| 其他摘要 | From the discovery of CC Com in 1967, which has the shortest period among contact binaries, during nearly half a century, contact binaries of W UMa type show a very well-defined short-period cut-off at about 0.22 days. This means, observationally, contact binaries have a shortest-period at 0.22 days, and no Mtype contact binaries have been found during this period. However, the advent of large-scale photometric surveys have led to the discovery of large number of contact binaries candidates with periods around 0.22 days. In this paper, based on the short-period contact binaries candidates found by the SDSS and SuperWASP, we present the follow-up detailed photometric studies of some samples, including two M-type contact binaries candidates and three late K-type contact binaries candidates. Meanwhile, we have collected all the well-studied contact binaries that have periods around 0.22 days, combined the results this paper obtained to conduct a statistical study. Specifically, the results are obtained as follows: 1. The period change study and the light curves analysis for the M-type contact binary candidates SDSS J001641-000925 and SDSS J012119.10-001949.9 are presented respectively. The period change study suggests that the observed minus calculated (O?C) diagram shows a cyclic oscillation with an amplitude of 0.00255 days and a period of 5.7 yr. The cyclic variation can be explained by the light travel time effect via the presence of a cool stellar companion with a mass of 0.14 M⊙. These results reveal that SDSS J001641-000925 is the first stable red dwarf contact binary, while the rapid decrease in the orbital period is not true. The light curve analysis of SDSS J012119.10-001949.9 suggests that the binary is in contact configuration with a moderate mass ratio. A third light contributing parts of the total luminosity was found, and the contribution of monochromatic third light increases with wavelength. This indicates that J012119.10-001949.9 is a triple system, and the third light comes from an additional cool component. 2. We present the first photometric study and the period change analysis of two late K-type contact binary candidates 1SWASP J074658.62+224448.5 and 1SWASP J075102.16+342405.3. It is found that 1SWASP J074658.62+224448.5 is a shallow contact system with a low mass ratio, while 1SWASP J075102.16+342405.3 is a deep overcontact system with a high mass ratio. The period change analysis suggests that the orbital period of both systems are undergoing increase at a rate of 10?7d.yr?1. Since this high rates of period change corresponds to very a large mass transfer rates, which may be physically impossible, we prefer to deem that the period increases of both systems are part of the cyclic variations due to the presence of a tertiary component, which means that these two systems may be triple systems. 3. Another late K-type contact binary candidate 1SWASP J024148.62+372848.3 was also studied with light curves solutions and period change analysis. Due to the large spread of the data used in the period analysis, the O?C curve is not enough to reveal any variations. The light curves solutions suggest that 1SWASP J024148.62+372848.3 is a middle contact system with a high mass ratio, and the two components are found to have large dark-spot coverage. The solutions also show that the third light contributes about 40% of the total luminosity, while the component with higher temperature contributes less luminosity than the component with lower temperature, this may be caused by the larger dark-spot coverage of the component with higher temperature. So, we deem that 1SWASP J024148.62+372848.3 may be a triple system, and due to the large dark-spot coverage of the two components, the contribution of the third light are relatively increased. 4. We have collected all the well-studied contact binaries that have periods less than 0.23 days. A total of twenty contact binaries were obtained, including five M-type and fifteen late K-type. It was found that most of contact binary systems are in shallow contact. More than 75% systems have evidences to proof that the third component exists in these systems, and others have circumstantial evidences to indicate it may be a triple systems. This suggests that the third components are ubiquitous in late-type close binary systems. The light curves of most of these samples show obvious asymmetry, this again signs that these late type close binaries are very active, the surfaces of the two components may exist strong magnetic field and intense magnetic activities. |
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