Hot Sub-dwarf Formation And Evolution

Hot subdwarf stars are exotic objects. They are believed to be helium-core-burning objects with very thin hydrogen-rich (or helium-rich) envelopes. Hot subdwarf stars populate a very narrow area which lies on a blueward extension of the horizontal branch (HB), the so called extreme horizontal branch (EHB). Their effective temperatures and logarithmic surface gravities are typically between 20,000K and 80,000K, 4.5 and 6.5 (cgs), respectively. Hot subdwarf dominate ultraviolet lights due to their high temperatures, and they are candidates of sources of ultraviolet radiation in elliptical galaxies. 'UV upturn' in elliptical galaxies may be resulted from hot subdwarf stars.Many hot subdwarf stars are in binaries (～2/3). The research of these stars is very important and significant. Three main binary evolution channels can lead to the formation of hot subdwarf stars (specially to sdB stars): the common-envelope (CE) ejection channel, the stable Roche lobe overflow (RLOF) channel, and the double helium white dwarfs (WDs) merger channel. We mainly investigate the stable Roche lobe overflow (RLOF) channel in this thesis.Using PPE stellar evolution code, in the conservative and nonconservative systems, we compute systemically stellar evolution respectively. We present parameters of initial binaries that can form hot subdwarf, and parameters of final binaries. The differences of natures of hot subdwarf stars formed through stable RLOF beginning in main sequence and Hertzsprung gap in the conservative and nonconservative systems are discussed. It is found that the progenitor of subdwarf B stars may be a star with big initial mass (for example: the initial mass of the primary M_li=3.55M_⊙) when mass transfer begins in main sequence. In the conservative condition, hot subdwarf stars can't be formed through stable RLOF when q≥4.5, however, in the nonconservative condition, hot subdwarf stars can be formed through this channel when q =4.5.It is found that the orbital period range of binary systems containing hot subdwarf formed through stable RLOF channel is from ten days to hundreds of days. In the conservative evolution, the peak of distribution of orbital period locates about 135.0 days and the extremum situates about 50.0 days. In the nonconservative evolution, the peak lies 30.0 days. From our results, We predict that binaries containing hot subdwarf stars with more long orbital periods will be observed in the future.