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Protoplanetary Disks as (Possibly) Viscous Disks

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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 internal stresses. Here we use a sample of 26 protoplanetary disks resolved by ALMA with measured (dust-based) masses and stellar accretion rates to derive the dimensionless α-viscosity values for individual objects, with the goal of constraining the angular momentum transport mechanism. We find that the inferred values of α do not cluster around a single value, but instead have a broad distribution extending from 10−4 to 0.04. Moreover, they correlate with neither the global disk parameters (mass, size, surface density) nor the stellar characteristics (mass, luminosity, radius). However, we do find a strong linear correlation between α and the central mass accretion rate M˙. This correlation is unlikely to result from the direct physical effect of M˙ on disk viscosity on global scales. Instead, we suggest that it is caused by the decoupling of stellar M˙ from the global disk characteristics in one of the following ways. (1) The behavior (and range) of α is controlled by a yet unidentified parameter (e.g. ionization fraction, magnetic field strength, or geometry), ultimately driving the variation of M˙. (2) The central M˙ is decoupled from the global viscous mass accretion rate as a result of an instability or mass accumulation (or loss) in the inner disk. (3) Perhaps the most intriguing possibility is that angular momentum in protoplanetary disks is transported non-viscously, e.g. via magnetohydrodynamic winds or spiral density waves.



accretion, accretion disks, planets and satellites: formation, protoplanetary disks

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Astrophysical Journal

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American Astronomical Society
Science and Technology Facilities Council (ST/L000636/1)
Science and Technology Facilities Council (ST/P000673/1)