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

TL;DR

Using dynamical ALMA measurements of $M_ullet$, $M_d$, and $R_c$, this study derives a broad range of $α$ spanning approximately $10^{-5}$ to $10^{-2}$ in protoplanetary discs and uncovers a significant correlation between $α$ (and $\\dot M_ullet$) and the stellar rotation period $P_{ m rot}$. Interpreting these results with magnetospheric accretion theory, the authors argue that the accretion state shifts between steady accretion ($R_M < R_{co}$) and propeller/intermittent regimes ($R_M \gtrsim R_{co}$) as the magnetospheric radius $R_M$ moves relative to the co-rotation radius $R_{co}$, with a transition around $P_{ m rot} \sim 6$ days. They test three scenarios for the origin of the $α$-dispersion and find evidence that the dispersion reflects accretion-state transitions rather than fixed disc properties, emphasizing the role of inner-disc regulation and star–disc coupling. These findings imply episodic accretion bursts and provide a framework for understanding disc evolution and planet formation timescales in the presence of spin-regulated accretion.

Abstract

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.

Figures (3)

• Figure 1: Scatter plots of $\alpha$ against the sample's properties, stellar accretion rate, star mass, disc mass, scale radius, aspect ratio and stellar rotation period (see Table \ref{['tab:properties']} for reference). On the top of each panel the value of the Spearman rank coefficient $\rho_S$ and $p$-value are shown.
• Figure 2: Accretion rate as a function of $P_{\rm rot}\sqrt{M_\star}$ for the nine sources in our sample with rotation period measurements available in the literature.
• Figure 3: Distribution of $\dot{M}_\star$ and $\dot{M}_{\rm dyn}$ normalised to their median value, showing that the spread in accretion rates is more than an order of magnitude larger than what would be expected due to the range of outer disc properties in the sample.