  Formation of supermassive stars (SMSs) with mass ~10^4 Msun is a promising
pathway to seed the formation of supermassive black holes in the early
universe. The so-called direct-collapse (DC) model postulates that such an SMS
forms in a hot gas cloud irradiated by a nearby star-forming galaxy. We study
the DC SMS formation in a fully cosmological context using three-dimensional
radiation hydrodynamics simulations. We initialize our simulations using the
outputs of the cosmological simulation of Chon et al. (2016), where two DC gas
clouds are identified. The long-term evolution over a hundred thousand years is
followed 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 the
multiplicity of the formed stars and affects the protostellar growth. In one
case, where a collapsing cloud is significantly stretched by strong tidal
force, multiple star-disk systems are formed via filament fragmentation.
Small-scale fragmentation occurs in each circumstellar disk, and more than 10
stars 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, the
gas cloud collapses nearly spherically under a relatively weak tidal field, and
a single star-disk system is formed. Only a few SMSs with masses ~ 10^4 Msun
are found already after evolution of a hundred thousand years, and the SMSs are
expected to grow further by gas accretion and to leave massive blackholes at
the end of their lives.
