Investigations Into Contact Binaries On The Post-main Sequence

Post-main sequence contact binaries(hereinafter called ”PSCBs”)refer to a kind of contact systems with one or two components evolved out of MS.In observations,they could have relatively low [Fe/H] and log(g).Compared with contact systems in MS(hereinafter called ”MSCBs”),PSCBs formed in early stages of contact might exhibit some weird behaviours(e.g.,deep contact,ultrashort orbital period)owing to the thermal instability on components.In addition,MS stage takes up most of the stellar lifetime,which means that PSCBs might be closer to the final merger.Hence,PSCBs formed in late stages of contact might be considered as ancestors of merged single stars,which could be provided to the merging models as the observational restrictions.In this thesis,we start from the general contact binaries and introduce the related research background,which is followed by the research significance of PSCBs.Then,we enter into the part of observational research,and introduce the possible observational characteristics of PSCBs and the photometric solutions to a candidate of PSCBs — LINEAR 199576.Finally,we go deep into two parts of theoretical research,which are related to the period changes and the thermal non-equilibrium of PSCBs,respectively.Major conclusions are as follows:1.Based upon the possible observational characteristics of PSCBs,LINEAR199576 is thought as a candidate of PSCBs.Through the photometric solutions,mass ratio and fill-out factor are determined as q = 0.374 and f = 10.0%,respectively.If this target truely in post-main sequence,components of MSCBs should have the ability to evolve into post-main sequence even in late stages of contact,and strong oscillations of common envelopes should also exist,which makes PSCBs fall into shallow contact configuration.2.According to Roche geometry and Kepler's third law,variations in fill-out factors f of MSCBs with high mass ratios should have great effects on orbital period P.Because of f representing the thickness of common envelope,this kind of effects should be attributed to the mass transfer between common envelope and components.Accordingly,a new mechanism on long-term period variations is proposed.In this mechanism,quasi-periodic oscillations of period influenced by fill-out factor occur on similar thermal timescales.As a result of this kind of oscillations,MSCBs with high mass ratios could be found in deep contact configuration,which corresponds to two rare samples in observations — NSVS 925605 and 1SWASP J075102.16+342405.3.3.In the cyclic evolution of a contact system,difference between thermal timescales of two components makes the thermal equilibrium of the whole contact system hardly reached.However,the real system still tries to approach the nonexistent thermal equilibrium,which eventually provides the driving force for the cyclic evolution.Through the investigations into the thermal non-equilibrium of MSCBs in current state,we find that the higher the mass ratio q,the weaker the deviation from thermal equilibrium.After an equation about energy conservation is introduced,it is found that MSCBs with high mass ratios might not enter into the broken-contact phase of TRO owing the lack of driving force of cyclic evolution;besides,it is almost impossible for this kind of systems to evolve into deep contact configuration.Hence,samples NSVS 925605 and 1SWASP J075102.16+342405.3mentioned above are exactly the PSCBs formed in early stages of contact.