2022PASJ...74..705M

We present ALMA archival data for 219-235 GHz continuum and line observations toward the hot molecular core (HMC) W 49 N MCN-a (UCHII region J₁) at a resolution of ∼${0.^{\prime \prime }3}$. The dust continuum emission, showing an elongated structure of ${1.^{\prime \prime }40} \times {0.^{\prime \prime }95}$ (position angle = 43.^°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, ¹³CS, HNCO, HC₃N, SO₂, DCN, H₂CO, OCS, CH₃OH, and C¹⁸O 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 ∝ R^0.44 ± 0.11^ for 2400 au ≲ R ≲ 14000 au, giving the dynamical mass as $M_{\rm dyn} = 57.0^{+24.5}_{-17.1}\, (R\, [{\rm au}]/3000)^{1.88}\, M_{\odot }$. The envelope mass independently estimated from the dust emission is 910 M_⊙ (for T_dust = 180 K) for R ≤ 7800 au and 32 M_⊙ (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_⊙ yr^–1, although inward motion was not detected. The envelope may become a non-self-gravitating Keplerian disk at R ≲ (300-1000) au. The formula is also consistent with the total mass ∼10⁴ M_⊙ of the entire HMC 0.15 pc (31000 au) in radius. Multiple transitions of CH₃CN, HNCO, and CH₃OH provide rotation temperatures of 278$^{+39}_{-30}$, 297$^{+52}_{-39}$, and 154$^{+73}_{-37}$ K, respectively, for R ≲ 1700 au, suggesting that the central source of MCN-a has an intrinsic bolometric luminosity of ∼10⁶ L_⊙. These results have revealed the structure and kinematics of MCN-a at its intermediate radii. With no broad-line H30α emission detected, MCN-a may be in the earliest phase of massive star formation.