| 其他摘要 | Double degenerates, a binary with two white dwarfs, are very common in the Galaxy. Many peculiar objects are closely related to double degenerates, e.g. type Ia su pernovae, millisecond Hertz (mHz) gravitational-wave (GW) radiation, etc. Studies of double degenerates help us understanding the evolution of universe, mass transfer pro cess in binaries, stellar evolution, etc. In recent years, a number of double degenerates with low-mass helium core white dwarf (≲ 0.3𝑀⊙; known as extremely low-mass white dwarf; ELM WD) have been discovered in ELM survey. Due to extremely short orbital periods, these binaries are supposed to be signifificant GW sources for future space-based GW detectors, e.g. the Laser Interferometer Space Antenna (LISA). Besides, in com parison to massive white dwarfs (such as Carbon-Oxygen white dwarf), ELM WDs have relatively thick hydrogen-rich envelope, which leads them to be detected by the electro magnetic (EM) telescopes more easily. In this thesis, we mainly explore the formation and evolution of ELM WDs in double degenerates, investigate the GW signals of such systems in the Galaxy. There are four chapters in this thesis as follows. In Chapter 1, we introduce the background in WD fifield, including the formation and evolution of double WDs, the structure and pulsation properties of ELM WDs, as well as the observational properties of ELM WDs. In Chapter 2, we fifirstly analyse the observational characteristics of ELM WDs and find that ELM WDs can be produced from two channel, i.e. the stable Roche lobe overflflow channel (RL channel) and common envelope ejection channel (CE channel), according to the ELM WD mass-orbital period relation. We then systemically study the formation and evolution of ELM WDs in double degenerates with the combination of binary evolution and binary population synthesis. The results show that most ELM WDs with mass less than ∼ 0.22𝑀⊙ are produced from the RL channel, and the ELM WDs produced in this way have a peak around 0.18𝑀⊙ with inclusion of selection effffects. The most common progenitor mass for these He WDs is in the range of 1.15 −1.45𝑀⊙. He WDs with mass larger than ∼ 0.22𝑀⊙ are likely to be produced from the CE channel, having a peak around 0.25𝑀⊙. Such a peak needs to be confifirmed by future observations. In Chapter 3, we further investigate the GW signals for double degenerates with ELM WD companions. We found that about 6×103 sources are expected to be detected by LISA, including 2 × 103 chirping sources. With the combination of EM and GW observations, about ∼13 (∼107) LISA detected sources can be observed by ELM Survey (Gaia). The summaries and conclusions are shown in Chapter 4. |
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