The Observation And Investigation On AF-type Contact Binary

The AF-type contact binary is a contact binary system with the spectral type of one component star to be A-type or F-type. AF-type contact binary is the bridge connecting early type and late type contact binary, which takes a very important role in contact binary system. AF-type contact binary has a very short period, usually less than 1 day, and it is a quite bright object in the universe, which makes it to be observed easily. In the present work,we have obtained the light curves and times of light minimum for five F-type contact binaries(V502 Oph,MQ UMa,II UMa,NO Cam,V776 Cas), an A-type contact binary(V868 Mon) and an A-type marginal contact binary(V921 Her)using the 60 cm and 1m reflecting telescopes in Yunnan Observatories and the85 cm reflecting telescopes at Xinglong Observation Base. The W- D program and the O- C method are used to analyze the observational data to get parameters of the binary system, information about interaction between the binary system and the possible tertiary component around the binary.1. It is found that MQ UMa, II UMa and V776 Cas are high fill-out, low mass ratio overcontact binaries, which may be the progenitors of blue straggles and other special objects. According to our statistics, AF-type stars have a very high rate of formation in high fill-out, low mass ratio overcontact binary. They are ideal targets to research the mechanism of coalescence in binary systems;2. Only the late F-type system, V502 Oph, shows asymmetry in its light curves and a spot model is used to deal with it. It may imply that the magnetic activity on the surface of AF-type contact binary are suppressed comparing with the late type contact binaries;3. Combining the photometric and spectroscopic analysis together, the absolute physical parameters of the the components in binary system can be determined. We collect all AF-type contact binaries that absolute physical parameters have been obtained. Then, the initial mass of the present primary and secondary stars are calculated according to their present masses and luminosity. It is found that A-subtype and W-subtype systems have different initial masses. Binaries with larger total masses(about 3.1M) will evolve to A-subtype systems and binaries with total masses about 2.5M will evolve to W-subtype systems. The initial more massive star will evolve to fill its critical Roche lobe and transfer masses to the initial less massive star, and the mass ratio will inverse. About30 % to 50 % of the mass loss from the initial more massive star will be gained by the initial less massive star, other masses will run away from the binary system via the magnetic stellar wind.4. It is confirmed that V502 Oph, MQ UMa, NO Cam, V776 Cas and V921 Her have tertiary components around them. The light travel time effect appearing in the O- C curve and the existence of third light in light curve analysis are the main methods to detect the tertiary components. And the light travel time effect method may be much more reliable. It is found that over 60 %of AF-type contact binary systems have tertiary components, which may imply that tertiary components are very common in AF-type contact binaries. The tertiary component around NO Cam is less than 2AU. It is the closest tertiary component in AF-type contact binaries.5. The long term period variation, both increase and decrease, reveals the interaction between the two components in binary system. In our targets, the period of V502 Oph shows long term decrease and the period of II UMa, NO Cam, V868 Mon and V921 Her show long term increase. A sample of 23 AF-type contact binaries which have long term variations are collected for the research in the future.