Pebble Accretion in Turbulent Protoplanetary Disks

Ziyan Xu1,2, Xue-Ning Bai3,4,5, and Ruth A. Murray-Clay6
Astrophysical Journal 846, 52 Link to Article [https://doi.org/10.3847/1538-4357/aa8620]
1Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
2Department of Astronomy, Peking University, Beijing 100871, China
3Institute for Advanced Study, Tsinghua University, Beijing 100084, China
4Tsinghua Center for Astrophysics, Tsinghua University, Beijing 100084, China
5Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-51, Cambridge, MA 02138, USA
6Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA

It has been realized in recent years that the accretion of pebble-sized dust particles onto planetary cores is an important mode of core growth, which enables the formation of giant planets at large distances and assists planet formation in general. The pebble accretion theory is built upon the orbit theory of dust particles in a laminar protoplanetary disk (PPD). For sufficiently large core mass (in the “Hill regime”), essentially all particles of appropriate sizes entering the Hill sphere can be captured. However, the outer regions of PPDs are expected to be weakly turbulent due to the magnetorotational instability (MRI), where turbulent stirring of particle orbits may affect the efficiency of pebble accretion. We conduct shearing-box simulations of pebble accretion with different levels of MRI turbulence (strongly turbulent assuming ideal magnetohydrodynamics, weakly turbulent in the presence of ambipolar diffusion, and laminar) and different core masses to test the efficiency of pebble accretion at a microphysical level. We find that accretion remains efficient for marginally coupled particles (dimensionless stopping time ${\tau }_{s}\sim 0.1\mbox{--}1$) even in the presence of strong MRI turbulence. Though more dust particles are brought toward the core by the turbulence, this effect is largely canceled by a reduction in accretion probability. As a result, the overall effect of turbulence on the accretion rate is mainly reflected in the changes in the thickness of the dust layer. On the other hand, we find that the efficiency of pebble accretion for strongly coupled particles (down to ${\tau }_{s}\sim 0.01$) can be modestly reduced by strong turbulence for low-mass cores.

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