SURFACE LAYER ACCRETION IN CONVENTIONAL AND TRANSITIONAL DISKS DRIVEN BY FAR-ULTRAVIOLET IONIZATION

Journal Article · Fri Jul 01 04:00:00 EDT 2011 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/735/1/8· OSTI ID:21576577

Perez-Becker, Daniel ^[1]; Chiang, Eugene ^[2]

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1. Department of Physics, University of California, Berkeley, CA 94720 (United States)
2. Departments of Astronomy and Earth and Planetary Science, University of California, Berkeley, CA 94720 (United States)

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 {approx}0.01 {mu}m 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{sup -2}, behaves in the ideal magnetohydrodynamic limit and can accrete at observationally significant rates at radii {approx}> 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 {approx}<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.

• OSTI ID: 21576577
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 735; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ACCRETION DISKS
AROMATICS
ELECTROMAGNETIC RADIATION
FAR ULTRAVIOLET RADIATION
FLUID MECHANICS
HYDROCARBONS
HYDRODYNAMICS
INSTABILITY
IONIZATION
IONIZING RADIATIONS
LAYERS
MAGNETOHYDRODYNAMICS
MECHANICS
ORGANIC COMPOUNDS
POLYCYCLIC AROMATIC HYDROCARBONS
PROTOPLANETS
RADIATIONS
STARS
SURFACES
ULTRAVIOLET RADIATION
X RADIATION