The Observation And Investigation On Dwarf Novae

As one of the subtypes of cataclysmic variable stars(CVs),dwarf novae(DN)display frequent and dramatic outbursts.They are semi-detached binary systems containing a white dwarf primary and a Roche-lobe filling low-mass secondary that transfers material to the primary star,forming an accretion disc.The collision of the gas stream with the accretion disc forms a so-called “hot spot”,a shock-heated region of emission at the edge of the disc.The complex structure leads to the rich and diverse photometric behaviors,these variations are seen on a variety of time scales,ranging from the several seconds to tens of years.In general,outbursts typically last for about a week or longer,and the interval between outbursts varies from ten days to many years.The outburst is explained by the disc instability at present.Moreover,there are the rapid brightness oscillations on the timescales from several seconds to tens of minutes,which is associated with the accretion events.Therefore,the study of these variations could be used to test the theory of outburst and accretion disc.On the other hand,the study for the rapid oscillations may shed new light on our understanding of the accretion process,in turn this also help us to understand other systems containing accretion disc.The main objects in the present paper are the eclipsing dwarf novae,the reason is that the eclipsing nature can offer a good opportunity to ascertain the geometric position and physical structure of each light source.Moreover,the eclipse also provide important clues concerning the long-term evolution of orbital periods and existence of circumbinary substellar objects.Combining small and medium sized telescopes at home and abroad with AAVSO database,the author mainly carry out the following areas of study: dwarf nova outbursts,rapid oscillations,evolution and circumbinary substellar objects.The results are summarized as follows:1.Based on the long-term AAVSO data,the outburst properties of two Z Cam-type stars,AY Psc and V729 Sgr,are explored.The analysis shows that there are diverse behaviors in these systems.First of all,we find thatthe duty cycle of AY Psc during outburst is close to 100% and the outburst is not strictly periodic but quasi-periodic.Second,the analysis indicates that the average magnitudes during standstill is brighter than during outburst.However,this standstill ends with an outburst.The unusual characteristic behavior is differ from most manners of Z Cam stars,which may be related to the mass transfer outbursts.There are a few outburst-like events during standstill,which have properties very similar to the stunted outbursts seen in nova-like stars(NLs).For V729 Sgr,its outburst behaviors are rich and complex.By comparing the outburst amplitude of both 77 DN and 33 NLs with the statistic data of V729 Sgr,we suggest that it may be an intermediate between DN and NLs.In addition,the rapid oscillations in V729 Sgr were detected and analyzed for the first time.2.By photometrically monitoring V2051 Oph,IY UMa and SJ1435,we discovered that their orbital periods show long-term decrease.These systems consist of a white dwarf and a fully convective main-sequence star.According to the standard model,when the secondary component becomes completely convective,the magnetic braking(MB)was assumed to stop and the evolution is driven purely by gravitational radiation(GR).However,the observed period decreases are much larger than purely GR-driven,indicating that GR is insufficient to explain the observations.Some investigations also showed that the fully convective objects can sustain significant magnetic and experience some spin-down due to MB.We conclude that MB may not be completely stop for fully convective stars.3.By analyzing the orbital periods of the period bouncer candidates WZ Sge and V4140 Sgr,we find that the orbital period of WZ Sge shows a long-term decrease but V4140 Sgr's period is increasing.Secular decrease,coupled with previous detection for WZ Sge's donor,suggest that it is a pre-bounce system.Further analysis indicates that the observed period decrease rate is about 1.53 times higher than pure GR driving.We also constructed that the evolutionary track of WZ Sge,which predicts Pmin? 77.98 min.If the period decrease at the current rate,it will evolve into a period bouncer after ? 25.3 Myr.The continuous period increase in V4140 Sgr is opposite to the secular decrease of normal CVs.Moreover,its observational properties are significant different from those of other CVs with similar orbital period.Therefore,V4140 Sgr is mostlikely a period bouncer.This is also the first period bouncer of CV evolution.4.The study of the changes in the orbital period find that the O-C curves of5 objects(i.e.V2051 Oph,DV UMa,V4140 Sgr,J1435 and EX Dra)show cyclic oscillations.To explain the cyclic period variations in close binaries containing at least one cool star,two main mechanisms are the solar-type magnetic activity cycle(Applegate mechanism)and the light time travel(LTT)effect.However,due to the required energies to drive the Applegate mechanism are too high in these systems.Thus,the periodic variations most likely arises from the LTT via the presence of an unseen companion.It is found that the third bodies in V2051 Oph and V4140 Sgr are giant planets,while the companions of the other three objects are brown dwarfs.In addition,we also analyzed and discussed two objects,V729 Sgr and GSC 4560-02157,which may have cyclic period oscillations.