The Observational Investigations On The Massive White-dwarf Binary Systems

According to the mass of the white dwarf component, a subgroup of closebinary systems, namely the massive white-dwarf binaries, is defned in this the-sis. The massive white-dwarf binary is composed of a quite massive (near orlarger than1M⊙) white dwarf primary and a secondary companion. In the non-degenerated case, the secondary may be a main sequence, red giant, asymptoticbranch giant (AGB), or post-AGB. It may be a He star or white dwarf in the de-generated case. In some extreme cases, it is even a neutron star or black hole. Inthe standard theory, a Carbon/Oxygen (C/O) white dwarf accumulates materialfrom the secondary star and fnally produces a type Ia supernova (SN Ia) whenits mass reaches the Chandrasekhar-mass limit. However, the progenitor system-s of SNe Ia still remain unclear. As commonly believed, SN Ia originates froma close binary system containing a massive C/O white dwarf, which can eitheraccrete mass from a non-degenerated companion or merge with a degeneratedcomponent to produce a SN Ia. But, of the massive white-dwarf binary systems,some containing a white dwarf with an adequately high temperature to triggerthe ignition of Carbon and Oxygen, may become a Oxygen/Neon/Magnesium(C/Ne/Mg) white dwarf and ultimately collapse as a neutron star.In this thesis, we in principle introduce some diferent types of the massivewhite-dwarf binary systems as well as their possible evolutionary ends and therelated models. On base of the published literatures and open data, several tensof such kind systems have been collected as a sample, from which six typicalmassive white-dwarf binaries are selected for the long-term monitoring and de-tailed investigations by using several ground-base telescopes located in China andArgentina. The main results and conclusions of our work are reported as below:1. In the long-period supersoft X-ray sources V Sge and WX Cen, theanalyses of the orbital-period variations suggest that mass transfer from the lobe-fll secondary onto the white dwarf sustains in these two systems, which resultsin the observed long-term decrease of the orbital periods. This is contradictory to the prediction of the wind-driven accretion mechanism. Hence, it is plausiblethat WX Cen is not a wind-driven supersoft X-ray source with a low mass ratio (q=M2/M1<1) as pointed out in the previous studies. Some acceptable rangesfor the basic systemic parameters are obtained from the dynamic analysis onWX Cen. In the short-period supersoft X-ray source V617Sgr, the O-C diagramshows a long-term increase of the orbital period, revealing the mass transfer inV617Sgr is driven by the wind-driven accretion mechanism. These supersoftX-ray sources are the most promising candidates of the SNe Ia progenitors2. The O-C analyses on the long-period nova-like BT Mon and V363Aurshow decreasing variations in their orbital period. The calculated rates of orbital-period variation by gravitational radiation and magnetic breaking are typicallysmaller than our observed results by1–2orders. The period variations henceshould be mainly attributed to the mass transfer from the main-sequence com-panions onto the white dwarfs. As the mass transfer from the donors onto thewhite dwarf primaries sustains, the white dwarfs can grow in mass, and mayfnally evolve into the SNe Ia. The photometric observations and orbital-periodanalysis of QZ Aur show that high mass-transfer process may persist in thissystem, and the mass of white dwarf should be very close to that of the main-sequence secondary star.3. The periodic oscillation components detected in the O-C analyses of BTMon, V363Aur and V617Sgr are more likely caused by the time-travel efectvia the third body, rather than the applegate mechanism. It is found that thetertiary component in the binary system V617Sgr may be a low-mass red dwarf,while that in BT Mon and V363Aur may be a brown dwarf with a mass only afew tens of MJup.4. In addition, a brown dwarf third body orbiting the pre-cataclysmic vari-able system, namely HW Vir-type star, HS0705+6700, is found from the O-Canalysis. Of particular interest, our result may suggest the presence of an addi-tional substellar object in a wider orbit than that of the third body.