The Photometry And Study Of W UMa-type Contact Binaries

The W UMa-type binary is a kind of overcontact binary where both components fill or exceed their critical Roche Lobe and share a common convection envolope. They have the dumbbell configuration which makes two companions eclipsing easily, showing a continuous change of the light curves and two almost equal light levels at the primary and secondary eclipsing moments. At the same time, due to the formation of common convection envelope, many complex physical processes, such as matter and energy exchange between two component stars and the mass, energy and angular momentum loss via the outer Lagrangian point, can occur. Because of their unique geometric structure and the complex physical processes, the W UMa-type contact binary have become the important observational targets which have attracted a wide attention and have been intensitely investigated. In this thesis, the periodic variation behaviors of a group of W UMa-type contact binaries have been investigated in detail, and the structural characteristics and evolutional states of several sample stars have been studied. The main results are summarized as follows:1. The orbital period investigations of the W UMa-type contact binary YY CrB suggest that its orbital period is either undergoing a continuous period decrease on which a periodic oscillation is superposed, or encountered a sudden period decrease 20 years ago. The long-term period decrease might be interpreted as the result of either mass transfer from the primary component to the secondary one or mass loss via stellar wind from the system. The sinusoidal oscillatory should be due to the magnetic activity cycles of the system, rather than the light-time effect of the third body. In addition, the abrupt period decrease might be due to the sudden mass transfer between the components. If the orbital period decrase is true, it would evolve into a single, rapid-rotation star due to the shrinking of the inner and outer critical Roche lobes.2. The orbital period changes of two deep, low mass-ratio overcontact binaries: V728 Her and AH Aur, have been investigated. It is revealed that the orbital period of V728 Her exhibits a secular increase, while the orbital period of AH Aur shows a long-term decrease. The secular period changes suggest that V728 Her and AH Aur are undergoing rapid mass transfer between the primary star and secondary companion. According to the merge mechanism and the photometric features of the blue straggler and deep, low mass-ratio overcontact binaries, both V728 Her and AH Aur might finally evolve into such single, rapid-rotation star as blue straggler and FK Com-type star.3. Based on the new CCD photometric observations of V508 Oph, we obtained six new times of light minimum and found that its light curves exhibit the significant asymmetry. At the same time, our photometric solutions suggested V508 Oph is a W UMa-type shallow contact binary with the mass ratio of q ?0.552 and the degree of contact of f ?15.4%, and the asymmetric light curves can be modeled by a hot spot on the surface of the secondary component. In addition, the investigation of its orbital period revealed that the orbital period may be undergoing multiple changes: a continuous period decrease and a small periodic variation. The multiple changes in its period indicate that there may exist cyclic magnetic activity, mass transfer between its two components, and the third body moving around the system.4. According to the first photometric study of V737 Per, we found it is a W-subtype shallow contact W UMa-type binary, and that the light curves show significant asymmetry which can be due to the presence of a cool spot on the secondary component of the system. In addition, we firstly determined its mass ratio, and the degree of contact. Currently, we can not say that the orbital period of V737 Per exist any variation because its all available times of light minimum spread over a short time interval and the corresponding O-C curve do not show any general trend for changes. Thus, to determine its evolutionary status, the long-term and depth photometric observations are required in the future.5. It has been revealed that the orbital period of the W UMa-type shallow contact binary YY Eri exist a long-term increase attached a small-amplitude cyclic oscillation. The continuous period increase may be rooted in the mass transfer from the less massive companion to the more massive one, and the periodic variation of its orbital period may be attributed to either the light-time effect of a faint third star or the magnetic activity cyclic of the system. Based on the analysis of long-term light level variation, the periodic period change is very likely caused by the cyclic magnetic activity.