| 其他摘要 | Cataclysmic variables (CVs) are interacting binary systems consisting of a white dwarf and a late-type Roche-lobe-filling red dwarf. Mass is transferred from the companion to the white dwarf via the inner Lagrange point L1 , which can form an accretion disk surrounding the primary, and a bright spot appears when the accretion stream from the secondary collides with the disk. Both sources are usually dominant in optical and ultraviolet radiation. Nova-like variables are a subclass of CVs that has never been observed to experience nova or dwarf nova outbursts. SW Sextantis-type nova-like stars have very high mass-transfer rates and particularly stable accretion disk structures, which maintains the system in a persistent bright state and blows out a strong disk wind. The complex structure and physical environment make this type of systems show many peculiar characteristics. The results are summarized as follows: 1. In hibernation scenario, systems after nova outburst will exhibit high mass transfer rates and change themselves into nova-like variables. The disk winds from the high-state disk bring away extra mass and angular momentum that affect their evolution. This has been lack of specific calculations. Assuming the evolution is stable, we calculated the mass transfer rate M2dot and the effective mass-radius index zeta of the donor stars for nova-likes with respect to the mass-loss proportion alpha based on their secular orbital period variations, and we located their positions on the tau_M2dot/tau_KH-zeta diagram, where tau_M2dot and tau_KH are the mass-transfer and thermal time-scales, respectively. V1315 Aql, a nova-like star surrounding by a nova shell. According to estimates of the radius of the shell and the explosion speed, it is speculated that it has experienced the nova eruption before 500-1200 years, so it is an exemplar for hibernation hypothesis. However, in comparison with other nova-likes, we found that the mass transfer in V1315 Aql is unstable due to the irradiation by the white dwarf and the accretion, and we estimated its magnitude-decline rate as 1E-4 mag/yr, which is consistent with its long-term optical observations. Thus, its time-scale of magnitude decline is comparable to the recurrence time-scale of nova eruption, which indicates that V1315 Aql will not enter into hibernation, but directly enter the next nova eruption. 2. Using the times of light minima of the prototype SW Sex, we found that the orbital period showed a possible cyclic wiggle from 1980 to 2015, then decreased severely from 2015 to 2020. The oscillation with an amplitude of 0.000973 days and a period of 36.57 yr may be due to the light time travel effect of a giant planet or the Applegate's mechanism. While the period decrease within the recent 5 yr is too fast to be produced by AML via MB, suggesting that there could be an additional AML mechanism. We found in this period, the system maintained the highest brightness, which indicates that the strong radiation may trigger the disk wind that severely takes away angular momentum, then leads the period of the system falls faster than expected. In addition, there is a flickering on the light curve of SW Sex, which oscillates periodically for about 20-40 minutes, that is consistent with the results of previous studies. Such short-time scale oscillation survives in both the lower and the higher state, which may be caused by the hot spot or the local unstable accretion of the accretion disk. The published data of DASCH and the AAVSO reveal that SW Sex experienced a series of state changes in the pattern of a slow increasing about 0.6 mag in about 9.7 yr following a fast decrease to the low state in less than 300 days. We believe that this kind of long-term variation exists not only in SW Sex, but also in other nova-like stars, although they have not been discovered yet due to their short history of observations. SW Sex was first discovered to have a maverick behavior, for which the interaction between the matter of inner disk and the magnetic field of the white dwarf may explain this phenomenon. 3. SW Sex stars are a important subclass of nova-like variables that are usually accumulate at the upper edge of the period gap, and they are characterized by the transit absorption in the optical emission lines near orbital phase 0.5. In order to confirm this phenomenon and further study the generation mechanism of the spectral line, we fitted the double-peaked emission lines of LAMOST spectra of SW Sex stars in two different manners. That is, double-peaked emission fitting and a core absorption plus a single emission line fitting, and then estimated the parameters (RV, FWHM, EW) of the emission lines. Based on the fitting results, we analysed the rationality of the three physics models proposed to explain the special features of the emission lines, and found that these models are not universal to produce the observed double-peaked emission lines. The distribution of the parameters also indicates that the double-peaked profile of the emission lines is caused by a central absorption in an emission line, rather than by two distinctive emission sources. And this feature exists in the entire orbital phase, not only in the 0.5 phase as previously known. We argued that a reliable model should include a disk overflow out of the orbital plane or/and a disk wind emanated from the whole disk, and the absorption source should be sufficiently extended to account for the absorption core of the double-peaked emission lines that presented in either orbital phase. |
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