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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 andsubmillimeter wavelengths of the young O-type protostar W51 North made with theSubmillimeter Array (SMA). We report the presence of a large (of about 8000 AU)and hot molecular circumstellar disk around this object, which connects theinner dusty disk with the molecular ring or toroid reported recently, andconfirms the existence of a single bipolar outflow emanating from this object.The molecular emission from the large disk is observed in layers with thetransitions characterized by high excitation temperatures in their lower energystates (up to 1512 K) being concentrated closer to the central massiveprotostar. The molecular emission from those transitions with low or moderateexcitation temperatures are found in the outermost parts of the disk andexhibits an inner cavity with an angular size of around 0.7$''$. We modeled alllines with a Local Thermodynamic Equilibrium (LTE) synthetic spectra. A detailstudy of the kinematics of the molecular gas together with a LTE model of acircumstellar disk shows that the innermost parts of the disk are alsoKeplerian plus a contracting velocity. The emission of the HCOCH$_2$OH revealsthe possible presence of a warm ``companion'' located to the northeast of thedisk, however its nature is unclear. The emission of the SO and SO$_2$ isobserved in the circumstellar disk as well as in the outflow. We suggest thatthe massive protostar W51 North appears to be in a phase before the presence ofa Hypercompact or an Ultracompact HII (HC/UCHII) region, and propose a possiblesequence on the formation of the massive stars.

Extremely large and hot multilayer Keplerian disk around the O-type protostar W51N: The precursors of the HCHII regions?