Accretion bottleneck in protoplanetary discs: the role of the stellar spin

We investigate angular momentum transport and accretion properties in a sample of protoplanetary discs with dynamical measurements of stellar masses, disc masses, and scale radii. From these data we infer effective $α$-viscosities, finding a remarkably broad range spanning over three orders of magnitude. This spread correlates with the stellar rotation period: systems with shorter periods exhibit significantly lower accretion rates, suggesting that they are undergoing at least temporary episodes of accretion bottleneck. We interpret this behaviour within the framework of magnetospheric accretion models, where the transition between steady accretion and the propeller regime is set by the relative locations of the co-rotation and magnetospheric radii. Our results indicate that stellar spin is a key parameter in regulating mass transfer from the disc to the star, and provide new evidence that the observed dispersion in $α$ reflects transitions between distinct accretion states rather than differences in global disc properties.

💡 Research Summary

This paper investigates the physical mechanisms driving angular momentum transport in protoplanetary discs, focusing on the origin of the wide dispersion in accretion efficiencies observed across different systems. The authors leverage a sample of discs with dynamically measured properties—stellar mass, disc mass, and scale radius—derived from high-resolution ALMA observations of 12CO and 13CO rotation curves. Combining these with literature measurements of stellar accretion rates, they compute the effective α-viscosity, a dimensionless parameter parameterizing angular momentum transport efficiency.

The key finding is that the inferred α-values span an astonishingly broad range, over three orders of magnitude from 10^-5 to 10^-2. To explain this large spread, the authors examine correlations between α and various system parameters. They find no strong correlation with invariant properties like stellar mass or with evolving disc properties like disc mass or scale radius. However, a statistically significant correlation (Spearman rank coefficient ρ_S=0.69, p=0.013) emerges with the stellar rotation period (P_rot). Systems with shorter rotation periods (approximately less than 6 days) exhibit significantly lower α-values and correspondingly lower accretion rates.

The authors interpret this correlation within the framework of magnetospheric accretion models. In this picture, accretion onto the star is mediated by its magnetic field, which truncates the disc at the magnetospheric radius (R_M). The flow regime is determined by the location of R_M relative to the co-rotation radius (R_co), where the disc’s orbital period matches the stellar spin period. When R_M < R_co, steady accretion occurs. When R_M approaches or exceeds R_co, the system enters the “propeller regime,” where the magnetic interaction injects angular momentum into the disc material, inhibiting or making accretion intermittent.

Rapidly rotating stars have a smaller R_co, making it easier for R_M to reach or surpass it, thus favoring the propeller state. The observed low accretion rates in fast rotators are consistent with this interpretation. Therefore, the wide dispersion in α is not primarily due to intrinsic differences in global disc properties but rather reflects transitions between distinct accretion states (steady vs. propeller/intermittent) regulated by the stellar spin.

The study concludes that stellar spin is a crucial parameter governing mass transfer from the disc to the star. This provides new evidence that the observed accretion properties and inferred transport efficiencies are strongly influenced by the star-disc interaction, particularly the magnetospheric coupling, shifting the perspective from a disc-centric to a star-disc interaction-centric view of accretion regulation in young stellar systems.