Forming spectroscopic massive proto-binaries by disk fragmentation
  The surroundings of massive protostars constitute an accretion disc which has
numerically been shown to be subject to fragmentation and responsible for
luminous accretion-driven outbursts. Moreover, it is suspected to produce close
binary companions which will later strongly influence the star's future
evolution in the Hertzsprung-Russel diagram. We present three-dimensional
gravitation-radiation-hydrodynamic numerical simulations of 100 Mo pre-stellar
cores. We find that accretion discs of young massive stars violently fragment
without preventing the (highly variable) accretion of gaseous clumps onto the
protostars. While acquiring the characteristics of a nascent low-mass
companion, some disc fragments migrate onto the central massive protostar with
dynamical properties showing that its final Keplerian orbit is close enough to
constitute a close massive proto-binary system, having a young high-mass and a
low-mass component. We conclude on the viability of the disc fragmentation
channel for the formation of such short-period binaries, and that both
processes -close massive binary formation and accretion bursts- may happen at
the same time. FU-Orionis-type bursts, such as observed in the young high-mass
star S255IR-NIRS3, may not only indicate ongoing disc fragmentation, but also
be considered as a tracer for the formation of close massive binaries -
progenitors of the subsequent massive spectroscopic binaries - once the
high-mass component of the system will enter the main-sequence phase of its
evolution. Finally, we investigate the ALMA-observability of the disc
fragments.
