Debris disks are huge belts of dust and rocky bodies that resemble the Solar System’s asteroid belt, but are far bigger. For nearly two decades, Fomalhaut’s uneven (or eccentric) debris disk has captured the attention of astronomers.
This morphology is referred to as a negative eccentricity gradient. You can picture the offsets between the star and the ring’s core much like Saturn’s rings, if Saturn weren’t sitting exactly at the center.
Our observations show, for the first time, that the disk’s eccentricity isn’t constant. It steadily drops off with distance, a finding that has never before been conclusively demonstrated in any debris disk.
Joshua Bennett Lovell, Study Lead Author and Submillimeter Array Fellow, Harvard-Smithsonian Center for Astrophysics
Lovell also serves as an ALMA Ambassador at the North American ALMA Science Center of the National Radio Astronomy Observatory of the United States National Science Foundation.
Using high-resolution ALMA images at 1.3mm wavelengths, the researchers fitted the data with a novel model setup that accounted for the disk's radius, width, and asymmetries, as well as an eccentric ring model that can change its eccentricity as it moves away from the star.
The best-fitting model indicated a sharp fall in eccentricity with distance, which is anticipated by dynamical models of how planets create debris disks but has yet to be observed anywhere in the cosmos.
This negative gradient provides information about hidden planets around Fomalhaut that scientists have yet to discover. The new model shows that a huge planet rotating inside Fomalhaut’s disk may have shaped its eccentricity profile early in the extrasolar system's existence.
The debris disk’s peculiar shape may have developed in the system’s infancy, during the protoplanetary disk phase, and has stayed this way for more than 400 million years, due to the planet's constant push and pull.
In the second study, conducted by Graduate Student Jay Chittidi of Johns Hopkins University, the team ruled out the hypothesis that the ring's eccentricity is set by its distance from the star.
Although the shift in brightness from the pericenter side of the disk, nearest to the star, to the apocenter side, furthest from the star, between the JWST and ALMA data was expected, the precise shifts that we measured in the disk brightness and the ring’s width could not be explained by the old models.
Jay Chittidi, Graduate Student, Johns Hopkins University
“Simply put: we couldn’t find a model with a fixed eccentricity that could explain these peculiar features in Fomalhaut’s disk. Comparing the old and new models, we are now able to better interpret this disk, and reconstruct the history and present state of this dynamic system,” added Chittidi.
Researchers plan to test this new model with further ALMA data, which were just authorized.
“And hopefully we’ll find new clues that will help us uncover that planet!” added Lovell.
The eccentricity model code created for this recently published study has been made available by the researchers so that other astronomers can use it on similar systems.