A newborn star’s disc moves gas inward and outward in stacked layers

Gas falling toward a forming star faces a rotational barrier. As a cloud contracts, its rotation becomes harder to ignore, and material settles into a disc rather than dropping directly onto the centre. For the star to keep growing, that orbiting gas must transfer angular momentum—the quantity associated with its rotation—to other material or magnetic fields.

An arXiv preprint by Antonin Borderies, Benoît Commerçon and Adnan Ali Ahmad examines how several mechanisms share that work. Its central finding is not a single dominant process everywhere, but a layered flow whose behaviour changes with the magnetic field’s connection to the gas.

Following collapse into a newborn system

The researchers used the Idefix simulation code to follow a rotating, magnetised cloud containing one solar mass through collapse and the formation of a protostar, then for about 64 further years. Near the stellar surface, the smallest radial cells measured roughly one ten-thousandth of an astronomical unit. One astronomical unit, or AU, is the average Earth–Sun distance.

The calculation combines gas motion, magnetic fields and the gas’s own gravity. It also includes two ways magnetic fields can slip relative to matter: Ohmic diffusion, associated with electrical resistance, and ambipolar diffusion, in which neutral gas moves relative to charged particles coupled to the field.

The collapse produces an unusually thick disc, with a vertical extent comparable to its radius. Early in the simulation, the disc is roughly an order of magnitude more massive than the protostar. Its own gravity therefore matters; treating it as a thin structure orbiting an overwhelmingly dominant central star would miss important physics.

To identify what moves angular momentum, the team separated the turning effects, or torques, associated with correlated gas motions, magnetic forces and gravity. Each was divided into radial transport and transport between different heights in the disc. Their main analysis covers a comparatively settled interval, 10–40 years after the protostar forms.

Magnetic forces at the ends, spirals between

Three broad regimes emerge. In the surrounding envelope, magnetic braking transfers angular momentum from rapidly rotating inner material toward more slowly rotating surroundings, helping gas approach the disc.

Across most of the disc, ambipolar diffusion weakens the field’s grip. This creates a magnetic “dead zone”: not an inactive region, but one where magnetic stresses contribute little to transport. Here, the leading contribution comes from correlated gas motions, known as Reynolds stresses.

Those motions are mainly carried by a two-armed spiral pattern, rather than small-scale turbulence. The distinction matters: although the disc’s self-gravity helps sustain the spirals, the angular momentum carried by their gas motions exceeds the direct contribution from gravitational stresses. The disc remains close to gravitational instability without breaking into separate objects during these runs.

Inside roughly 0.1 AU, high temperatures cause dust grains to vaporise in the adopted microphysical model, reducing magnetic diffusion. The field reconnects dynamically with the gas, and magnetic stresses become the largest contributor.

The researchers also find evidence for the magneto-rotational instability, which can turn differential rotation into magnetic turbulence. Quasi-periodic field reversals migrating away from the disc’s middle plane support that interpretation. However, the instability is not sufficiently resolved for its transport strength to be considered a converged measurement.

Why neighbouring layers flow opposite ways

Between approximately 0.05 and 1 AU, inward and outward flows coexist in vertically stacked layers. Where torques remove angular momentum, gas moves inward; where they deposit it, gas can move outward. Transport between layers is as important as radial transport.

The protostar nevertheless gains mass overall. During the relatively quiet phase, its total angular momentum stays nearly constant while its mass increases, so its angular momentum per unit mass declines.

Averaging the opposing flows into a single net rate hides much of this activity. Applying a commonly used steady-state accretion formula to that net rate gives a negative equivalent viscosity parameter, called alpha, between about 0.09 and 0.35 AU. This is a warning about that simplified representation—not a discovery of a material with literal negative viscosity.

A detailed model, with consequential restrictions

The preprint’s strongest limitations concern heat and geometry. Temperature is prescribed from density rather than calculated using a full energy budget and radiative transfer. The prescription is also adjusted to produce a tractable protostellar size, so the simulation cannot predict the system’s thermal evolution reliably.

The calculation seeds a two-armed perturbation and models only half the azimuthal circle, excluding one-armed and other odd-numbered patterns. The prominence of two arms must be interpreted within that restriction. Magnetic transport in the inner disc also changes with numerical resolution and solver choice, while the short duration samples relatively few outer-disc orbits.

The result is therefore a physically informative preprint model, not a universal portrait of newborn stars. It shows why understanding stellar growth can require following angular momentum through a disc’s height, not just along its radius.