A Photometric Study Of The Eclipsing Dwarf Novae

Dwarf novae are erupting cataclysmic variable stars in which a Roche-lobeflling late-type main sequence star(Secondary star) transfers matter to an accre-tion disc around a white dwarf(Primary star). The gas stream from the secondaryfeeds material into the disc at a point near its outer rim where a bright spot isformed. The existence of accretion disc makes it have complex physical changesand observed features, including the outburst of the frequent phenomenon andthe transitions from a high state to a low state, etc. It will greatly promotethe understanding to the theory of the accretion disc and the mass transfer tostudy these phenomenon. The eclipse provides an excellent opportunity to deter-mine the relative brightness of each light source, geometric position and physicalstructure. And the times of mid-eclipse also reveals that the orbital period ischanging. Therefore, the study of eclipsing dwarf novae is important and mean-ingful. In the present paper, the photometric observations of a typical eclipsingdwarf novae OY Car are observed and analyzed, and the parameters of the dwarfnovae were also studied statistically. Some results are obtained as follows:1、The orbital period variations of OY Car are analyzed. It is shown thatthe orbital period of OY Car is decreasing at the rate ofP˙=3.77(±0.15)×1012. Since the system consist of a less massive donor star and a more massivegainer star, we would expect the orbital period to increase rather than decrease.Gravitational radiation loss to explain the observed change. The Rappaport etal.(1983) prescription to calculate the orbital period change caused by magneticbraking are found to be able to explain the observed period decrease. However,according to the CV standard model, a donor star of0.07M⊙should not havemagnetic braking. It is therefore possible that the observed downward parabolicchange is simply a part of a long-period cyclic variation. On the other hand,recently, a new view that fully convective donors below the period gap are capableof generating signifcant magnetic felds and can produce MB strong enough。This idea is supported by some observational evidence, and our calculation is consistent with this possibility. These evidences pose a severe challenge to theCV standard model. In addition, a cyclic change with a small amplitude of27.8sand a period of14.0yr is detected. We attempted to interpret the a cyclic changewith the light travel-time efect due to an invisible third star in the system. Basedon the analysis of the light travel-time efect, we fnd that the third star may bea critical substellar object between brown dwarf and giant planet.2、The eclipsing light curves of OY Car are studied, the results reveal thatOY Car has the transitions from a high state to a low state in quiescence, whichis similar to the magnetic cataclysmic variable and nova-likes. We observed thatthe magnitude change amplitude between the highest state and the lowest stateis about1mag. From the light curve, we obtained the following information:hot spot is larger than the previous, and brighter(more faint) than before、theexpansion of Accretion disk、 ficking and the profle of boundary layer. Themain reason for the high state is the sudden increase of matter transfer rate andthe accretion discs is unstable, and the lower state is mainly caused by masstransfer rate suddenly drops. Starspots theory can well explain the variation ofmass transfer, a lower mass transfer rate may be refect starspots growing nearL1or form elsewhere and drift across it, a higher mass transfer is caused by fare.So far, only a few dwarf novae have been observed to have behavior resemblinghigh/low states.3、 The outburst of the frequent phenomenon have been observed from28November2008to14January2009, which should be a normal outburst. Andthe light curves on23November2009and24November2009are in the declinedstages of a superoutburst. Comparing with the AAVSO’s long-term observationresults of OY Car, it is found that our observations are in complete accord withthe results of the AAVSO.4、By collecting the latest cataclysmic variable star catalogs and online data, a latest dwarf novae catalogue is obtained. Based on this catalogue, we fndthat (1), the distribution of orbital periods of dwarf novae shows that the peri-od gap has a disappear trend. There is a gradual process in the23h range.The distribution of orbital period of eclipsing dwarf novae is consistent with the total distribution, so the orbital inclination should be random and uniform dis-tribution.(2), According to the statistical study of the relationship between thesecondary mass and the orbital period indicates that this relation is universallyapplicable.(3), Absolute magnitude at maximum of normal outbursts-orbitalperiod relation was verifed the statistical results of1987Warner.(4), The sta-tistical results of mass ratio-orbital relation and the secondary spectral classare consistent with the theoretical predictions and observations recently.