Observation And Study On Deep Low-mass Ratio Contact Binaries

The deep, low-mass ratio overcontact binary is a system whose mass ratio is lower than 0.25 and whose degree of overcontact is higher than 50%. In this paper, deep, low-mass ratio overcontact binaries were observed, analyzed and studied statistically. Meanwhile, the observational and theoretical studies for overcontact binary are also summarized briefly. Through photometrically analyzing nine deep low-mass ratio contact binaries and statistically studying 27 those systems, the results are obtained as follow:1. In photometric solutions, it is discovered that V857 Her is a binary with the lowest mass-ratio of q = 0.065 and that GSC 0619-0232 is a binary with the highest degree of overcontact degree of f = 91.7% up to now. The photometric analysis of V410 Aur has identified the existence of the third body discovered from spectroscopic observations. For four systems (i.e., FG Hya, IK Per, GSC 0619-0232 and V410 Aur), the asymmetric or distorted light curves are fitted fairly well by the dark-spot model. The variations of light curve for FG Hya may be attributed to the cyclic magnetic activity. Combining the photometric solutions with spectroscopic elements for five systems (i.e., FG Hya, GR Vir, TV Mus, V410 Aur and XY Boo), their absolute parameters were determined.2. Analyzing orbital periods for three binaries (i.e., V410 Aur, XY Boo and V857 Her), it is found that their orbital periods show secularly increasing. Their period increase rates are +8.22 × 10^-7dyr^-1 for V410 Aur, +2.90× 10^-7dyr^-1 for V857 Her and +6.25 × 10^-7dyr^-1 for XY Boo, respectively. For each of V410 Aur and XY Boo, the mass transfer timescale is always larger than the thermal timescale due to much large mass transfer rate, implicating that the mass transfer from the secondary component to the primary one is enough larger to result in the orbital period increase without mass loss from the binary system.3. For five overcontact binaries (i.e., CU Tau, FG Hya, GR Vir, IK Per, TV Mus), their orbital periods seem to display long-term decreases. Their period decrease rates are -1.81 × 10^-6dyr^-1 for CU Tau, -4.32 × 10^-7dyr^-1 for FG Hya...