Fomalhaut: A star with a debris disc

The star Fomalhaut hosts a well-known debris disc - a collection of planetesimals and dust in a disc shape, like the Asteroid Belt and Kuiper Belt in our Solar System. This disk has been known in the outer regions of the system for 40 years, being imaged by several telescopes, including the Atacama Large Millimeter/submillimeter Array (ALMA) (see image below left).

However, last year, the James Webb Space Telescope (JWST) discovered another ‘intermediate’ disc, interior to the main disc (see image below right). Unlike the main disc, which has material in various sizes from small micrometre dust (seen by JWST) to large millimetre grains (seen by ALMA), the intermediate disc only contains small dust. It also has a well-defined, ring-like structure. This has led to speculation that unseen planets could cause it. Simulating the Fomalhaut system can help resolve this case.

What might form the intermediate disc?

There are two main questions to consider: how did the dust get to the location of the intermediate disc, and how did it form into the well-defined structure we see with JWST?

We think the dust in the intermediate disc originates further out in the main disc. Therefore, we need a mechanism to transport that dust from the main disc to the approximate location of the intermediate disc. Poynting-Robertson drag (P-R drag) is a well-known process where light from the star causes dust to spiral inwards. The susceptibility of the dust to P-R drag depends on its size. The smaller the dust, the more susceptible it is. This presents a way to transport the small dust to the position of the intermediate ring, whilst leaving the large grains in the main disc.

Now, we need to reproduce the structure seen. Planet interactions with the dust could offer a solution. When the orbital period of a body is an integer ratio with another body’s, they are in resonance with each other. The Neptune-Pluto system is an example of a system in a 3:2 resonance, which protects the two bodies from colliding. Dust in resonance with a planet can form resonance structures, which look like the structure seen in the intermediate disc. This gives us a clue about where potential planets may be in the Fomalhaut system, even if we do not see any directly.

Image: Simulation of a resonant dust ring in the inner Solar System. The structure formed in the simulation looks like the intermediate disc discovered around Fomalhaut.

Simulating the Fomalhaut system

Using Rebound, an N-body integrator, we simulated the Fomalhaut system, aiming to reproduce observations. The simulation comprised the star, dust affected by P-R drag, and hypothetical planets for the dust to interact with. The dust was initialised with the same orbit as the main disc. We then ran the simulations to let the dust spiral inwards and interact with the planets.

Image: Our simulated JWST (left) and ALMA (right) images of the Fomalhaut system with the inclusion of two hypothetical planets.

We initially investigated if one planet could reproduce the intermediate disc of Fomalhaut. Whilst this planet could capture dust in the right location, it had trouble separating the two discs. Adding another planet between the two discs helped to separate them. We found that having the following configuration produced similar observations to JWST and ALMA:

1. A Uranus mass planet near the inner edge of the intermediate disc.

2. A Mars mass planet between the intermediate and main discs.

Whilst not perfect, our simulations can reproduce a ring at the location of the intermediate disc. This demonstrates a favourable conclusion that P-R drag and planet interactions are a possible mechanism behind the formation of the intermediate disc.

The future of Fomalhaut

The results from our simulations show an exciting prospect for the Fomalhaut system. It demonstrates the possible presence of planets, and future observations may lead to the discovery of new planets.

For more details, see this academic poster about our research.

Acknowledgement

This research was conducted as part of the University of Warwick’s Undergraduate Research Support Scheme (URSS).

References

• Gáspár et al. Nature Astronomy 7, 790 (2023)

• MacGregor et al. ApJ 842, 8 (2017)

• Sommer et al. A&A 635, A10 (2020)