Surface Layer Accretion in Conventional and Transitional Disks Driven by Far-Ultraviolet Ionization

TL;DR

This paper demonstrates that far-ultraviolet (FUV) radiation ionizes disk surface layers sufficiently to enable MRI-driven accretion in protoplanetary disks, especially at radii greater than 1-10 AU, impacting disk evolution models.

Contribution

It reveals that FUV ionization creates dense plasma in disk surface layers, allowing MRI turbulence and accretion rates consistent with observations, especially in transitional disks.

Findings

FUV ionization produces dense plasma in disk surface layers.

MRI turbulence can drive significant accretion at >1-10 AU.

FUV-driven accretion explains increasing rates with larger hole sizes in transitional disks.

Abstract

Whether protoplanetary disks accrete at observationally significant rates by the magnetorotational instability (MRI) depends on how well ionized they are. Disk surface layers ionized by stellar X-rays are susceptible to charge neutralization by small condensates, ranging from ~0.01-micron-sized grains to angstrom-sized polycyclic aromatic hydrocarbons (PAHs). Ion densities in X-ray-irradiated surfaces are so low that ambipolar diffusion weakens the MRI. Here we show that ionization by stellar far-ultraviolet (FUV) radiation enables full-blown MRI turbulence in disk surface layers. Far-UV ionization of atomic carbon and sulfur produces a plasma so dense that it is immune to ion recombination on grains and PAHs. The FUV-ionized layer, of thickness 0.01--0.1 g/cm^2, behaves in the ideal magnetohydrodynamic limit and can accrete at observationally significant rates at radii > 1--10 AU. Surface layer accretion driven by FUV ionization can reproduce the trend of increasing accretion rate with increasing hole size seen in transitional disks. At radii < 1--10 AU, FUV-ionized surface layers cannot sustain the accretion rates generated at larger distance, and unless turbulent mixing of plasma can thicken the MRI-active layer, an additional means of transport is needed. In the case of transitional disks, it could be provided by planets.