ISSN 1063-7729, Astronomy Reports, 2008, Vol. 52, No. 4, pp. 318–326. c  Pleiades Publishing, Ltd., 2008. Original Russian Text c  D.V. Bisikalo, D.A. Kononov, P.V. Kaigorodov, A.G. Zhilkin, A.A. Boyarchuk, 2008, published in Astronomicheski˘ ı Zhurnal, 2008, Vol. 85, No. 4, pp. 356–365. The Matter-Flow Structure in the SS Cyg System in Its Quiescent State from Comparisons of Observational and Synthetic Doppler Tomograms D. V. Bisikalo 1 , D. A. Kononov 1 , P.V. Kaigorodov 1 , A. G. Zhilkin 1, 2 , and A. A. Boyarchuk 1 1 Institute of Astronomy, Russian Academy of Sciences, Pyatnitskaya ul. 48, Moscow, 109017 Russia 2 Chelyabinsk State University, ul. Brat’ev Kashirinykh 129, Chelyabinsk, 454021 Russia Received October 11, 2007; in final form, October 26, 2007 Abstract—Doppler tomograms are constructed for the quiescent state of the SS Cyg system based on Hβ and Hγ spectral-line observations carried out in August 2006 with the 2-m telescope at Terskol Peak. Gas- dynamical simulations combined with the Doppler tomograms enable identification of the main features of the flow. Comparisons of synthetic tomograms with observations indicate that an accretion disk is present in the quiescent system. In the tomograms, the luminosity is maximum at the arms of the spiral tidal shock at the shock front due to the interaction between the gas of the circum-binary envelope and material in the stream issuing from the Lagrangian point L 1 (the “hot line”), and in the region behind the bow shock due to the motion of the accretor and disk in the gas of the envelope. The contribution of this last element results in appreciable asymmetry of the tomograms. PACS numbers: 97.80.Gm, 97.30.Qt, 95.85.Kr, 97.10.Fy, 97.10.Gz DOI: 10.1134/S1063772908040069 1. INTRODUCTION SS Cyg is among the brightest dwarf novae in the Northern sky, and has been observed for more than a century. The components of the semi-detached SS Cyg binary are an K(4–5)V red dwarf with a mass of ∼0.56 M ⊙ and a radius of ∼0.68 R ⊙ and a white dwarf with a mass of ∼0.97 M ⊙ and a radius of ∼0.007 R ⊙ . The red dwarf loses matte at a rate of ∼10 −9 −10 −8 M ⊙ per year. The component separa- tion is 2.05 R ⊙ , and the orbital period of the system is 6.6 hr. A wealth of information on this system has been gathered over more than a century of observa- tions. However, many issues concerning its physi- cal properties remain unresolved. In accordance with the morphological features of SS Cyg, it has been assigned to a subclass of U Gem stars. However, there are several observational reasons for classifying SS Cyg as an intermediate polar star with a magnetic field from ∼3 × 10 4 −3 × 10 5 G [1] to ∼10 6 G [2]. Since the presence of this magnetic field substantially affects the flow, information on the flow structure will be advantageous for our understanding of the physical processes in the system. Information about the basic features of the gas- dynamical flow pattern can be obtained using Doppler-tomography techniques [3]. This method transforms the orbital variability of emission-line intensities into luminosity maps in two-dimensional velocity space (V x , V y ). In some cases, the resulting Doppler maps can be interpreted more straightfor- wardly than the original spectrograms; in addition, the tomogram can indicate (or at least give some idea of) some characteristic features of the matter- flow structure. In particular, lines with double-peaked profiles, corresponding to circular motions (e.g., in an accretion disk), are transformed into a blurred ring in the Doppler map. In other words, the components of a binary can be resolved in velocity space, while they cannot be spatially resolved by direct obser- vations; therefore, the Doppler-tomography tech- nique is a powerful tool for investigating binaries. Unfortunately, the problem of reconstructing the spatial distributions of emission-line intensities from a Doppler map does not have a general solution, since points that are very distant from one another can have the same velocities and contribute to the same location in the Doppler map. Thus, the transform I (V x ,V y ) → I (x, y) is not possible unless some a priori assumptions about the velocity-field structure are made. Gas-dynamical computations can be used to find a solution to this problem. The computed fields of the density, ρ(x, y), and temperature, T (x, y), can be used to obtain the emission-intensity distribution. 318