Observations And Investigations Of Southern Sky Contact Binaries At Different Stages Of Evolution

Close binaries are composed of two component stars,where one star's evolution is affected by the other star in the binary system.There are many classifications of close binaries,one is morphology by Roche model:(1)detached system,(2)semi-detached system and(3)contact system.Contact binaries are a special type of close binary stars where both components are filling the critical Roche lobe and are sharing a common envelope.Their formation and evolution are unsolved questions in astrophysics.Contact binaries in the northern hemisphere are usually well observed and investigated because telescopes are mainly located in this hemisphere.On the other hand,contact binary systems in southern hemisphere are usually ignored because the lack of telescopes.In this thesis,several neglected contact binaries(e.g.TY Pup,RW Dor,YZ Phe,AV Pup and AY Pup)in southern hemisphere were investigated.New photometric data were obtained by using the 60-cm telescope(PROMPT-8)at CTIO in Chile,the 1-m and the 60-cm telescopes at Yunnan Observatories in China.By analyzing the light curves and the O-C curves,their structures,evolutionary states,magnetic activities and triplicities are investigated.The main research results are as following:1.It is found that TY Pup is a low-mass ratio(0.184)and deep-contact binary with a high fill-out factor of 84.3 %.These results suggest that the binary may be at the end evolutionary stage of contact binaries.It will merge into a single rapidly rotating star and may produce a luminous red nova during merging process which is similar to the case of V1309 Sco.Meanwhile,a close-in third body is detected to be orbiting the deep contact binary star that may play an important role for the formation and evolution of the central binary.2.Two sets of complete multi-color light curves were obtained for RW Dor.It is found that the binary is a shallow-contact binary with a fill-out factor about 11 % and a high mass ratio(> 0.4).Meanwhile,the orbital period of the binary is detected to be decreasing while it undergoes a small-amplitude cyclic variation.Both the shallowcontact configuration and the long-term period decrease indicate that it is a newly formed contact binary via Case A mass transfer and it will evolve into a deeper normal contact binary.The small-amplitude cyclic variation reveals that the third body may be a brown dwarf candidate.This will provide us more information on the formation and evolution of substellar objects.3.As for YZ Phe,several sets of complete multi-color light curves were obtained.By combining those photometric data obtained by several surveys,it is discovered that the light curve of YZ Phe shows a cyclic change with period of 1532.78 days(or 4.20yr).This indicates the presence of a magnetic activity cycle similar to that of our Sun.The variation of the O'Connell effect was explained as the activity of dark spot on photosphere of one or both components with a period of 466.16 days(or 1.28 yr).As those of RW Dor,the period of YZ Phe also shows a combination of a long-term decrease and a cyclic change with a period of 40.76 years and a semi-amplitude of 0.0081 days.The cyclic variation reveals the presence of a cool third body in the system.4.New photometric data were obtained for the two neglected binaries AV Pup and AY Pup with similar orbital periods.By analyzing their light curves,it is found both of them are W-type contact binaries.AV Pup is a shallow-contact system with a mass ratio of 0.853,while AY Pup is an intermediate contact binary with a mass ratio of 0.43.At the same time,the period of AV Pup is increasing,while that of AY Pup is decreasing.These observational properties suggest that AV Pup and AY Pup may be on different evolutionary stages,i.e.,AV Pup is on the TRO-controlled stage,while AY Pup is on the AML-controlled stage.As in the cases of those targets aforementioned,the periods of the two binaries may also show cyclic variations that could be interpreted as the presence of third bodies.All of the sample that we investigated may have third bodies indicating that tertiary components may be common for contact binary systems.