ALMA observations reveal gravitational instability in planet-forming disk

ALMA observations reveal gravitational instability in planet-forming disk
by Clarence Oxford
Los Angeles CA (SPX) Sep 06, 2024

Planet formation has traditionally been described as a "bottom-up" process, where dust grains gradually clump together over millions of years, forming larger structures step by step. However, a competing theory suggests that planets can form much faster via a "top-down" process, where material in a protoplanetary disk fragments due to gravitational instability.

An international team of astronomers, led by Jessica Speedie, a PhD candidate at the University of Victoria, Canada, has found new evidence supporting this alternative theory. Using the Atacama Large Millimeter/submillimeter Array (ALMA), the team observed the protoplanetary disk around the star AB Aurigae, uncovering signs of gravitational instability.

The National Radio Astronomy Observatory (NRAO) and ALMA, which is partly managed by the NRAO, played a vital role in this discovery. "ALMA's sensitivity and high velocity resolution enabled us to probe the gas deep within the disk and measure its motion precisely. It was the only tool for the job," said Speedie.

Several developing protoplanets, including one nine times more massive than Jupiter, have already been identified within the spiral arms of AB Aurigae's disk. These spiral structures rotate counterclockwise around the star, which has a mass roughly 2.4 times that of the Sun and is about 4 million years old. This creates a puzzle for the traditional "bottom-up" theory, as the star's young age suggests there hasn't been enough time for planets to form through slow accumulation.

Speedie, along with her PhD advisor Ruobing Dong, and their team used ALMA to study the gas motion in the star's spiral arms. According to Dr. Cassandra Hall, Assistant Professor of Computational Astrophysics at the University of Georgia, who co-authored the research, "Disks that are gravitationally unstable should have distinctive 'wiggles' in their velocity field, unlike disks that are stable." Hall explained that in 2020, she led advanced simulations predicting this signature. "It was clear, it was testable, and it was a bit scary - if we didn't find it, then something had to be very, very wrong with our understanding of these disks."

Using ALMA's 12-meter array, Speedie mapped the movement of specific gases in AB Aurigae's disk and discovered clear evidence of these predicted velocity "wiggles." Cristiano Longarini, a postdoctoral researcher at the University of Cambridge and co-author of the study, explained, "Spiral arms form in the disk when the disk-to-star mass ratio is sufficiently high. Within those arms, changes in density lead to changes in gravity, which in turn lead to variations in the velocities of gas in the local area around and within the arms. We see these variations in the velocity as wiggles."

Longarini further noted that the size of these wiggles could be used to estimate the mass ratio between the star and its surrounding disk.

"Our detection of gravitational instability in the disk around AB Aurigae is a direct observational confirmation of this 'top-down' pathway to planet formation," Speedie summarized.

ALMA's advanced interferometry measurements allowed Speedie and her team to produce a three-dimensional map of the gas velocities within the disk. Through careful analysis, they identified the velocity wiggle that indicates gravitational instability.

"We worked with one of the deepest ALMA datasets ever collected for a protoplanetary disk at such high velocity resolution," Speedie said. "The ALMA data provides a clear diagnosis of gravitational instability in action. There is no other mechanism we know of that can create the global architecture of spiral structure and velocity patterns that we observe."

Reflecting on Hall's earlier predictions, Speedie added, "This is a classic science story of, 'we predicted it, and then we found it'. The Hall-mark of gravitational instability." Speedie, who is part of the NSF NRAO ALMA ambassador program, plans to continue her work with ALMA and is training with other astronomers to share the observatory's resources with the broader community.

Research Report:Gravitational instability in a planet-forming disk