W 49 N MCN-a: A disk-accreting massive protostar embedded in an early-phase hot molecular core

Abstract

Abstract 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 Vrot ∝ R0.44 ± 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 Tdust = 180 K) for R ≤ 7800 au and 32 M⊙ (for Tdust = 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.