Colloquium

Protoplanetary Disks and Planet Formation: A Microphysical Perspective

Planet formation is intimately connected to the gas dynamics in protoplanetary disks (PPDs), where a central role is played by magnetic fields. Due to extremely weak level of ionization, PPDs suffer from strong non-ideal magnetohydrodynamic (MHD) effects including Ohmic resistivity, the Hall effect and ambipolar diffusion, and they are the key to understanding the level of disk turbulence, angular momentum transport, and the overall disk structure and evolution. Via local simulations that self-consistently include all non-ideal MHD effects, we show that in the inner region of PPDs (<10 AU), the magneto-rotational instability (MRI) is suppressed, and disk accretion is mainly driven by magnetocentrifugal wind. The gas dynamics also strongly depends on the polarity of external magnetic field threading the disk as a result of the Hall effect. We further predict that PPD wind is heavily loaded, with wind mass loss rate a substantial fraction of disk accretion rate. In the outer region of PPDs (>15 AU), the MRI operates in the surface layer due to far-UV ionization, and is damped near the midplane due to ambipolar diffusion. In addition, external magnetic flux strongly concentrates into thin, axisymmetric shells, leading to enhanced radial density variations known as zonal flows. Implications on planet formation at large separation will be discussed. Our simulation results provide key ingredients for a new paradigm on PPD gas dynamics, and shed new lights on the theory of planet formation.

Followed by wine and cheese in Pupin 1402.

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