A passing star can make a planet-forming disc look more warped than it is

When motion masquerades as shape

How can astronomers tell whether a planet-forming disc is warped? One approach measures how its gas moves. In a simple disc dominated by the central star’s gravity, gas follows approximately Keplerian rotation: material closer to the star travels faster. Departures from that pattern can reveal disturbances, including changes in the disc’s orientation.

But those departures need not have a unique explanation. In an arXiv preprint submitted to Monthly Notices of the Royal Astronomical Society, Vasundhara Prasad and colleagues investigate a particular ambiguity: a passing star can both bend a disc and disturb its gas directly. Their numerical experiments show that interpreting all the resulting motion as a warp can misrepresent the disc’s actual shape.

This is a simulation study, not a new detection of a stellar encounter or an experimentally confirmed explanation for a particular observed disc.

Five encounters with a model disc

The researchers used the three-dimensional hydrodynamics code PHANTOM to follow gas and dust around a star with the Sun’s mass. The initial disc extended from 10 to 150 astronomical units, where one astronomical unit is Earth’s average distance from the Sun. Its adopted mass was one hundredth of the Sun’s mass.

A second, equally massive star passed within 200 astronomical units on a parabolic trajectory. Across five simulations, its orbital inclination ranged from zero to 180 degrees in 45-degree steps. This covered encounters in the disc’s plane, inclined encounters and passages moving against the disc’s rotation. In every case, closest approach occurred in the original disc plane.

The team then calculated how the simulated systems would emit and scatter radiation. Synthetic observations of carbon monoxide and its carbon-13-bearing form were smoothed to a resolution comparable to high-resolution ALMA observations. These were modelled as smoothly rotating discs; subtracting that model exposed the leftover velocity patterns, or residuals.

Crucially, the researchers could compare shapes inferred from these artificial observations with the actual geometry inside their simulations.

A flattened disc can retain disturbed gas

The inclined encounters initially produced warps and twists. A warp means that different radial regions have different orbital orientations; a twist describes how the direction of that orientation changes with radius. A disc tilted uniformly as a whole is not necessarily warped.

By the main comparison snapshot, 5,402 years after closest approach, much of the radial variation in orientation had relaxed, especially in the inner disc. Some discs remained globally tilted, but their geometry was considerably simpler than immediately after the encounter.

Their gas motions were not. Spiral-like velocity residuals persisted in the inclined cases and in the coplanar encounter moving against the disc’s rotation. That last case provides an important control: it developed no warp, yet still displayed spiral kinematic structure. The coplanar encounter moving with the rotation showed disturbances but no clear residual spiral.

Accounting for the local disc orientation also left velocity substructure visible. Together, these comparisons demonstrate that the encounter perturbed the gas beyond merely changing the plane in which it orbited.

What a warp-only fit gets wrong

The team fitted the residual motions with a simple model that attributes them entirely to warped geometry. It inferred much stronger radial changes in twist than the simulations actually contained. Even the unwarped coplanar cases acquired apparent tilts of a few degrees.

The mismatch was not uniform across every measure. In the inclined cases, the inferred dimensionless warp amplitude was fairly similar to the simulated value, despite the inaccurate twist profiles. In the inner regions of the coplanar cases, however, that amplitude was overestimated by roughly an order of magnitude. Twist estimates also become poorly defined when the tilt approaches zero.

Additional views, with important limits

Synthetic scattered-light images offered complementary clues. The inclined cases showed spirals and, when viewed edge-on, lateral offsets between the two illuminated surfaces. The coplanar cases did not show those features. Synthetic millimetre dust-continuum images lacked visible substructure under the adopted conditions.

These are useful distinctions within the models, not universal proofs of a flyby. Other mechanisms can warp discs, and the study does not establish that encounters explain particular ALMA targets.

The timing also requires caution. The simulations use relatively high effective viscosity, which accelerates warp damping; real, less viscous discs could remain warped longer. Only five encounter orientations and one stellar-mass and closest-approach configuration were explored. The robust lesson is therefore about interpretation, not a universal lifetime: unusual gas velocities alone cannot establish a warped shape.