| 其他摘要 | About half of the stars are in binary systems, which orbit around each other under the influence of gravity. Binary evolution determines the evolutionary fate of stars, resulting in the formation of important objects such as Type Ia supernovae and gravitational wave-producing double compact objects (double black holes/neutron stars). The hot stars generated by binary evolution, such as hot subdwarf stars, contribute significantly to the ultraviolet radiation of early-type galaxies. Also, Wolf-Rayet stars (WR) play an important role in cosmic reionization. However, the basic properties of binary population and the common envelope evolution are still very unclear, which greatly affects the studies of star/binary evolution and formation of important objects such as double black holes and double neutron stars. In this project, we have conducted a series of observational studies on the properties of binary population and the common envelope process in binary evolution, which are summarized as follows. (1) The observational properties of the binary population such as binary fraction, mass ratio distribution and period distribution, play an important role in understanding and studying binary evolution, the formation of binary-related objects, as well as the evolution of the Milky Way. However, the statistical properties of binaries are still poorly understood. The LAMOST medium-resolution survey (MRS) has provided a large sample of stars to study the properties of binary populations, especially for the mass ratio distributions and the binary fractions. Therefore, we have investigated the properties of 3000 binaries by using the data from the LAMOST-MRS DR6 & DR7. We have devised a Peak Amplitude Ratio (PAR) approach to derive the mass ratio of a binary system based on results obtained from its spectrum. By utilizing spectral observations obtained from LAMOST-MRS DR6 & DR7, we applied the PAR approach to form distributions of the derived mass ratio of the binary systems to the spectral types. We selected the mass ratio within the range of 0.6 − 1.0 for investigating the mass-ratio distribution. Through a power-law fitting, we obtained the power index 𝛾 values of −0.42 ± 0.27, 0.03 ± 0.12, and 2.12 ± 0.19 for A-, F-, and G-type stars identified in the sample, respectively. The derived 𝛾-values display an increasing trend toward lower primary star masses, and G-type binaries tend to be more in twins. The close binary fractions (for 𝑃 ≲ 150 d and 𝑞 ≳ 0.6) in our sample for A, F, and G binaries are 7.6 ± 0.5%, 4.9 ± 0.2% and 3.7 ± 0.1%, respectively. Through this study, we reveal the properties of binary systems and devise a PAR approach for estimating the mass ratio. The PAR approach can be applied to large spectroscopic surveys of stars. (2) The common envelope evolution in binaries is the key to understanding their interactions and is widely used to explain the formation of important phenomena such as Type Ia supernovae (cosmic standard candles) and gravitational-wave sources such as double black holes and double neutron stars. However, this critical process has never been observationally confirmed for almost half a century. Hot subdwarf binaries are of great importance for studying the common envelope process because they are commonly thought to be formed through the common envelope evolution. Here, we report the discovery of the binary system J1920-2001 (only two such systems have been found in previous studies), consisting of a hot subdwarf star and an accreting white dwarf (WD), based on the WiFeS spectroscopic data and photometric data from SkyMapper, K2, and ZTF. It consists of an O-type hot subdwarf (sdO) star and a WD with an orbital period of 3.495 hours and an orbital shrinkage of 0.1 s in 6 yr. The sdO star overfills its Roche lobe and likely transfers mass to the WD via an accretion disk. From spectroscopy, we obtain an effective temperature of 𝑇eff = 54240 ± 1 840 K and a surface gravity of log 𝑔 = 4.841 ± 0.108 for the sdO star. From the light curve analysis, we obtain a sdO mass of 𝑀sdO = 0.55 M⊙ and a mass ratio of 𝑞 = 𝑀WD/𝑀sdO = 0.738 ± 0.001. Also, we estimate that the disk has a radius of ∼ 0.41 R⊙ and a thickness of ∼ 0.18 R⊙. This binary star is probably formed through a common envelope ejection channel, where the progenitor of the sdO star is either a red giant branch star or, more likely, an early asymptotic giant branch star; the sdO star will subsequently evolve into a WD and merge with its WD companion, likely resulting in an R Coronae Borealis (R CrB) star. The outstanding feature in the spectrum of this object is strong Ca II H&K lines, and likely originate from the recently ejected common envelope, which is of great significance for understanding binary evolution, thermonuclear supernovae, the formation of double black holes, etc. (3) Observational studies of post common envelope binaries can place strong constraints on the common envelope evolution and are therefore of great importance for studying binary evolution. However, the observations of common envelope evolution are challenging because of the very short timescale of this phase. In this work, we stud ied the possible existence of disks or dust around hot subdwarf star candidates in the low-resolution spectroscopic survey (LRS) data of LAMOST DR7. We calculated the equivalent widths of the Ca II K lines in the spectra using Gaussian fitting and compared them with the intensity of the Ca II K lines produced by the interstellar medium. Using the spectral line features and the galactic position of the star, we identified 368 hot subdwarf candidates that may be surrounded by a disk, and these samples were possible to have undergone a common envelope phase. The results can help us understand the formation and evolution of hot subdwarfs, as well as have important implications for subsequent studies of the common envelope phase. |
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