thus not the mass accreting star. The equation of state is that of an ideal gas characterized by a specific heat ratio γ, and the case with γ = 5/3 is mainly studied. A system of eddies appears on the surface of the companion star: an eddy in the low pressure region near the L1 point, one around the high pressure at the north pole, and one or two eddies around the low pressure at the opposite side of the L1 point. Gas elements starting near the pole region rotate clockwise around the north pole (here the binary system rotates counter-clockwise as seen from the north pole). Because of viscosity, the gas drifts to the equatorial region, switches to the counter-clockwise eddy near the L1 point and flows through the L1 point to finally form the L1 stream. The flow field in the L1 region and the structure of the L1 stream are also considered.

Numerical simulation of the hydrodynamic behavior of an accretion disk in a close binary system is reported. Calculations were carried out for a region including a compact star and its gas-supplying companion. The equation of state is that of an ideal gas characterized by a specific heat ratio γ. Two cases, with γ = 1.01 and γ = 1.2, are studied. Our calculations show that the gas, flowing from the companion via a Lagrangian L1 point towards the accretion disk, forms a fine gas beam (L1 stream), which penetrates into the disk. Thus, no hot spot forms in these calculations. Another result is that the gas rotating with the disk forms -upon collision with the L1 stream-a bow shock wave, which we call an 'L1 shock'. The disk becomes hot because the L1 shock heats the disk gas in the outer parts of the disk, so that the spiral shocks wind loosely, even with γ = 1.01. The L1 shock enhances axial asymmetry of the density distribution in the disk, and therefore angular momentum is transferred through the tidal torque more effectively. The maximum value of the effective α becomes ∼ 0.3. A 'hot spot' is not formed in our simulations, but our results suggest the formation of a 'hot line', which is the L1 shock elongated along the penetrating L1 stream.

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Abstract. We present the results of a large international spectroscopic campaign on the δ Scuti star BN Cnc. Combining observations from five observatories taken over more than two weeks, we calculate line indices of the Hα line. A line index is the integrated line flux in a software filter divided by the continuum flux. We demonstrate that this can be used in combination with simultaneous photometry to classify the oscillation modes. We recover all the frequencies also found from photometry and assign likely mode identifications, which differ slightly from previously published values, but are found to be consistent with simple models. The difference in identification is found to have very little effect on the derived luminosity and temperature.