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Protoplanetary disks are believed to evolve on Myr timescales in a diffusive(viscous) manner as a result of angular momentum transport driven by internalstresses. Here we use a sample of 26 protoplanetary disks resolved by ALMA withmeasured (dust-based) masses and stellar accretion rates to derive thedimensionless $\alpha$-viscosity values for individual objects, with the goalof constraining the angular momentum transport mechanism. We find that theinferred values of $\alpha$ do not cluster around a single value, but insteadhave a broad distribution extending from $10^{-4}$ to $0.04$. Moreover, theycorrelate with neither the global disk parameters (mass, size, surface density)nor the stellar characteristics (mass, luminosity, radius). However, we do finda strong linear correlation between $\alpha$ and the central mass accretionrate $\dot M$. This correlation is unlikely to result from the direct physicaleffect of $\dot M$ on disk viscosity on global scales. Instead, we suggest thatit is caused by the decoupling of stellar $\dot M$ from the global diskcharacteristics in one of the following ways. (1) The behavior (and range) of$\alpha$ is controlled by a yet unidentified parameter (e.g. ionizationfraction, magnetic field strength, or geometry), ultimately driving thevariation of $\dot M$. (2) The central $\dot M$ is decoupled from the globalviscous mass accretion rate as a result of an instability or mass accumulation(or loss) in the inner disk. (3) Perhaps the most intriguing possibility isthat angular momentum in protoplanetary disks is transported non-viscously,e.g. via magnetohydrodynamic winds or spiral density waves.

Protoplanetary Disks as (Possibly) Viscous Disks