Extremely large and hot multilayer Keplerian disk around the O-type
  protostar W51N: The precursors of the HCHII regions?
  We present sensitive high angular resolution (0.57$''$-0.78$''$) SO, SO$_2$,
CO, C$_2$H$_5$OH, HC$_3$N, and HCOCH$_2$OH line observations at millimeter and
submillimeter wavelengths of the young O-type protostar W51 North made with the
Submillimeter Array (SMA). We report the presence of a large (of about 8000 AU)
and hot molecular circumstellar disk around this object, which connects the
inner dusty disk with the molecular ring or toroid reported recently, and
confirms the existence of a single bipolar outflow emanating from this object.
The molecular emission from the large disk is observed in layers with the
transitions characterized by high excitation temperatures in their lower energy
states (up to 1512 K) being concentrated closer to the central massive
protostar. The molecular emission from those transitions with low or moderate
excitation temperatures are found in the outermost parts of the disk and
exhibits an inner cavity with an angular size of around 0.7$''$. We modeled all
lines with a Local Thermodynamic Equilibrium (LTE) synthetic spectra. A detail
study of the kinematics of the molecular gas together with a LTE model of a
circumstellar disk shows that the innermost parts of the disk are also
Keplerian plus a contracting velocity. The emission of the HCOCH$_2$OH reveals
the possible presence of a warm ``companion'' located to the northeast of the
disk, however its nature is unclear. The emission of the SO and SO$_2$ is
observed in the circumstellar disk as well as in the outflow. We suggest that
the massive protostar W51 North appears to be in a phase before the presence of
a Hypercompact or an Ultracompact HII (HC/UCHII) region, and propose a possible
sequence on the formation of the massive stars.
