7,952 research outputs found
Wind-driven Accretion in Protoplanetary Disks. I: Suppression of the Magnetorotational Instability and Launching of the Magnetocentrifugal Wind
We perform local, vertically stratified shearing-box MHD simulations of
protoplanetary disks (PPDs) at a fiducial radius of 1 AU that take into account
the effects of both Ohmic resistivity and ambipolar diffusion (AD). The
magnetic diffusion coefficients are evaluated self-consistently from a look-up
table based on equilibrium chemistry. We first show that the inclusion of AD
dramatically changes the conventional picture of layered accretion. Without net
vertical magnetic field, the system evolves into a toroidal field dominated
configuration with extremely weak turbulence in the far-UV ionization layer
that is far too inefficient to drive rapid accretion. In the presence of a weak
net vertical field (plasma beta~10^5 at midplane), we find that the MRI is
completely suppressed, resulting in a fully laminar flow throughout the
vertical extent of the disk. A strong magnetocentrifugal wind is launched that
efficiently carries away disk angular momentum and easily accounts for the
observed accretion rate in PPDs. Moreover, under a physical disk wind geometry,
all the accretion flow proceeds through a strong current layer with thickness
of ~0.3H that is offset from disk midplane with radial velocity of up to 0.4
times the sound speed. Both Ohmic resistivity and AD are essential for the
suppression of the MRI and wind launching. The efficiency of wind transport
increases with increasing net vertical magnetic flux and the penetration depth
of the FUV ionization. Our laminar wind solution has important implications on
planet formation and global evolution of PPDs.Comment: 23 pages, 13 figures, accepted to Ap
Dynamics of Solids in the Midplane of Protoplanetary Disks: Implications for Planetesimal Formation
(Abridged) We present local 2D and 3D hybrid numerical simulations of
particles and gas in the midplane of protoplanetary disks (PPDs) using the
Athena code. The particles are coupled to gas aerodynamically, with
particle-to-gas feedback included. Magnetorotational turbulence is ignored as
an approximation for the dead zone of PPDs, and we ignore particle self-gravity
to study the precursor of planetesimal formation. Our simulations include a
wide size distribution of particles, ranging from strongly coupled particles
with dimensionless stopping time tau_s=Omega t_stop=1e-4 to marginally coupled
ones with tau_s=1 (where Omega is the orbital frequency, t_stop is the particle
friction time), and a wide range of solid abundances. Our main results are: 1.
Particles with tau_s>=0.01 actively participate in the streaming instability,
generate turbulence and maintain the height of the particle layer before
Kelvin-Helmholtz instability is triggered. 2. Strong particle clumping as a
consequence of the streaming instability occurs when a substantial fraction of
the solids are large (tau_s>=0.01) and when height-integrated solid to gas mass
ratio Z is super-solar. 3. The radial drift velocity is reduced relative to the
conventional Nakagawa-Sekiya-Hayashi (NSH) model, especially at high Z. We
derive a generalized NSH equilibrium solution for multiple particle species
which fits our results very well. 4. Collision velocity between particles with
tau_s>=0.01 is dominated by differential radial drift, and is strongly reduced
at larger Z. 5. There exist two positive feedback loops with respect to the
enrichment of local disk solid abundance and grain growth. All these effects
promote planetesimal formation.Comment: 25 pages (emulate apj), accepted to Ap
A New Godunov Scheme for MHD, with Application to the MRI in disks
We describe a new numerical scheme for MHD which combines a higher order
Godunov method (PPM) with Constrained Transport. The results from a selection
of multidimensional test problems are presented. The complete test suite used
to validate the method, as well as implementations of the algorithm in both F90
and C, are available from the web. A fully three-dimensional version of the
algorithm has been developed, and is being applied to a variety of
astrophysical problems including the decay of supersonic MHD turbulence, the
nonlinear evolution of the MHD Rayleigh-Taylor instability, and the saturation
of the magnetorotational instability in the shearing box. Our new simulations
of the MRI represent the first time that a higher-order Godunov scheme has been
applied to this problem, providing a quantitative check on the accuracy of
previous results computed with ZEUS; the latter are found to be reliable.Comment: 11 pages, style files included, Conference Proceedings: "Magnetic
Fields in the Universe: from Laboratory and Stars to Primordial Structures",
More information on Athena can be found at
http://www.astro.princeton.edu/~jstone/athena.htm
Nonlinear Evolution of the Magnetohydrodynamic Rayleigh-Taylor Instability
We study the nonlinear evolution of the magnetic Rayleigh-Taylor instability
using three-dimensional MHD simulations. We consider the idealized case of two
inviscid, perfectly conducting fluids of constant density separated by a
contact discontinuity perpendicular to the effective gravity g, with a uniform
magnetic field B parallel to the interface. Modes parallel to the field with
wavelengths smaller than l_c = [B B/(d_h - d_l) g] are suppressed (where d_h
and d_l are the densities of the heavy and light fluids respectively), whereas
modes perpendicular to B are unaffected. We study strong fields with l_c
varying between 0.01 and 0.36 of the horizontal extent of the computational
domain. Even a weak field produces tension forces on small scales that are
significant enough to reduce shear (as measured by the distribution of the
amplitude of vorticity), which in turn reduces the mixing between fluids, and
increases the rate at which bubbles and finger are displaced from the interface
compared to the purely hydrodynamic case. For strong fields, the highly
anisotropic nature of unstable modes produces ropes and filaments. However, at
late time flow along field lines produces large scale bubbles. The kinetic and
magnetic energies transverse to gravity remain in rough equipartition and
increase as t^4 at early times. The growth deviates from this form once the
magnetic energy in the vertical field becomes larger than the energy in the
initial field. We comment on the implications of our results to Z-pinch
experiments, and a variety of astrophysical systems.Comment: 25 pages, accepted by Physics of Fluids, online version of journal
has high resolution figure
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