The First Bird’s-eye View of a Gravitationally Unstable Accretion Disk in High-mass Star Formation

Abstract

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 Millimeter/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 ∼5–7 M ☉ and accretion rate onto the stellar surface is 3 × 10 −3 M ☉ 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 ☉ , 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 gravitational instability, and are consistent with 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.