Fragmentation and disk formation in high-mass star formation: The ALMA
  view of G351.77-0.54 at 0.06" resolution
  Aims: We resolve the small-scale structure around the high-mass hot core
region G351.77-0.54 to investigate its disk and fragmentation properties.
  Methods: Using ALMA at 690GHz with baselines exceeding 1.5km, we study the
dense gas, dust and outflow emission at an unprecedented spatial resolution of
0.06" (130AU@2.2kpc).
  Results: Within the inner few 1000AU, G351.77 fragments into at least four
cores (brightness temperatures between 58 and 197K). The central structure
around the main submm source #1 with a diameter of ~0.5" does not show
additional fragmentation. While the CO(6-5) line wing emission shows an outflow
lobe in the north-western direction emanating from source #1, the dense gas
tracer CH3CN shows a velocity gradient perpendicular to the outflow that is
indicative of rotational motions. Absorption profile measurements against the
submm source #2 indicate infall rates on the order of 10^{-4} to
10^{-3}M_sun/yr which can be considered as an upper limit of the mean accretion
rates. The position-velocity diagrams are consistent with a central rotating
disk-like structure embedded in an infalling envelope, but they may also be
influenced by the outflow. Using the CH_3CN(37_k-36_k) k-ladder with excitation
temperatures up to 1300K, we derive a gas temperature map of source #1
exhibiting temperatures often in excess of 1000K. Brightness temperatures of
the submm continuum never exceed 200K. This discrepancy between gas
temperatures and submm dust brightness temperatures (in the optically thick
limit) indicates that the dust may trace the disk mid-plane whereas the gas
could be tracing a hotter gaseous disk surface layer. In addition, we conduct a
pixel-by-pixel Toomre gravitational stability analysis of the central rotating
structure. The derived high Q values throughout the structure confirm that this
central region appears stable against gravitational instability.
