Formation of supermassive stars (SMSs) with mass ~10^4 Msun is a promisingpathway to seed the formation of supermassive black holes in the earlyuniverse. The so-called direct-collapse (DC) model postulates that such an SMSforms in a hot gas cloud irradiated by a nearby star-forming galaxy. We studythe DC SMS formation in a fully cosmological context using three-dimensionalradiation hydrodynamics simulations. We initialize our simulations using theoutputs of the cosmological simulation of Chon et al. (2016), where two DC gasclouds are identified. The long-term evolution over a hundred thousand years isfollowed from the formation of embryo protostars through their growth to SMSs.We show that the strength of the tidal force by a nearby galaxy determines themultiplicity of the formed stars and affects the protostellar growth. In onecase, where a collapsing cloud is significantly stretched by strong tidalforce, multiple star-disk systems are formed via filament fragmentation.Small-scale fragmentation occurs in each circumstellar disk, and more than 10stars with masses of a few times 10^3 Msun are finally formed. Interestingly,about a half of them are found as massive binary stars. In the other case, thegas cloud collapses nearly spherically under a relatively weak tidal field, anda single star-disk system is formed. Only a few SMSs with masses ~ 10^4 Msunare found already after evolution of a hundred thousand years, and the SMSs areexpected to grow further by gas accretion and to leave massive blackholes atthe end of their lives.
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