The first bird's-eye view of a gravitationally unstable accretion disk
  in high-mass star formation
  We report on the first bird's-eye view of the innermost accretion disk around
the high-mass protostellar object G353.273+0.641, taken by Atacama Large
Millimter/submillimeter Array long-baselines. The disk traced by dust continuum
emission has a radius of 250 au, surrounded by the infalling rotating envelope
traced by thermal CH$_3$OH lines. This disk radius is consistent with the
centrifugal radius estimated from the specific angular momentum in the
envelope. The lower-limit envelope mass is $\sim$5-7 M$_{\odot}$ and accretion
rate onto the stellar surface is 3 $\times$ 10$^{-3}$ M$_{\odot}$ yr$^{-1}$ or
higher. The expected stellar age is well younger than 10$^{4}$ yr, indicating
that the host object is one of the youngest high-mass objects at present. The
disk mass is 2-7 M$_{\odot}$, depending on the dust opacity index. The
estimated Toomre's $Q$ parameter is typically 1-2 and can reach 0.4 at the
minimum. These $Q$ values clearly satisfy the classical criteria for the
gravitational instability, and are consistent with the recent numerical
studies. Observed asymmetric and clumpy structures could trace a spiral arm
and/or disk fragmentation. We found that 70$\%$ of the angular momentum in the
accretion flow could be removed via the gravitational torque in the disk. Our
study has indicated that the dynamical nature of a self-gravitating disk could
dominate the early phase of high-mass star formation. This is remarkably
consistent with the early evolutionary scenario of a low-mass protostar.
