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V Sge: a Hot, Peculiar Binary System

Smak J., Belczyński K., Zoła S.

Acta Astronomica

2001

Vol. 51, No. 2

117--150

Five sets of mean UBV light curves of V Sge covering 2 mag of its large scale variations are analyzed. The mass ratio adopted in the analysis q=M2/M1=3.76 is that obtained by Herbig et al. (1965) from radial velocity curves based on fluorescent O III lines (arising from the surfaces of the two components). Models with an accretion disk around the white dwarf primary (or a very massive neutron star secondary) fail completely to reproduce the shapes of the observed light curves. Successful solutions are obtained with a model involving contact configuration, modified by the presence of a hot, gaseous envelope (needed to explain the behavior of colors and the variable depth of the primary eclipse). At inclination i≈71° the resulting masses of the components are: M1≈0.9 Msolar and M2≈3.3 Msolar. In the faintest state the secondary is a main sequence star with R2≈1.2 Rsolar and T2≈12 000 K, while the main parameters of the primary are: R1≈2.1 Rsolar, T1≈70 000 K, and L1≈1×1038 erg/s. Due to the high radiation pressure from the primary an expanding gaseous envelope is formed, leading to the mass outflow from the system. Large scale variations involve significant increase of the temperatures of both components, up to about 140 000 K for the primary and about 50 000 K for the secondary, and a considerable thickening of the gaseous envelope, which contributes up to 20-30% of the total UBV flux. These variations are interpreted as being due - in part - to the instability and large variations in the rate of mass outflow from the secondary. No obvious explanation, however, is offered for the major increase of the temperature and luminosity of the primary component in the brightest state. The temperature of the primary in the faint and intermediate states (T1≈70 000 K) is too low to explain the supersoft X-ray flux (observed only during those states), the only alternative being that it must come from the envelope surrounding the two stellar components. Such a hypothesis can also explain the origin of the O III and O VI lines. The distance and interstellar reddening, resulting from the solution, are d=4 kpc and EB-V≈0.30-0.36 mag. The far ultraviolet fluxes, calculated with model parameters obtained from the solution, do not agree with the observed IUE fluxes, corrected for interstellar extinction using the standard extinction law. The agreement becomes satisfactory, however, when arbitrarily chosen examples of non-standard extinction curves are used instead.

On Light Curves Modeling of Low Inclination Binary Systems with Accretion Disks

Zoła S.

Acta Astronomica

1998

Vol. 48, No. 2

373--382

Computer generated light curves of a low inclination, semidetached binary system containing an accretion disk, were analyzed with the Wilson-Devinney code (which does not account for the disk effects), to search for the best solution. The following inclinations were considered: 77°.4, 75°.0 and 60°.0 as well as three different disk contributions to the total light: 2%, 15% and 30%. For the disk contribution being small (2%), the resulting configuration agreed with the input one. For higher disk light detached geometry was derived. The parameters obtained from modeling greatly differ from the input ones and the fits are quite good.