Modeling disk fragmentation and multiplicity in massive star formation
  We investigate the formation and early evolution and fragmentation of an
accretion disk around a forming massive protostar. We use a grid-based
self-gravity-radiation-hydrodynamics code including a sub-grid module for
stellar and dust evolution. On purpose, we do not use sink particles to allow
for all paths of fragment formation and destruction, but instead keeping the
spatial grid resolution high enough to properly resolve the physical length
scales of the problem. We use a 3D grid in spherical coordinates with a
logarithmic scaling in the radial direction and cosine scaling in the polar
direction. Because of that, roughly 25% of the total number of grid cells,
corresponding to $\sim$ 26 million grid cells, are used to model the disk
physics. They constitute the highest resolution simulations performed up to now
on disk fragmentation around a forming massive star with the physics considered
here. We study the convergence of our results by performing the same simulation
for 5 different resolutions. We start from the collapse of a molecular cloud; a
massive (proto)star is formed in its center, surrounded by a fragmenting
Keplerian-like accretion disk with spiral arms. The fragments have masses of
$\sim 1 M_\odot$, and their continuous interactions with the disk, spiral arms
and other fragments results in eccentric orbits. Fragments form hydrostatic
cores, surrounded by secondary disks with spiral arms that also produce new
fragments. We identified several mechanisms of fragment formation, interaction
and destruction. Central temperatures of the fragments can reach the hydrogen
dissociation limit, form second Larson cores and evolve into companion stars.
Based on this, we study the multiplicity predicted by the simulations and find
$\sim 6$ companions at different distances from the primary: from possible
spectroscopic multiples, to companions at distances between 1000 and 2000 au.
