| 其他摘要 | Near Contact Binary(NCB) is a kind of close binary where both componentsfill or near fill their critical Roche Lobe. They are not in contact like W UMa systems, and show EB-type light variation. These special configurations make NCBs interesting. NCBs can be in the intermediate stage between detached or semi-detached state and contact state. They are the important observational targets which may be lying in key evolutionary states. In the present paper, a group of NCBs, a few W UMa and Algol systems were observed, analyzed and studied statistically. Meanwhile, we collected and studied available properties of light curves, orbital period variations and some absolute parameters of NCBs. Some results are obtained as follows: (1) We find that BL And is a semidetached system with the primary component filling its Roche lobe and the secondary one almost filling but still detached, and the orbital period of BL And is decreasing at the rate of dp/dt =-2.36 * 10^-8 d/yr. The derived configuration and secular period decrease for BL And combined with the asymmetry of the light curve indicate that this system may evolve from the present semidetached phase into a contact stage, with mass transfer from the primary component to the secondary one or that it might just undergo the broken stage predicted by the theory of thermal relaxation oscillations(TRO). (2) We find that GW Tau, UU Lyn and AS Ser are marginal-contact binary systems with a small degree of contact (f = 10.9%, 2.3%, 2.3%, respectively ) and a large temperature difference of about 3100 K, 1900 K, 2500 K, respectively.It is shown that the orbital periods of these three systems are decreasing. With the period decrease, GW Tau, UU Lyn and AS Ser will evolve into a contact binary systems with true thermal contact or may be at the beginning of the contact phase predicted by TRO. (3) RU UMi is found to be a semi-detached system with the secondary component filling its Roche lobe. While, the orbital period analysis shows thatthe orbital period of this system undergoes a continuous decrease at a rate of dp/dt =-1.72*10^-8 d/yr. Since the secondary component is filling the Roche lobe, the conservative mass transfer between the two components can not explain the present orbital period variation. The continuous period decrease may demonstrate that the system undergoes a secular mass and angular momentum loss, and it may evolve from the present short-period near-contact system into an A-type contact binary. Meanwhile, photometric solutions of two short-period Algol type binaries of BF Vel and DF Pup are derived. They are semi-detached systems with the secondary components filling their Roche lobe and primary components near filling with filling factors of 78% and 82% respectively. They may be the transitional systems between NCBs and Algols. (4) We find that GSC3658-0076 and KM UMa are marginal detached near contact binaries with the primary almost filling their Roche Lobe. Both of them may be at the beginning of the mass transfer phase and may evolve from the present detached systems into contact binaries. According to the mass-radius relation of unevolved (ZAMS) detached binaries, the primary component is more evolved for GSC3658-0076 and the secondary one is more evolved for KM UMa. (5) It is found that the periods of VZ Psc and HL Aur show small-amplitude cyclic oscillations, while the period of HL Aur undergoes a secular decrease at a rate of dp/dt = -1.08 * 10^-7 d/yr. The cyclic period oscillation can be interpreted either by the light-time effect of a third body or by magnetic activity cycles of the components. Since the third-body assumption is in agreement with the presence of a large amount of third light in the system discovered by Gray et al. (1997), we think that HL Aurigae may be a true triple system. And since both components of VZ Psc are strong chromospherically active, the period of this system may be more plausibly explained by cyclic magnetic activity. (6)We find that SS Com is an A-type W UMa contact binary with mass ratio of 0.29 and PP Lac is a W-type W UMa contact binary system with mass ratio of 2.3. Long-time period increases of SS Com is detached. With the period increase, SS Com will evolve toward extreme mass ratio and finally to coalesce to single rapidly rotating star. The period of PP Lac showed a small-amplitudecyclic oscillation with a period of 19.7 years and an amplitude of 0.0058 days, which can be explained as the light-time effect of an unseen third body. (7)It is discovered that NCBs with the orbital inclinations larger than 70 degree can be observed easily and most of the O'Connell effect existing in the light curves of NCBs are the result of the mass transfer between the components. (8) The orbital period variations of NCBs are investigated. It is shown that the period of all primary filling NCBs exhibit decreasing orbital periods with order of magnitude of 10^-7 to 10^-8 d/yr, and that of other type NCBs may be decreasing, increasing or exhibit more complex variations. Especially, the dP/dt - P relation of the secondary filling NCBs is agree well with that of Alogl systems with secular decreasing period. These systems are separated into two groups with period larger and smaller than 0.73 (0.24) days. The orbital evolution of systems with period larger than this value are dominated by the mass transfer between two components, while the evolution of the other group may be dominated by the mass and angular momentum loss from systems. Moreover, NCBs with periods showing cyclic oscillations occupy 40% of NCBs with available period investigations. This percentage may indicate that most NCBs may exist in multiple systems. (9)According to the statistical study of the absolute parameters of NCBs, we find that (i) Both components of most of NCBs have similar average densities and near the ZAMS in the HR diagram, which implies that NCBs may evolve from the Case A mass transfer or just undergoing Case A mass transfer. (ii) NCBs may evolve toward extreme mass ratio. (iii) The evolutionary direction of secondary filling NCBs to W UMa contact systems needs of angular momentum loss from the system. While, other types of NCBs can evolve to W UMa contact binaries or Algols under the mass and angular momentum conservation. |
修改评论