  We present ALMA archival data for 219-235 GHz continuum and line observations
toward the hot molecular core (HMC) W49N MCN-a (UCHII region J1) at a
resolution of ~0."3. The dust continuum emission, showing an elongated
structure of 1."40x0."95 (PA=43.5deg) perpendicular to the outflow seen in SiO
and SO, represents a rotating flattened envelope, or torus, with a radius of
7,800au inclined at 47.5deg or larger. The emissions from CH3CN and 11
molecular lines 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
Vrot prop R^0.44+-0.11 for 2,400au < R < 14,000au, giving the dynamical mass as
Mdyn = 57.0+24.5-17.1 (R [au]/3, 000)^1.88 Msun. The envelope mass
independently estimated from the dustemission is 910Msun (for Tdust =180K) for
R<7,800au and 32Msun (for Tdust=300K) for R<1,700 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 order 10^-2 Msun yr^-1, although inward motion was not detected.
The envelope may become a non self-gravitating Keplerian disk at R<(300-1,000)
au. The formula is also consistent with the total mass ~10^4 Msun of the entire
HMC 0.15 pc (31,000 au) in radius. Multiple transitions of CH3CN, HNCO and
CH3OH provide the rotation temperatures, suggesting that the central source of
MCN-a has an intrinsic bolometric luminosity of ~10^6 Lsun. These results have
revealed the structure and kinematics of MCN-a at its intermediate radii. With
no broad-line H30alpha emission detected, MCN-a may be in the earliest phase of
massive star formation.
