Gravitational instability in a planet-forming disk

TL;DR

This paper presents observational evidence of gravitational instability in a planet-forming disk around AB Aurigae, supporting the theory that giant planets can form directly from collapsing disk fragments.

Contribution

The study provides the first kinematic evidence of gravitational instability in a protoplanetary disk, using ALMA observations to estimate a high disk-to-star mass ratio.

Findings

Detected non-Keplerian gas motions consistent with gravitational instability

Estimated disk mass up to one-third of the stellar mass

Observed spiral arm features match simulation predictions

Abstract

The canonical theory for planet formation in circumstellar disks proposes that planets are grown from initially much smaller seeds. The long-considered alternative theory proposes that giant protoplanets can be formed directly from collapsing fragments of vast spiral arms induced by gravitational instability -- if the disk is gravitationally unstable. For this to be possible, the disk must be massive compared to the central star: a disk-to-star mass ratio of 1/10 is widely held as the rough threshold for triggering gravitational instability, inciting significant non-Keplerian dynamics and generating prominent spiral arms. While estimating disk masses has historically been challenging, the motion of the gas can reveal the presence of gravitational instability through its effect on the disk velocity structure. Here we present kinematic evidence of gravitational instability in the disk around AB Aurigae, using deep observations of 13CO and C18O line emission with the Atacama Large Millimeter/submillimeter Array (ALMA). The observed kinematic signals strongly resemble predictions from simulations and analytic modelling. From quantitative comparisons, we infer a disk mass of up to 1/3 the stellar mass enclosed within 1" to 5" on the sky.