Numerical Study Of Axion Stars

Axion stars also called axion boson stars,are a kind of stars composed of axion.Axion is a kind of boson particle with spin 0.The concept of boson stars comes from J.A.Wheeler's attempt to explore the coupling of classical electromagnetic field with gravitational field.In such a theory,Wheeler called the locally stable solutions as geons.However,such solutions are unstable.Later,D.J.Kaup and R.Ruffini explored the coupling of massive complex scalar field with gravitational field,and proposed the pos-sibility of Klein-Gordon geons as stars,which are boson stars.Boson stars are a kind of solution of the Einstein-Klein-Gordon equation(or the Einstein-Proca equation).Due to the similarity between Klein-Gordon equation and Schr?dinger equation,boson star solution is similar to that of a hydrogen atom and can be distinguished into ground state and excited state according to the node number of scalar field.Therefore,boson stars are also called gravitational atoms.At present,free scalar boson stars in the ground state,excited state and multistate were studied.Due to the significance of axion in cos-mology,we generalize these works to rotating axion boson stars.We construct different types of rotating axion boson stars,including the first excited,multistate,and hybrid stars.Here,hybrid stars are composed of free scalar field and axion field.In this paper,we mainly study the property of axion stars by ADM mass and angular momentum.In the second chapter,we firstly construct the model of rotating hybrid axion mini-boson stars by Einstein-Klein-Gordon thery.Then,we obtain two types of solution,including~1S~2S state and~1S~2S state and plot the distribution of matter field.Since fa is considered as a measure of self-interaction of the axion field,Under different interaction strengths,we analyze the relationship between ADM mass,angular momentum and axion field frequency by the choose of fa.We also study the behavior of hybrid stars under the synchronized frequency condition and nonsynchronized frequency condition.We find that,field frequencies?from large to small,the hybrid star transforms from a free scalar boson star in the excited state to a hybrid star in which the free scalar field is mixed with the axion field,and to an axion star in the ground state when the frequency reaches its maximum.The effect of decay constant fa and mass?a of the axion field on the frequency range is also studied.In the third chapter,we construct axion stars in the excited~2S states and~2P states,and multistate solutions,including~1S~2S states and~1S~2P states.The relations between their ADM masses,angular momentum and axion field frequencies for different fa are given,and we compared with rotating mini-bosons in the multistate states.We find that the ground state can have higher mass and angular momentum than the excited state when fa is small enough,and even the mass and angular momentum of the multistate solution composed of the ground state and the first excited state can be higher than those of the ground state and the first excited state.Moreover,as fa decreases,a second branch(i.e.,more than one correspondence between mass and field frequency)appears in the multistate solution.These properties are not found in the miniboson model composed.In addition,we analyzed the ergo surface of axion stars and found that they always appear at the inflection point between the first and second branches.For axion stars in multistate states,the ergo surface appears at the inflection point when fa is small enough,which means that the second branch of multistate axion stars is unstable.