47 research outputs found
Spiral-driven accretion in protoplanetary discs - I. 2D models
We numerically investigate the dynamics of a 2D non-magnetised protoplanetary
disc surrounded by an inflow coming from an external envelope. We find that the
accretion shock between the disc and the inflow is unstable, leading to the
generation of large-amplitude spiral density waves. These spiral waves
propagate over long distances, down to radii at least ten times smaller than
the accretion shock radius. We measure spiral-driven outward angular momentum
transport with 1e-4 1e-8
Msun/yr. We conclude that the interaction of the disc with its envelope leads
to long-lived spiral density waves and radial angular momentum transport with
rates that cannot be neglected in young non-magnetised protostellar discs.Comment: 4 pages, 4 figures, accepted in A&A Letter
A simple toy model of the advective-acoustic instability. II. Numerical simulations
The physical processes involved in the advective-acoustic instability are
investigated with 2D numerical simulations. Simple toy models, developped in a
companion paper, are used to describe the coupling between acoustic and
entropy/vorticity waves, produced either by a stationary shock or by the
deceleration of the flow. Using two Eulerian codes based on different second
order upwind schemes, we confirm the results of the perturbative analysis. The
numerical convergence with respect to the computation mesh size is studied with
1D simulations. We demonstrate that the numerical accuracy of the quantities
which depend on the physics of the shock is limited to a linear convergence. We
argue that this property is likely to be true for most current numerical
schemes dealing with SASI in the core-collapse problem, and could be solved by
the use of advanced techniques for the numerical treatment of the shock. We
propose a strategy to choose the mesh size for an accurate treatment of the
advective-acoustic coupling in future numerical simulations.Comment: 9 pages, 10 figures, ApJ in press, new Sect. 5 and Fig.
Dynamics of an Alfven surface in core collapse supernovae
We investigate the dynamics of an Alfven surface (where the Alfven speed
equals the advection velocity) in the context of core collapse supernovae
during the phase of accretion on the proto-neutron star. Such a surface should
exist even for weak magnetic fields because the advection velocity decreases to
zero at the center of the collapsing core. In this decelerated flow, Alfven
waves created by the standing accretion shock instability (SASI) or convection
accumulate and amplify while approaching the Alfven surface. We study this
amplification using one dimensional MHD simulations with explicit physical
dissipation. In the linear regime, the amplification continues until the Alfven
wavelength becomes as small as the dissipative scale. A pressure feedback that
increases the pressure in the upstream flow is created via a non linear
coupling. We derive analytic formulae for the maximum amplification and the non
linear coupling and check them with numerical simulations to a very good
accuracy. We also characterize the non linear saturation of this amplification
when compression effects become important, leading to either a change of the
velocity gradient, or a steepening of the Alfven wave. Applying these results
to core collapse supernovae shows that the amplification can be fast enough to
affect the dynamics, if the magnetic field is strong enough for the Alfven
surface to lie in the region of strong velocity gradient just above the
neutrinosphere. This requires the presence of a strong magnetic field in the
progenitor star, which would correspond to the formation of a magnetar under
the assumption of magnetic flux conservation. An extrapolation of our analytic
formula (taking into account the nonlinear saturation) suggests that the Alfven
wave could reach an amplitude of B ~ 10^15 G, and that the pressure feedback
could significantly contribute to the pressure below the shock.Comment: 18 pages, 14 figures, accepted for publication in ApJ. Added a
discussion of the energy budget in subsection 7.
Midplane sedimentation of large solid bodies in turbulent protoplanetary discs
We study the vertical settling of solid bodies in a turbulent protoplanetary
disc. We consider the situation when the coupling to the gas is weak or
equivalently when the particle stopping time tau_{st} due to friction with the
gas is long compared to the orbital timescale Omega^{-1}. An analytical model,
which takes into account the stochastic nature of the sedimentation process
using a Fokker-Planck equation for the particle distribution function in phase
space, is used to obtain the vertical scale height of the solid layer as a
function of the vertical component of the turbulent gas velocity correlation
function and the particle stopping time. This is found to be of the same form
as the relation obtained for strongly coupled particles in previous work.
