A Study Of The Formation Of Bimodal Dust Species In Nova Ejecta

Nova eruptions are very important and complicated physical processes,which include white dwarf accreting matter,the mixing,thermonuclear runaway,ejecta and dust formation.On the observations,these processes correspond to light curve with the increase by 104 times,X-ray emissions,Gamma-ray emissions and infrared radiation.Dust formation can produce infrared radiation.Recently,the bimodal dust species(namely the silicate and amorphous carbon dust grains)are observed in a nova eruption.It is a challenge for popular theory for dust formation.Using Modules for Experiments in Stellar Astrophysics code(MESA),this thesis introduces a parameter,the mixing depth,and constructs the model for nova eruption.This model can give the initial density,temperature and chemical abundances for nova ejecta.Based on the free-expansion model,the radiative shock model and dust nucleation model,this thesis discusses the probability for the formation of bimodal dust species,and calculates the yield of dust.We find that the C/O(the ratio of the carbon number density to the oxygen number density)of nova ejecta is affected by the mixing depth.For a model with a small mixing depth,the C/O of nova ejecta can evolve from greater than 1.0 to less than 1.0 over the course of an eruption.In the free-expansion model,the nova ejecta is not an idea environment for dust nucleation.However,it can efficiently produce dust in the radiative shock model.We estimate that every nova can produce C grains with an average mass of about 10-9 and 10-8 M?,and Mg2SiO4 grains with an average mass of about 10-8 and 10-7 M?.Based on the mass of ejected gas,the ratio of dust to gas is about 1%.In fact,based on the chemical environment,nova ejecta can produce many species of dust,such as olivine-type,pyroxene-type,quartz-type,iron,SiC-type dust grains,and so on.There is still long way to understand the formation and growth of bimodal species dust in the nova ejecta.