W49 N MCN-a: A disk-accreting massive protostar embedded in an early-phase hot molecular core

We present ALMA archival data for 219-235 GHz continuum and line observations toward the hot molecular core (HMC) W49 N MCN-a (UCHII region J(1)) at a resolution of similar to 0 ''.3. The dust continuum emission, showing an elongated structure of 1 ''.40 x 0 ''.95 (position angle = 43 degrees.5) perpendicular to the outflow seen in SiO and SO, represents a rotating flattened envelope, or torus, with a radius of 7800 au inclined at 47:5 or larger. The emissions from CH3CN, (CS)-C-13, HNCO, HC3N, SO2, DCN, H2CO, OCS, CH3OH, and (CO)-O-18 exhibit a consistent velocity gradient as a result of rotation. The magnitude of each velocity gradient is different, reflecting that each line samples a specific radial region. This allows us to derive a rotation curve as V-rot proportional to R-0.44 +/- 0.11 for 2400 au less than or similar to R less than or similar to 14000 au, giving the dynamical mass as M-dyn = 57.0(-17.1)(+24.5) (R[au]/3000)(1.88) M-circle dot. The envelope mass independently estimated from the dust emission is 910 M-circle dot (for T-dust = 180 K) for R <= 7800 au and 32 M-circle dot (for T-dust = 300 K) for R <= 1700 au. The dynamical mass formula agrees well with these mass estimates within an uncertainty of a factor of three in the latter. The envelope is self-gravitating and is unstable to form spiral arms and fragments, allowing rapid accretion to the inner radii with a rate of the order of 10(-2) M-circle dot yr(-1), although inward motion was not detected. The envelope may become a non-self-gravitating Keplerian disk at R less than or similar to (300-1000) au. The formula is also consistent with the total mass similar to 10(4 )M(circle dot) of the entire HMC 0.15 pc (31000 au) in radius. Multiple transitions of CH3CN, HNCO, and CH3OH provide rotation temperatures of 278(-3)(0)(+39), 297(-39)(+59), and 154(-37)(+)(73)K, respectively, for R less than or similar to 1700 au, suggesting that the central source of MCN-a has an intrinsic bolometric luminosity of similar to 10(6) L-circle dot. These results have revealed the structure and kinematics of MCN-a at its intermediate radii. With no broad-line H300/ emission detected, MCN-a may be in the earliest phase of massive star formation.