We compare the predictions of this model with results obtained from local
shearing box MHD simulations of solid particles embedded in a vertically
stratified disc in which there is turbulence driven by the MRI. We find that
the ratio of the dust disc thickness to the gas disc thickness satifies
H_d/H=0.08 (Omega tau_{st})^{-1/2}, which is in very good agreement with the
analytical model. By discussing the conditions for gravitational instability in
the outer regions of protoplanetary discs in which there is a similar level of
turbulence, we find that bodies in the size range 50 to 600 metres can
aggregate to form Kuiper belt-like objects with characteristic radii ranging
from tens to hundreds of kilometres.Comment: 8 pages, 4 figures, accepted in MNRA
Numerical simulations of type I planetary migration in nonturbulent magnetized discs
Using 2D MHD numerical simulations performed with two different finite
difference Eulerian codes, we analyze the effect that a toroidal magnetic field
has on low mass planet migration in nonturbulent protoplanetary discs. The
presence of the magnetic field modifies the waves that can propagate in the
disc. In agreement with a recent linear analysis (Terquem 2003), we find that
two magnetic resonances develop on both sides of the planet orbit, which
contribute to a significant global torque. In order to measure the torque
exerted by the disc on the planet, we perform simulations in which the latter
is either fixed on a circular orbit or allowed to migrate. For a 5 earth mass
planet, when the ratio \beta between the square of the sound speed and that of
the Alfven speed at the location of the planet is equal to 2, we find inward
migration when the magnetic field B_{\phi} is uniform in the disc, reduced
migration when B_{\phi} decreases as r^{-1} and outward migration when B_{\phi}
decreases as r^{-2}. These results are in agreement with predictions from the
linear analysis. Taken as a whole, our results confirm that even a subthermal
stable field can stop inward migration of an earth--like planet.Comment: 17 pages, 12 figures, accepted in MNRAS. A version with full
resolution, colour figures is available at
http://www.maths.qmul.ac.uk/~rpn/preprints
On the aerodynamic redistribution of chondrite components in protoplanetary disks
Despite being all roughly of solar composition, primitive meteorites
(chondrites) present a diversity in their chemical, isotopic and petrographic
properties, and in particular a first-order dichotomy between carbonaceous and
non-carbonaceous chondrites. We investigate here analytically the dynamics of
their components (chondrules, refractory inclusions, metal/sulfide and matrix
grains) in protoplanetary disks prior to their incorporation in chondrite
parent bodies. We find the dynamics of the solids, subject to gas drag, to be
essentially controlled by the "gas-solid decoupling parameter" , the ratio of the dimensionless stopping time to the
turbulence parameter. The decoupling of the solid particles relative to the gas
is significant when exceeds unity. is expected to increase with time
and heliocentric distance. On the basis of (i) abundance of refractory
inclusions (ii) proportion of matrix (iii) lithophile element abundances and
(iv) oxygen isotopic composition of chondrules, we propose that non-matrix
chondritic components had
when the other chondrites accreted. This suggests that accretion of
carbonaceous chondrites predated on average that of the other chondrites and
that refractory inclusions are genetically related to their host carbonaceous
chondrites.Comment: 13 pages, 6 figures, accepted to Icaru
The ionization fraction in alpha-models of protoplanetary disks
We calculate the ionization fraction of protostellar alpha disks, taking into
account vertical temperature structure, and the possible presence of trace
metal atoms. Both thermal and X-ray ionization are considered. Previous
investigations of layered disks used radial power-law models with isothermal
vertical structure. But alpha models are used to model accretion, and the
present work is a step towards a self-consistent treatment. The extent of the
magnetically uncoupled (``dead'') zone depends sensitively on alpha, on the
assumed accretion rate, and on the critical magnetic Reynolds number, below
which MHD turbulence cannot be self-sustained. Its extent is extremely
model-dependent. It is also shown that a tiny fraction of the cosmic abundance
of metal atoms can dramatically affect the ionization balance. Gravitational
instabilities are an unpromising source of transport, except in the early
stages of disk formation.Comment: 25 pages including 8 figures, Latex in the MN style - Accepted by
MNRA
Kinematic Dynamos using Constrained Transport with High Order Godunov Schemes and Adaptive Mesh Refinement
We propose to extend the well-known MUSCL-Hancock scheme for Euler equations
to the induction equation modeling the magnetic field evolution in kinematic
dynamo problems. The scheme is based on an integral form of the underlying
conservation law which, in our formulation, results in a ``finite-surface''
scheme for the induction equation. This naturally leads to the well-known
``constrained transport'' method, with additional continuity requirement on the
magnetic field representation. The second ingredient in the MUSCL scheme is the
predictor step that ensures second order accuracy both in space and time. We
explore specific constraints that the mathematical properties of the induction
equations place on this predictor step, showing that three possible variants
can be considered. We show that the most aggressive formulations (referred to
as C-MUSCL and U-MUSCL) reach the same level of accuracy as the other one
(referred to as Runge-Kutta), at a lower computational cost. More
interestingly, these two schemes are compatible with the Adaptive Mesh
Refinement (AMR) framework. It has been implemented in the AMR code RAMSES. It
offers a novel and efficient implementation of a second order scheme for the
induction equation. We have tested it by solving two kinematic dynamo problems
in the low diffusion limit. The construction of this scheme for the induction
equation constitutes a step towards solving the full MHD set of equations using
an extension of our current methodology.Comment: 40 pages, 10 figures, accepted in Journal of Computational Physics. A
version with full resolution is available at
http://www.damtp.cam.ac.uk/user/fromang/publi/TFD.pd