| 其他摘要 | Hot subdwarf-B stars (sdBs) have been identi?ed as helium(He)-core burning stars with very thin hydrogen-rich envelopes. They are located on the extreme horizontal branch (EHB) in the Hertzsprung-Russell diagram (HRD). SdBs play a crucial role in stellar evolution, asteroseismology and contribute to the far-UV radiation of early type galaxies, and have been intensively studied in both observational and theorical circles. This dissertation ?rst provides a review of the observational aspect of sdB studies, such as their observational properties, statistical properties, and recent major surveys which came up with samples of sdBs. Then, we introduce a few theoretial models concerning the formation of sdBs, including the single sdBs evolution model, the binary evolution model and the ignition mechanism of He burning. Finally, our research for the formation of sdB+NS binaries is expounded in detail. Binary population syndissertation(BPS) predicts the existence of sdBs with neutron star (NS) companions. Several works have been dedicated to ?nding such systems, but none have been found yet. We systematically investigate the formation of sdB+NS binaries using binary evolution, then obtain their population properties by combining our models with BPS methods. For our binary evolution studies, the initial MS star masses we adopt range from 0.8 to 5M⊙ , that is, 0.8M⊙, 1.0M⊙, 1.26M⊙, 1.6M⊙, 2.0M⊙, 2.5M⊙, 3.2M⊙ , 4.0M⊙ , and 5.0M⊙. The NS mass is set to be 1.4M ⊙ , and various NS accretion e?ciencies and NS masses have been examined to investigate their in?uences. In the BPS study, we adopt two prescriptions for NS natal kicks. Di?erent values of αCE and λ, where αCE is the common envelope ejection e?ciency and λ is the stellar structure parameter, are chosen to examine the e?ect of common envelope evolution on the results. Based on the above work, the population properties of sdB+NS binaries and the model grid for producing sdB+NS binaries are obtained. We show the characteristics of the produced sdB+NS systems such as the mass of components, orbital period, the semi-amplitude of radial velocity (K) and the spin of the NS component. Finally, we provide some suggestions on how future observations should be conducted. The main results of our study are as follows:(1) The sdB+NS binaries can be produced from either stable RLOF or CE ejection. In the stable RLOF channel, sdBs can be formed if the donor starts mass transfer close to the tip of the red giant branch if the donor has an initial mass of ≤ 2.0M ⊙ . For more massive donors, sdBs can be formed if the donor starts mass transfer during the HG phase or near the TAMS.(2) The orbital period of sdB+NS binaries produced from stable RLOF ranges from several days to more than 1000 days and decreases with increasing initial MS mass. The largest K is around 150km/s .(3) The sdB+NS systems resulting from CE ejection have very short orbital periods and large values of K (up to 800km/s). Gravitational wave radiation may lead them to become contact binaries again on a timescale of only ～ Myr. Consequently, they are rare and di?cult to discover. Such systems, ～ 100 ? 300 of which ought to exist in the Milky Way according to our studies, are potential gravitational wave sources which could be resolved by the Laser Interferometer Space Antenna (LISA) in the future.(4) For the sdB+NS systems resulting from stable RLOF, the NSs in sdB binaries can be spun up to become millisecond pulsars, since the accreted material carries angular momentum which is then bestowed upon the NS during the accretion process. When this happens, in the event that He is not ignited following the mass transfer, a potential sdB progenitor evolves directly into a He WD, whereas in the event that it is ignited, an actual sdB is formed, which in turn eventually evolves into a CO WD. Thus, such millisecond pulsars usually have He/CO WD companions. Our results show that the spin period and orbital period of some sdB+NS binaries are generally consistent with observations of binary pulsars with such companions.(5) In the Galaxy, the birthrate of sdB+NS binaries ought to be on the order of 10^-4/yr , and their current numbers amount to ～ 7000 ? 21000, contributing at most 0.3-0.5% of the total sdB binary population. Most of the Galactic sdB+NS binaries (>60%) evolved via the stable mass transfer channel.(6) Regardless of whether they are produced by the stable RLOF channel or the CE ejection channel, the sdB+NS binary populations have similar delay time distributions, which peak at around 0.2Gyr. This indicates that sdB+NS binaries are predominantly found in very young populations, probably in the Galactic disk. |
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