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

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-µm-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.

Magnetorotational-Instability-Driven Accretion in Protoplanetary Disks

“…However, since the ionization rate depends on the column gas density from the disk surface, this effect is compensated by the reduced ionization rate and enhanced recombination rate (at given disk height), giving only order unity corrections to the predicted accretion rate. In fact, the predicted accretion rate does not sensitively depend on the density distribution across disk height, and the approximate calculations by Perez-Becker & Chiang (2011b) that are free from the vertical density distribution are generally consistent with ours within order unity.…”

“…The upper limit on the magnetic field strength characterized by β (blue solid) is controlled by AD, where smaller Am value requires weaker magnetic field. The value of Am above the lower boundary is generally greater than 0.1, and reaches up to 10 in the disk upper layers, which agrees with PAH-free model calculations in Perez-Becker & Chiang (2011a), and the minimum value of β ranges from about 10 to 800. The two constraints combined together give the MRI permitted region in the z-B plane at a given radius the PPD, as painted in gray.…”

“…One closely related work is by Wardle (2007), who performed similar chemistry calculations to obtain magnetic diffusivities of all non-ideal MHD effects with a simpler reaction network, but the extent of the active layer and accretion rate was not addressed in detail which may be partly due to the unavailability of numerical simulation results. Another closely related work to ours is by Perez-Becker & Chiang (2011a), who were motivated by the accretion problem in transitional disks and the role of tiny grains. They have considered both Ohmic resistivity and AD, although their adopted criteria were more simplified and did not account for the role of magnetic field strength.…”

“…The importance of PAHs on the disk conductivity has been raised by Perez-Becker & Chiang (2011a) recently. They argue that PAHs in T-Tauri disks may be equally abundant as in Herbig Ae/Be disks, but they are undetected because T-Tauri stars are much less luminous in ultraviolet (UV) radiation to excite fluorescence emission from the PAHs.…”

Searching for Baby Planets in a Star’s Dusty Rings

“…The ionized region is predicted to have high turbulence velocities and quickly accrete onto the central star, possibly leading to the formation of a large central hole [37]. At larger distances the ionization is nonthermal and caused mostly by stellar ultraviolet and x-ray photons and interstellar cosmic rays, which are absorbed over columns of 0.1, 10, and 100 g cm −2 , respectively [38][39][40]. The ionized regions experience MRI, which leads to turbulent speeds that increase with height above the disk midplane in proportion to the Alfven speed, reaching the local sound speed above several disk scale heights [41,42].…”