| 其他摘要 | According to the mass of the white dwarf component, a subgroup of close binary systems, namely the massive white-dwarf binaries, is de?ned in this the- sis. The massive white-dwarf binary is composed of a quite massive (near or larger than 1 M⊙) white dwarf primary and a secondary companion. In the non- degenerated case, the secondary may be a main sequence, red giant, asymptotic branch 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. In the standard theory, a Carbon/Oxygen (C/O) white dwarf accumulates material from the secondary star and ?nally produces a type Ia supernova (SN Ia) when its mass reaches the Chandrasekhar-mass limit. However, the progenitor system- s of SNe Ia still remain unclear. As commonly believed, SN Ia originates from a close binary system containing a massive C/O white dwarf, which can either accrete mass from a non-degenerated companion or merge with a degenerated component to produce a SN Ia. But, of the massive white-dwarf binary systems, some containing a white dwarf with an adequately high temperature to trigger the 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 di?erent types of the massive white-dwarf binary systems as well as their possible evolutionary ends and the related models. On base of the published literatures and open data, several tens of such kind systems have been collected as a sample, from which six typical massive white-dwarf binaries are selected for the long-term monitoring and de- tailed investigations by using several ground-base telescopes located in China and Argentina. 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, the analyses of the orbital-period variations suggest that mass transfer from the lobe- ?ll secondary onto the white dwarf sustains in these two systems, which results in the observed long-term decrease of the orbital periods. This is contradictory to the prediction of the wind-driven accretion mechanism. Hence, it is plausible thatWX 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 ranges for the basic systemic parameters are obtained from the dynamic analysis on WX Cen. In the short-period supersoft X-ray source V617 Sgr, the O-C diagram shows a long-term increase of the orbital period, revealing the mass transfer in V617 Sgr is driven by the wind-driven accretion mechanism. These supersoft X-ray sources are the most promising candidates of the SNe Ia progenitors 2. The O-C analyses on the long-period nova-like BT Mon and V363 Aur show decreasing variations in their orbital period. The calculated rates of orbital- period variation by gravitational radiation and magnetic breaking are typically smaller than our observed results by 1–2 orders. The period variations hence should 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 the white dwarf primaries sustains, the white dwarfs can grow in mass, and may ?nally evolve into the SNe Ia. The photometric observations and orbital-period analysis of QZ Aur show that high mass-transfer process may persist in this system, 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 BT Mon, V363 Aur and V617 Sgr are more likely caused by the time-travel e?ect via the third body, rather than the applegate mechanism. It is found that the tertiary component in the binary system V617 Sgr may be a low-mass red dwarf, while that in BT Mon and V363 Aur may be a brown dwarf with a mass only a few tens of MJup. 4. |
修改评论