Accreting White Dwarfs And The Related Objects

Accreting white dwarfs is a class of white dwarfs that accretes material from their companion stars in binary systems.The mass-accreting white dwarfs play an important role in the study of stellar evolution,usually relating to some objects or astrophysical processes,such as,supernovae,cataclysmic variable,symbiotic stars,millisecond pul-sars,supersoft X-ray sources,gravitational wave sources,etc.However,researches on the evolution of mass-accreting white dwarfs still remain uncertain,such as the mass-accumulation and mass-loss processes,the burning conditions of accreted material and the final fates of the white dwarfs.In this thesis,we introduced the backgrounds and significances of mass-accreting white dwarfs,especially for two of the related objects:type Ia supernovae and accretion-induced-collapse supernovae.We conducted a series of investigations on the evolution of mass-accreting white dwarfs.We investigated the process of double white dwarf mergers,and found that the evolutionary outcomes of the white dwarfs are determined by mass-accretion rate during the merger process.We investigated the process of off-center carbon burning in helium-accreting carbon-oxygen white dwarf,and found that the white dwarf can evolve to an oxygen-silicon core and then collapse to a neutron star.We investigated the process of helium-novae outbursts,and the obtained parameters can provide the initial input physics for the binary population synthesis studies.We investigated the long-term evolution of oxygen-neon white dwarf accreting material,and found that the final outcome of accreting oxygen-neon white dwarf is to collapse into neutron star through the process of e-capture.Our results are provided as follows:(1)We investigated the process of double carbon-oxygen white dwarf mergers,and found that the evolutionary outcomes of the white dwarfs are determined by mass-accretion rate during the merger process.The double degenerate model is not only a popular progenitor model of type Ia supernova,but also a source of gravitational wave.However,previous studies suggested that the outcome of the WD may be collapsing to a neutron star through the e-capture supernova explosion.Hence,the final outcomes of double WD mergers still remain uncertain.We found that the final outcomes of double white dwarfs depend on the mass-accretion rate.As the mass-accretion rate increases,the outcomes can be type Ia supernovae,oxygen-silicon white dwarfs(subsequently evolves to an iron-core-collapse supernova),oxygen-neon white dwarf(subsequently evolves to an e-capture supernova),or off-center oxygen/neon ignition(subsequently evolves to a particular kind of thermonuclear supernovae).This study indicates the diversity of outcomes of double WD mergers,and there also exists a parameter space which can produce type Ia supernovae.(2)We simulated the process of off-center carbon burning in helium-accreting carbon-oxygen white dwarf,and found that the white dwarf can evolve to an oxygen-silicon core and then collapse to a neutron star.Generally,CO WD+He star systems tend to explain the formations of young type Ia supernovae in galaxies.Recent stud-ies suggested that the white dwarf may ignite its carbon on the surface rather than in its center if the mass-accretion rate is higher than a critical value.Previous works usu-ally assumed that the off-center carbon flame would transform the white dwarf into an oxygen-neon core,and then the hot core would undergo e-capture process to form a neutron star.We found that the temperature of inwardly propagating carbon flame is extremly high,resulting in that neon is transformed into silicon once it appears.As the carbon flame propagates inwardly,the CO WD will be burned into an oxygen-silicon core,and finally forms an OSi WD(subsequently evolves to an iron-core-collapse su-pernova),rather than the traditional point of view(to form a oxygen-neon white dwarf and then experiences e-capture supernova).Our results implied the existence of oxygen-silicon in the Unverise and the increase of birthrates of iron-core-collapse supernovae.Moreover,we found that the white dwarf can increase in mass to the Chandrasekhar mass limit and explode as a type Ia supernova if the mass-accretion rate is below the critical mass-accretion rate.(3)We simulated the process of helium-novae and obtained parameters of the mass-retention efficiencies and nova outburst durations of the white dwarfs with different mass-accretion rates.We also found that the helium novae can produce type Ia super-novae.The present results can provide the initial input physics for the binary population synthesis studies.Helium nova is a flash phenomenon that is originate from the unsta-ble helium burning on the surface of white dwarf.Helium novae are relate to some objects like supersoft X-ray source,cataclysmic variable,type Ia supernova,etc.How-ever,the mass-retention efficiencies during He nova outbursts are still uncertain,which may have influence on the studies of binary evolutions,supernovae and physical pro-cesses under extreme conditions.By simulating the He nova outbursts,we obtained the mass-retention efficiencies and nova cycle durations,and found that the carbon-oxygen white dwarfs can increase in mass to the Chandrasekhar mass limit and explode as type Ia supernovae due to the successive helium outbursts.Based on the new results of mass-retention efficiencies,we investigated the carbon-oxygen white dwarfs+helium stars model,and found that this model can explain the type Ia supernovae with short delay times.(4)We simulated the long-term evolution of oxygen-neon white dwarf accreting material,and found that the final outcome of accreting oxygen-neon white dwarf is to collapse into neutron star through the process of e-capture.Generally speaking,oxygen-neon white dwarfs are orginate from the evolutions of 8-10 M_?main sequence stars.Previous studies suggested that the oxygen-neon white dwarf would collapse into neu-tron star through the e-capture supernova when the oxygen-neon white dwarf increases in mass to the Chandrasekhar mass limit in binary system.However,some recent studies argued that the oxygen-neon white dwarfs may undergo explosive neon burning to form a thermonuclear runaway supernova.We simlated the long-term evolutions of mass-accreting oxygen-neon white dwarfs with different mass-accretion rates,and found that thermal energy released by the electron-capture process cannot keep balance with the neutrino energy loss when it increases in mass to the Chandrasekhar mass limit,which cannot trigger the explosive neon burning.Our results indicates that the final outcome of mass-accretion oxygen-neon white dwarfs is to form neutron stars through electron-capture supernovae.