A Study of Ten Early-type Contact Binary Stars in the Small Magellanic Cloud

To study early-type binary systems in different evolutionary stages or environments, the Small Magellanic Cloud (SMC) is an ideal laboratory due to its low metallicity compared to that of the Milky Way. We conduct a study on the period changes of the close binary systems with B-type spectral classifications in the SMC using OGLE collections. Ten B-type binaries that show no significant periodic variations based on the current observational data were analyzed. Through O - C analysis, four of the ten early-type binaries show a long-term period decrease, one shows a long-term period increase, and the other five systems are ambiguous due to the limitations of the observational data. Among the period-decreasing systems, two may be mainly caused by mass transfer, while the rest may be caused by angular momentum loss. The Wilson-Devinney code (W-D method) is used to analyze their I-band photometric light curves. According to the W-D results, five early-type binaries are in the deep contact state, three are in the shallow contact state, while two are in the medium contact state, and the temperature ratios of these early-type binaries are all close to unity. The five deep contact binaries are highly unstable systems and therefore serve as important objects for the study of binary mergers. Finally, a discussion on the evolution of early-type binary stars is conducted by combining the analyses of the light curves and the periodic changes of ten early-type binaries, that implies that the majority of early-type binaries in the SMC may form contact binaries from a phase of rapid mass transfer.