PI: Richard Alexander

Young stars are surrounded by cold discs of dust and gas, which form because gas clouds invariably “spin up” as they collapse to form stars. These so-called protoplanetary discs are the sites of exoplanet formation. In recent years observations with telescopes such as ALMA have revealed a wealth of structure in planet-forming discs. We have traditionally considered these discs to be smooth and featureless, but we now know that most discs are structured, and gaps, rings and spirals are common. These structures offer us a unique window into the processes of stellar growth and planetary formation, but we cannot observe these processes in real time. Instead we must rely on theoretical models and computer simulations to understand the physics of these planetary nurseries.

We have previously used DiRAC’s Data Intensive at Leicester (DIaL) cluster to develop a new, state-of-the-art approach to simulating protoplanetary discs, combining 3-D numerical hydrodynamics (using the PHANTOM code) with detailed Monte Carlo radiative transfer (using MCFOST). Coupling these two codes allows is calculate the heating and cooling processes self-consistently as the disc evolves, which is critical to understanding disc dynamics.

We applied this method to the problem of a protoplanetary disc around a misaligned binary star. The effect of the binary’s gravity is to break the disc into two distinct sections: an inner part which precesses rapidly; and an outer part which retains its original orientation. The interface between the inner and outer discs launches spiral waves into the outer disc, and these spirals “wind” in the same direction as the disc’s rotation (so-called “leading” spirals). This is the opposite to the “trailing” spirals we see launched by embedded planets, or disc self-gravity, and is the first time that leading spirals have been seen in protoplanetary disc simulations.

Leading spirals require the direction of angular momentum transport to be reversed, so we see that – in certain configurations – the inner disc in our simulations is “feeding” the outer disc (usually it’s the other way around). Moreover, the presence of both leading and trailing spiral arms at different times implies that the rotation of the disc cannot be assumed based on the orientation of the spiral arms alone. This approach (using observed spirals to infer the direction of rotation) is commonly used to interpret observations of planet-forming discs, but our simulations show that this method is fraught with uncertainty.

The complex dynamics in binary systems offer a unique insight into the processes of star and planet formation, but only through simulations like these – made possible by the power of DiRAC – will we be able to understand what we are seeing.

These results were published in Leading and trailing spiral arms in a nearly broken protoplanetary disc, S.Rowther et al., Monthly Notices of the Royal Astronomical Society, 542, 1430 (2025).

Animations and other visualisations of these simulations are available HERE.