Simulation Of The Neutron Star Binaries In Globular Clusters

Neutron star binaries(NSBs)are important objects of astrophysical studies.Laser Interferometer Gravitational-Wave Observatory(LIGO)and Virgo detected the first gravitational wave event GW180817 in August 2017,which was produced by a neutron star binary merger.NSB are the essential sources of gravitational waves.The studies of NSB are useful for well-understanding the formation and evolution of gravitational wave sources.The study of NSBs also plays a significant role in other fields of astronomy.The simulation of NSBs in star clusters and galaxies can provide a theoretical foundation for detecting gravitational waves.As a kind of compact binaries,NSBs are one of the destinations of stellar evolution and binary interactions.Previous works showed that the evolution of massive binaries and dynamical process are the primary formation modes of NSBs.Because globular clusters are the collection of numerous stars,they are the ideal formation sites of NSBs.This work makes use of the stellar population synthesis and dynamical simulation technique to study the formation and fraction of NSBs in globular clusters.Firstly,we build two globular cluster models with the same primordial binary fraction.They are named Model 1 and Model 2.Model 1 includes 100000 stars and eight metallicities(Z=0.0001?0.03).Model 2 includes 10000 stars.According to the parameters of the host galaxy NGC4993 of gravitational event GW170817,the metallicity of Model 2 is set to Z=0.02.Then we apply a new stellar population synthesis,Advanced Stellar Population Synthesis(ASPS),to study Model 1 and obtain the fraction of NSBs,taking different metallicities.We next use a high computational efficiency code,NBODY6++GPU,to simulate Model 2 and gain the result of dynamical formation.Finally,we compare the results and make a summary.The results can be summarized as follows:(1)NSBs can be generated by both stellar evolution and dynamical simulation in globular clusters.(2)The NSB fraction,which is simulated by evolutionary stellar population synthesis,is related to stellar metallicities.The smaller the metallicity,the larger the NSB fraction.(3)When the primordial binary fraction is set to be 0.5,the NSB fractions in stellar population synthesis simulation are 0.000786,0.000737,0.000638,0.000442,0.000442,0.000344,0.000442 and 0.000344,for metallicities of Z=0.0001,0.0003,0.001,0.004,0.008,0.01,0.02 and 0.03,respectively.The result of Nbody simulation is 0.000117 for metallicity of Z=0.02.(4)The results of stellar population synthesis are about four times larger than those of dynamical simulation.The formation time in the former case is very short(?0.10Gyr),while that it is very long in the latter(?4.46Gyr).In conclusion,the effect of stellar evolution plays a much more important role than that of dynamical process among stars formation of NSBs.Stellar evolution is the major formation mode of NSBs in globular clusters.However,the effect of dynamical interaction can not be ignored.We should consider the combination of the two formation modes to obtain the reliable results in future works.