From large-scale to protostellar disk fragmentation into close binary
  stars
  Recent observations of young stellar systems with the Atacama Large
Millimeter/submillimeter Array (ALMA) and the Karl G. Jansky Very Large Array
(VLA) are helping to cement the idea that close companion stars form via
fragmentation of a gravitationally unstable disk around a protostar early in
the star formation process. As the disk grows in mass, it eventually becomes
gravitationally unstable and fragments, forming one or more new protostars in
orbit with the first at mean separations of 100 astronomical units (AU) or even
less. Here we report direct numerical calculations down to scales as small as
$\sim 0.1$ AU, using a consistent Smoothed Particle Hydrodynamics (SPH) code,
that show the large-scale fragmentation of a cloud core into two protostars
accompanied by small-scale fragmentation of their circumstellar disks. Our
results demonstrate the two dominant mechanisms of star formation, where the
disk forming around a protostar, which in turn results from the large-scale
fragmentation of the cloud core, undergoes eccentric ($m=1$) fragmentation to
produce a close binary. We generate two-dimensional emission maps and simulated
ALMA 1.3 mm continuum images of the structure and fragmentation of the disks
that can help explain the dynamical processes occurring within collapsing cloud
cores.
