Photometric Study Of Eclipsing Cataclysmic Variable Stars

Cataclysmic Variable Stars (CVs) are interactive semi-contact binary systems, which consist of a filled (K/G/M-type) main sequence dwarf or giant and an evolved white dwarf. During the process that materials are transferred from main sequence dwarf to white dwarf, the material stream can not fall on the surface of white dwarf, but go round the white dwarf due to its large angular momentum, and extends to a disk because of viscosity and friction. When the new stream hits the disk, a hot area on the rim of disk can be produced with high luminosity - hot spot. Thus, a physical image for CVs can be constructed by the white dwarf, accretion disk, hot spot and red dwarf. Except for polar (a magnetic subtype of CV), almost all observed characters of CVs are from the physical processes and interaction between all parts of CVs. In recent 20 years, the studies for orbital period changes have been neglected, which is caused by the invalid results from the low precision of photometric data and the observational difficulty for the faint CVs. On the other hand, the spectroscopic data are also disappointed because CVs include many light sources which are hardly distinguished and cause badly pollution, deviation and broadening in spectrum and even stem from the accurate analysis of radial velocity curve. Therefore, in order to make a valid study at present, the photometric or spectroscopic data with high precision are needed. Using the telescopes in China, this paper makes photometric observations for the analysis of orbital period changes of the eclipsing CVs. At first, a complete catalogue of eclipsing CVs including 108 objects are obtained via lots of investigations and statistics. Then, Based on this catalogue, ten representative eclipsing CVs (Novae DQ Her and T Aur, Dwarf Novae U Gem, Z Cha and V2051 Oph, Novae-like AC Cnc, V363 Aur, RW Tri, SW Sex and UU Aqr) are observed and analyzed. At the same time, for the better understand of CVs, this paper still investigates the eclipsing pre-CVs in detail, and makes an analysis of orbital period changes and absolute parameters for one of important object - HW Vir. Based on the works of analysis of orbital period changes and statistics for the 11 samples, we obtain three conclusions as follows:1. According to the studys of orbital period changes on two SW Sex-type stars - SW Sex and UU Aqr, which locate in the range of 3h≤P≤4.5h, we derive that the orbital period of both objects are increasing. And the orbital periods of another two non-SW Sex-type stars - DQ Her and U Gem, which locate in the same range, also present secular increase. Although the orbital period increase of the latter can be explained by another mechanisms, they are still considered as the results of enhanced mass transfer as SW Sex-type stars. However, this disagrees with the requirement of the disturbed magnetic braking mechanism, which the evolution of CVs should be toward the shorter orbital period. This conclusion is well preparation for the continuing works.2. We discuss the orbital period changes for Pre-CVs HW Vir, Novae-like AC Cnc, V363 Aur and RW Tri and Dwarf Novae V2051 Oph, which almost cover the complete evolution of CVs. All these five objects present their orbital periods are decreasing. Moreover, all these five CVs are needed to introduce a valid mechanism of angular momentum loss, such as gravitational radiation and magnetic braking, because of q < 1. This conclusion is seemly agree with mechanism of disturbed magnetic braking, but the discussions and calculations for both objects HW Vir and V2051 Oph indicate that gravitational radiation can not make the observed period decreasing rate. This means that the prediction of disturbed magnetic braking that the evoluton of CVs with ultra-short orbital period should be dominated by gravitational radiation, is not corrected. Thus, we reconsider the effects of magnetic braking in the objects with ultra-short orbital period and deduce that magnetic braking can well interpret the observed period decreasing rate. This means the the assumption that magnetic braking would be stopped when the dwarf became complete convection (P～3h), is hard to be supported. Considering the many other works about the magnetic field and stellar wind of complete convection dwarf and our works presented in item 1, we regard that magnetic braking should naturally cross the orbital period gap, and never be disturbed. In addition, the newest distribution of orbital periods of CVs implies that the gap is slowly disappearing. Thus, if the gap indeed disappeared, then magnetic braking can naturally cross the gap and the magnetic braking would disappear when the complete convection star became brown dwarf at the end of evolution of CVs. This also is a nature explanation for the minimum period in distribution of orbital periods of CVs.3. When analyzing the orbital period changes of 11 objects, we detect that the O-C diagrams of 9 objects has cyclical period changes. Thus, two plausible mechanisms - Applegate mechanism (the solar-type magnetic activities) and the light travel-time effect are discussed in detail. Since all 9 CVs present a same conclusion that Applegate mechanism requires too large energy to afford by dwarf, we conclude that the latter is a better interpretation for the cyclical period changes in O-C diagram. Based on this argument, lots of brown dwarfs including massive exterior-planets are detected in CVs situation. Since CVs are low mass binaries, we predict that more and more brown dwarf like faint stars can be found by using the analysis method of orbital period changes in future.