| 其他摘要 | Cataclysmic variables with accretion disks are commonly semi-detached binary systems composed of a weakly magnetized white dwarf and a late main-sequence star filling its Roche lobe. The main-sequence companion star, overflowing with material into its Roche lobe, continually transfers matter to the weakly magnetic white dwarf via the inner Lagrangian point, thus forming an accretion disk around the white dwarf. In comparison to other accretion systems involving more massive central bodies, such as X-ray binary, the accretion disks in cataclysmic variables have smaller radii and are relatively closer to Earth, allowing for detailed study. The study of these accretion disks and their accretion processes is instrumental in understanding a broad range of accreting celestial bodies and systems. Large-scale domestic and international sky surveys are currently providing increasingly detailed information about accretion disks in cataclysmic variables. Analyzing continuous, high-precision observation data of cataclysmic variables reveals a variety of complex patterns in their light curves, including outbursts, periodic oscillations, and eclipses, all closely related to activities within the accretion disk. Based on the current abundance of survey data, the author has conducted research on the physical characteristics of accretion disks in various types of cataclysmic variables, achieving the following principal results:1. By analyzing the light curve obtained from TESS observations, the study delved into the classical long-period dwarf nova system EX Dra. In the quiescent light curve, simultaneous detections of orbital period signals, negative superhump signals, and superorbital signals were observed. The superorbital period of 4.39(7) days is quite distinctive, where the primary and secondary maxima display sinusoidal-like changes, while the secondary minima exhibit periodic yet non-sinusoidal variations in brightness. Remarkably, the primary minimum brightness remains constant, indicating that the accretion disk is completely eclipsed by the companion star during quiescence. The superorbital period reflects the precession period of the accretion disk, serving as the optimal observational window to infer the precession period. The O - C curve of eclipsing minimum occultation, influenced by precession, exhibits a periodic oscillation, with an oscillation period of 3.9(5) days. Negative superhumps vanish during outbursts, giving way to quasi-periodic oscillations (QPOs) of approximately 40 minutes in the out-of-eclipse light curve. These QPOs exhibit the highest coherence near outburst peaks and fade away as the outburst subsides, due to activities on the accretion disk during outbursts. The system also displays a unique bimodal outburst pattern, which appears to challenge the disk instability model, the research ultimately suggests that the bimodal outbursts are closely linked to disk precession. From a thirty-year-long AAVSO light curve dataset, marked changes in the amplitude and frequency of outbursts were detected. Short-time Fourier transform analysis showed that in January 2003, the outburst period transitioned from 20-25 days to 10-15 days, accompanied by a decrease in amplitude of about one stellar magnitude. The system reverted back to the original mode in January 2017. This 'switch-like' behavior in the outburst pattern is generally attributed to changes in the accretion rate, and this phenomenon has only been observed in this specific object so far.2. To compare the distinct properties of accretion disks in long-period and short-period cataclysmic variables, the author also thoroughly analyzed the prototype short-period cataclysmic variable, the SU UMa-type star SU UMa. TESS recorded the complete superoutburst light curve for the first time, and spectral analysis revealed a unique phenomenon: during outbursts, both superhumps and eclipsing features were observed. The outer edge of the precessing accretion disk experiences eclipses, while the central white dwarf remains unobscured. An O - C analysis at the eclipse minimum phase during the B stage of superhumps showed periodic oscillations, and the oscillation period was found to be largely consistent with the precession period, echoing the findings in EX Dra. Employing stage A superhump method, the mass ratio of this system was calculated to be 0.137(1). Unlike in EX Dra, QPOs were not detected during outbursts in SU UMa, but a pronounced long-period QPO of 0.19 days was evident during quiescent periods. The research further established that negative superhumps are more common in long-period systems, whereas short-period systems predominantly exhibit positive superhumps. This difference provides valuable insight into the dynamical behavior and matter transport mechanisms of accretion disks in cataclysmic variables with different periods3. From the extensive photometric data provided by TESS, a batch of objects experiencing super-outbursts and superhumps were identified, and among them, ten SU UMa-type stars with relatively high signal-to-noise ratios in their light curves were carefully selected, all of which were non-eclipsing to ensure more accurate O - C analysis, given that eclipses can influence the profile of superhumps. Upon analyzing the patterns of period and amplitude changes in the superhumps of these ten targets, it was found that most superoutbursts indeed exhibit the typical three stages of superhumps. However, there were no signs of all three stages in ER UMa. Virtually all superhumps showed an overall negative period change rate, with stage B having a positive rate. Calculations revealed that the mass ratios for these targets were in the range of 0.1-0.2, with precession periods around two days, which are shorter than the 4.39(7) day precession period of the long-period cataclysmic variable EX Dra. Statistical analysis of the superhump information was performed, and empirical relationships between orbital periods and superhump periods were established, which led to the determination of the orbital periods for ten SU UMa systems with previously unknown orbital periods.4. According to theoretical models, it is speculated that up to 40% to 70% of cataclysmic variables in the Milky Way should theoretically have evolved to the period bounce stage; however, the number of such systems actually observed falls significantly short of this estimate. To bridge this discrepancy, the author integrated TESS satellite data with the VSX catalog, identifying 47 objects that exhibited superhumps. By applying methods utilizing stage A superhumps, the mass ratios of these targets were calculated, and their evolutionary stages were determined. Four candidates were recognized as potentially being in the period bounce stage, each with a mass ratio less than 0.1, consistent with the characteristics of period bouncer systems. Specifically, the mass ratios for DI UMa and RX Vol systems were calculated to be 0.036(2) and 0.063(7), respectively, making them strong suspects for period bouncers. Systems 1RXS J185310 and RZ LMi displayed traits indicative of either being at their shortest periods or at the period bounce stage. Additionally, the study revealed a novel finding: in SU UMa-type dwarf novae, systems with smaller mass ratios tend to have longer precession periods of their accretion disks. |
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