Hot Subdwarf Stars From Common Envelope Ejection Channel

Hot subdwarf B(sd B)stars,also known as extreme horizontal-branch(EHB)stars,are located on the hot and blue end of HB in a colour-magnitude diagram.They are generally considered to be core helium burning stars with extremely thin hydrogen envelope.Sd B stars are important in several aspects.(1)The study of their origin significantly improved our knowledge of stellar and binary evolution theory.(2)Many sd B stars show multiperiodic pulsations(having both p-mode and g-mode),and those stars are important objects of asteroseismology study.(3)The formations and evolutions of sd B stars determine The properties of globular clusters(GC)'s EB.(4)Sd B stars are also considered to be crucial sources of far-ultraviolet radiation in earlytype galaxies.The major scenario in sd B's origin is binary scenario,as a high fraction of sd B stars are determined as sd B binaries(2/3 for field sd B stars).In this thesis,firstly we review the observations and formation models of sd Bs.Then we introduce the studies of the properties of the sd Bs formed by common envelop(CE)ejection channel and the related objects.The thesis is mainly divided into the following aspects.(1)The properties of sd Bs from CE ejection channel.we study stars with initial stellar masses(the mass of donor): Mi= 0.8M,1.0M,1.26 M,and 1.5M.For each mass,we systematically investigate a series of positions on the first giant branch(FGB)where we assume CE begins,i.e.the core mass increases from the minimum mass allowed for He ignition to the tip of FGB in steps of 0.002 M.An artificial high mass-loss rate(10-3M yr-1)is then employed to remove the envelopes until the star begins to collapse.We follow the evolution of remnant until it becomes a cool CO WD.The produced sd B stars are clearly separated into two groups on the Teff-logg diagram and a gap between two groups is left.One group has almost no H-rich envelope and is crowded at the hottest temperature end of the EHB,very close to zero-age helium main sequence(ZAHe MS,the thick-solid lines).The other group has a significant H-rich envelope and is spread throughout the entire canonical EHB region.For the first group,the star enters the CE process earlier and has a lower He core mass.The first helium flash occurs when the star descends the WD cooling curve and is followed by a violent hydrogen flash,which is triggered by convective element mixing between H-rich envelope and He-burning region.Hydrogen in the envelope is then almost exhausted and the products are almost naked He cores.For the second group,the star enters the CE process later and has a more massive He core.The first helium flash occurs much earlier,and the convection induced by the helium flash never penetrates the H-rich envelope.The products then remain significant Hrich envelope and are spread throughout the entire canonical EHB region.Some observational short-period sd Bs,however,are located in the gap between the two groups,the number density is obviously higher than the theoretical value from our stimulation.The treatment of convection and the modelling of the CE ejection process will greatly change the parameter spaces for the two typical groups of sd B stars.A falling-back process after the CE ejection and mass losses during flash can make the two group of stars to fall into the gap,respectively.They are likely to explain this contradiction.(2)The period range of sd B stars produced from CE ejection channel.The motivation is to constraint the CE ejection parameters,?CEby compared with observations.We stimulate seven stars with initial masses from 0.7 to 1.3M,and obtain the binding energies and the internal energies of envelopes,Egrand Eth,at positions near the tip of FGB where the stars are assumed to overfill their Roche Lobes and begins the common envelope phase.Then we may get the initial orbital separations by a given companion mass.The period,P,is calculated through CE energy conservation:by a given ?CEand ?th.Our results generally agree with observations and previous studies.The maximum P from our study is shorter,by an order of magnitude,than that of Han et al.(2002)[1].This is due to the different definitions of the core boundary.(3)Blue hook stars(BHk)in GC.Blue hook stars occupy a very hot position on the HB with helium abundance higer than that of EHB stars.Two scenarios,i.e.He self-enrichment scenario and late He flasher scenario,have been proposed for producing BHk stars.Lei et al.(2015)[2]reproduced the BHk stars through tidally enhanced stellar wind in binaries where the late He flash occur in some models.We found that products of the CE ejection channel also suffered the late He flashes,then contribute to the formation of BHk stars.We inferred that stable mass transfer may contribute to BHk stars as well.Since the appearance of the late He flashes does not depend on the detailed mass loss process.