Observational Evidence and Analysis of the Coupling between the Dynamical and the Thermodynamic Processes for Close Binaries

In the present paper, the analysis methods of the orbital period, the types of its variations, the mechanisms caused the changes in the orbital period, and problems and progresses in the orbital period study are introduced. The properties of the variations in the orbital period and in the light curve, and the connections between the changes of the orbital period and the light curve in different type of close binaries are also reviewed. The changes of the orbital period, the variations of the light curve and their connections in some sample stars are studied. The statistical relation of the parameters in contact binaries are also investigated. The main results are: 1. The orbital periods of three RS CVn-type binaries ERVul, UV Psc and AR Lac are analyzed. It is found that the orbital period of UV Psc oscillates with a period of 61 years; the orbital periods of ER Vul and AR Lac show periodic variations with periods of 31 years and 47 years respectively while their periods undergo secular decrease. The mechanisms that could explain the changes in the orbital periods of the three systems are studied. The variation in the orbital period of UV Psc is caused by the cyclical magnetic activity in the primary component; the cyclical magnetic activity in one or both of the components combined with the angular momentum loss via magnetic breaking can explain the changes in the orbital periods of ER Vul and AR Lac. Meanwhile, it is found that the mean breightness outside the eclipse of ER Vul may show periodic change with the same period (31 years) as its orbital period. This result may demonstrate that the coupling between the dynamical and the thermodynamic processes in the system is existed.2. It is found that the orbital periods of two Algol-type binaries RW Crb and RX Ari are decreased, and two sudden jumps in the orbital period of TU Her are also found while it is undergoing secular period decrease. The secondary component of Algol-type close binary is filling its critical Roche-lobe. The secular decreases in the orbital period of the three systems may demonstrate that they are undergoing secular mass and angular momentum loss.3. It is found that the orbital periods of 7 AF-type short-period near-contact binaries (STAqr, AV Hya, BF Vir, BF Vir, SW Lyn, BV Eri, RU Eri, and AV Hya,) are decreased. Combined with the collected decrease rate in the orbital period of other systems with same type the following statistical relation: dP/dt =-5.574×10^-7P+1.767×10^-7days/year is obtained. These results show that in the near contact stage, these systems may evolve into contact binary via the secular angular momentum loss (AML), the more orbital period is decreased, the more its decrease rate of change is small; In the contact stage, the angular momentum loss (AML) is mainly caused by the decrease of the mass ratio q via mass transfer from the secondary to the primary and the orbital period of the system will increase. These conclusions are satisfactorily coincided with the predictions of the angular momentum loss (AML)theory.4. It is found that the orbital period of the contact binary AU Ser shows a sudden decrease in the time interval 1987～1988, and several sudden jumps in the orbital period are also found. When compared the jumps in the orbital periods with the variations in the light curves of the two systems a possible connection is found, i.e., after each sudden change in the orbital period the light curve will change from the negative OConnell effect to the positive OConnell effect or vice versa.5. The orbital period of the contact binary CK Boo is analyzed. It is found that the orbital period of the system shows the periodic variation with period of 14 year while it undergoes a secular increase. When compared the change in the orbital period with the variation in the light curve, it is found that the brightness at the phases Min. I, Max. I and II show periodic variations with the same period (14 years)and in the same phase as that of the periodic component in the orbital period. This kind of connection is not the same as that in the RS CVn type and in Algol type close binaries. Whether this kind of relation is popular in contact binaries or not ?What is the mechanism caused this relation? Further study is needed to solve these problems.6. The M₁-P diagram of the contact binaries shows a strong linear correlation between the mass of primary and the orbital period. This is possibly caused by the secular mass loss from the primary component or the secular free mass transfer between the two components. The distribution of contact binaries in the diagrams J_orb-M₁, J_orb-M₂ and J_orb-P also show that the secular mass transfer in contact binaries is existent while they undergo secular mass and angular momentum loss. These results indicate that the evolution of contact binaries may be thermal relaxation oscillation (TRO)plus angular momentum loss (AML).This need further investigation.7. The energy transfer rates ΔL for 78 contact binaries are calculated. The following results are obtained. First, the strong correlation between the energy transfer rate ΔL and mass ratio q indicates that the energy transfer is correlation with Roche equipotential structure. When q =0.5, the structure can make the energy effectively transferred from the primary to secondary. Second, the ΔL-DT diagram shows the more increase the thermal degree DT is, the less the energy transfer rate ΔL is. When DT is increased to 0.2, the energy transfer is stopped, at this time the system is not in thermal contact. Third, the energy transfer rate in the W-type contact binary is usually larger than that in A-type. Finally, the correlation between the energy transfer rate ΔL and orbital period P and orbital angular momentum J_orb W-type contact binary are different from those in A-type contact binary. This indicates that the mechanism of mass transfer in W-type may be different from that in A-type.