| 其他摘要 | A contact binary is a strongly interacting binary system with both components filling the Roche lobes and share a common envelope. As we know, the evolution of a single star is already very complicated. The formation and evolution of a contact binary becomes more complicated because of the special configuration. They are also excellent astrophysical laboratories for studying some special astronomical phenomena, such as the merger of binary stars, the formation of rapidly rotating single stars, and the bright red nova outbursts. Therefore, it is very important to study the contact binaries. In this paper, the photometric observation of contact binaries is briefly introduced. Physical parameters of contact binaries can be obtained through the photometric study. Then, some special properties of contact binaries are discussed in detail through the statistical analyses of a large number of samples. The results are as follows: 1. The photometric solution of CSS J075415.6+191052 suggests that it is a low mass ratio (𝑞 = 0.178), A-subtype contact binary with 𝑓 = 34.9%. For R and I wave bands, the light curves show weakening around the left shoulder of secondary minimum, however, the light curves of B and V bands are totally symmetric. It is unreasonable if the dark spot is caused by magnetic activity or mass transfers between two components. Therefore, a possible explanation is mass transferring from primary component to com mon convective envelope through the inner Lagrangian point, and this part of the mass, for some reason, weakens R and I bands of light from secondary component. If it is true, according to the conservation of angular momentum, the transferred mass should appear around 0.25 to 0.5 phase. This is in a great agreement with light curves, which makes this assumption credible. 2. Based on the photometric study of NW Leo, it is concluded that it is a high mass ratio (𝑞 = 0.707), shallow contact binary (𝑓 = 2.3%). Its 𝑂 − 𝐶 curve shows periodic changes, and the modulation period is only about 4.7 years. The cyclic oscillation may be due to the light travel effect of a third body whose mass is calculated to be about 0.48 𝑀⊙. The 𝑂 − 𝐶 trend also indicates that there is a long-term decrease in its orbital period, which may be caused by the mass transfer from the primary to the secondary component. However, the number of current minima is still limited, and the long-term decreasing trend is not obvious. Therefore, more observations are needed to check this trend. 3. Secondary components of contact binaries (CBs) have many special properties, two of them are excess in radius and luminosity. By comparing the radial density distribution between secondary components of CBs and main-sequence stars, we find that there is a great difference between the two subtypes of CBs. The secondaries of A-subtype CBs are evolved from stars have initial masses higher than 1.8 𝑀⊙ and experience mass ratio reverse. Therefore, the secondary components of the A-subtype CBs are no longer the main sequence stars, and their expansion and overluminosity are mainly due to evolution. In W-subtype CBs, there is a linear relationship between the luminosity loss of the primary components and the overluminosity of the secondary components, which indicates that the energy transfer occurs between the two components. According to thermal relaxation oscillation (TRO) theory, the energy transfer will cause the secondary component to expand and transfer mass to the primary, then the 𝑀 − 𝑅 relation of the secondary component will leave the main sequence. 4. The period cutoff is a very special property of CBs. It has been believed that the period cutoff of CBs is 0.22 days. The relationship between orbital period and mass ratio is derived by studying the relationship among four physical parameters (orbital period 𝑃 , mass ratio 𝑞, mass of primary star 𝑀1 and the seperation between two components a). The lower limit of the orbital period (0.15 days) is obtained under the restriction of the mass ratio. This result is also supported by observation. In addition, we find that the distributions of high-mass ratio and low-mass ratio CBs in the 𝑃 −𝐽′ 𝑜𝑟𝑏 diagram are quite different. Further study shows that high mass ratio CBs follow TRO theory, while low mass ratio CBs follow angular momentum loss (AML) theory. The results indicate that TRO and AML may play important roles in the evolution of CBs. With the decrease of mass ratio, AML gradually takes the dominant position, which promotes the merging of the contact binaries. 5. Several methods for calculating the critical mass ratio under different conditions are discussed. By using these methods, we can estimate the emerging mass ratio of CBs. However, the mass ratios of several CBs are already less than the calculated critical mass ratios. We think the reason is that the influence of the differential rotation and other factors on the dimensionless radius of stars is not taken into account, 𝜂𝑘2 instead of 𝑘2 should be more reasonable. Generally, 𝜂 < 1, take V1187 Her for example, its 𝜂 < 50%. Then, we also analyzed the distribution of the two subtypes CBs in the 𝑞 − 𝐽′ 𝑜𝑟𝑏 and 𝑞 − 𝑓 diagrams. The results show that the A-subtype and W-subtype CBs have different evolution modes, which supports the results analyzed in Chapter 5. |
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