Disk Kinematics and Stability in High-Mass Star Formation: Linking
  Simulations and Observations
  In the disk-mediated accretion scenario for the formation of the most massive
stars, gravitational instabilities in the disk can force it to fragment. We
investigate the effects of inclination and spatial resolution on observable
kinematics and stability of disks in high-mass star formation. We study a
high-resolution 3D radiation-hydrodynamic simulation that leads to the
fragmentation of a massive disk. Using RADMC-3D we produce 1.3 mm continuum and
CH3CN line cubes at different inclinations. The model is set to different
distances and synthetic observations are created for ALMA at ~80 mas resolution
and NOEMA at ~0.3''. The synthetic ALMA observations resolve all fragments and
their kinematics well. The synthetic NOEMA observations at 800 pc (~300 au
resolution) are able to resolve the fragments, while at 2000 pc (~800 au
resolution) only a single slightly elongated structure is observed. The
position-velocity (PV) plots show the differential rotation of material best in
the edge-on views. As the observations become less resolved, the inner
high-velocity components of the disk become blended with the envelope and the
PV plots resemble rigid-body-like rotation. Protostellar mass estimates from PV
plots of poorly resolved observations are therefore overestimated. We fit the
emission of CH3CN lines and produce maps of gas temperature with values in the
range of 100-300 K. Studying the Toomre stability of the disks in the resolved
observations, we find Q values below the critical value for stability against
gravitational collapse at the positions of the fragments and the arms
connecting the fragments. For the poorly resolved observations we find low Q
values in the outskirts of the disk. Therefore we are able to predict that the
disk is unstable and fragmenting even in poorly resolved observations. This
conclusion is true regardless of knowledge about the inclination of the disk.
