237 research outputs found
Magnetically driven accretion in protoplanetary discs
We characterize magnetically driven accretion at radii between 1 au and 100
au in protoplanetary discs, using a series of local non-ideal
magnetohydrodynamic (MHD) simulations. The simulations assume a Minimum Mass
Solar Nebula (MMSN) disc that is threaded by a net vertical magnetic field of
specified strength. Confirming previous results, we find that the Hall effect
has only a modest impact on accretion at 30 au, and essentially none at 100 au.
At 1-10 au the Hall effect introduces a pronounced bi-modality in the accretion
process, with vertical magnetic fields aligned to the disc rotation supporting
a strong laminar Maxwell stress that is absent if the field is anti-aligned. In
the anti-aligned case, we instead find evidence for bursts of turbulent stress
at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear
instability. The presence or absence of these bursts depends upon the details
of the adopted chemical model, which suggests that appreciable regions of
actual protoplanetary discs might lie close to the borderline between laminar
and turbulent behaviour. Given the number of important control parameters that
have already been identified in MHD models, quantitative predictions for disc
structure in terms of only radius and accretion rate appear to be difficult.
Instead, we identify robust qualitative tests of magnetically driven accretion.
These include the presence of turbulence in the outer disc, independent of the
orientation of the vertical magnetic fields, and a Hall-mediated bi-modality in
turbulent properties extending from the region of thermal ionization to 10 au.Comment: accepted to MNRAS after very minor revision
The subcritical baroclinic instability in local accretion disc models
(abridged) Aims: We present new results exhibiting a subcritical baroclinic
instability (SBI) in local shearing box models. We describe the 2D and 3D
behaviour of this instability using numerical simulations and we present a
simple analytical model describing the underlying physical process.
Results: A subcritical baroclinic instability is observed in flows stable for
the Solberg-Hoiland criterion using local simulations. This instability is
found to be a nonlinear (or subcritical) instability, which cannot be described
by ordinary linear approaches. It requires a radial entropy gradient weakly
unstable for the Schwartzchild criterion and a strong thermal diffusivity (or
equivalently a short cooling time). In compressible simulations, the
instability produces density waves which transport angular momentum outward
with typically alpha<3e-3, the exact value depending on the background
temperature profile. Finally, the instability survives in 3D, vortex cores
becoming turbulent due to parametric instabilities.
Conclusions: The subcritical baroclinic instability is a robust phenomenon,
which can be captured using local simulations. The instability survives in 3D
thanks to a balance between the 2D SBI and 3D parametric instabilities.
Finally, this instability can lead to a weak outward transport of angular
momentum, due to the generation of density waves by the vortices.Comment: 12 pages, 17 figures, Accepted in A&
Vortex migration in protoplanetary disks
We consider the radial migration of vortices in two-dimensional isothermal
gaseous disks. We find that a vortex core, orbiting at the local gas velocity,
induces velocity perturbations that propagate away from the vortex as density
waves. The resulting spiral wave pattern is reminiscent of an embedded planet.
There are two main causes for asymmetries in these wakes: geometrical effects
tend to favor the outer wave, while a radial vortensity gradient leads to an
asymmetric vortex core, which favors the wave at the side that has the lowest
density. In the case of asymmetric waves, which we always find except for a
disk of constant pressure, there is a net exchange of angular momentum between
the vortex and the surrounding disk, which leads to orbital migration of the
vortex. Numerical hydrodynamical simulations show that this migration can be
very rapid, on a time scale of a few thousand orbits, for vortices with a size
comparable to the scale height of the disk. We discuss the possible effects of
vortex migration on planet formation scenarios.Comment: 13 pages, 13 figures, accepted for publication in Ap
Nonaxisymmetric stability in the shearing sheet approximation
Aims: To quantify the transient growth of nonaxisymmetric perturbations in
unstratified magnetized and stratified non-magnetized rotating linear shear
flows in the shearing sheet approximation of accretion disc flows. Method: The
Rayleigh quotient in modal approaches for the linearized equations (with
time-dependent wavenumber) and the amplitudes from direct shearing sheet
simulations using a finite difference code are compared. Results: Both
approaches agree in their predicted growth behavior. The magneto-rotational
instability for axisymmetric and non-axisymmetric perturbations is shown to
have the same dependence of the (instantaneous) growth rate on the wavenumber
along the magnetic field, but in the nonaxisymmetric case the growth is only
transient. However, a meaningful dependence of the Rayleigh quotient on the
radial wavenumber is obtained. While in the magnetized case the total
amplification factor can be several orders of magnitude, it is only of order
ten or less in the nonmagnetic case. Stratification is shown to have a
stabilizing effect. In the present case of shearing-periodic boundaries the
(local) strato-rotational instability seems to be absent.Comment: 8 pages, 7 figures, A&A (in press
Turbulence in Global Simulations of Magnetized Thin Accretion Disks
We use a global magnetohydrodynamic simulation of a geometrically thin
accretion disk to investigate the locality and detailed structure of turbulence
driven by the magnetorotational instability (MRI). The model disk has an aspect
ratio , and is computed using a higher-order Godunov MHD
scheme with accurate fluxes. We focus the analysis on late times after the
system has lost direct memory of its initial magnetic flux state. The disk
enters a saturated turbulent state in which the fastest growing modes of the
MRI are well-resolved, with a relatively high efficiency of angular momentum
transport . The accretion stress
peaks at the disk midplane, above and below which exists a moderately
magnetized corona with patches of superthermal field. By analyzing the spatial
and temporal correlations of the turbulent fields, we find that the spatial
structure of the magnetic and kinetic energy is moderately well-localized (with
correlation lengths along the major axis of and respectively),
and generally consistent with that expected from homogenous incompressible
turbulence. The density field, conversely, exhibits both a longer correlation
length and a long correlation time, results which we ascribe to the importance
of spiral density waves within the flow. Consistent with prior results, we show
that the mean local stress displays a well-defined correlation with the local
vertical flux, and that this relation is apparently causal (in the sense of the
flux stimulating the stress) during portions of a global dynamo cycle. We argue
that the observed flux-stress relation supports dynamo models in which the
structure of coronal magnetic fields plays a central role in determining the
dynamics of thin-disk accretion.Comment: 24 pages and 25 figures. MNRAS in press. Version with high resolution
figures available from
http://jila.colorado.edu/~krb3u/Thin_Disk/thin_disk_turbulence.pd
On the Stability of Elliptical Vortices in Accretion Discs
(Abriged) The existence of large-scale and long-lived 2D vortices in
accretion discs has been debated for more than a decade. They appear
spontaneously in several 2D disc simulations and they are known to accelerate
planetesimal formation through a dust trapping process. However, the issue of
the stability of these structures to the imposition of 3D disturbances is still
not fully understood, and it casts doubts on their long term survival.
Aim: We present new results on the 3D stability of elliptical vortices
embedded in accretion discs, based on a linear analysis and several non-linear
simulations.
Methods: We derive the linearised equations governing the 3D perturbations in
the core of an elliptical vortex, and we show that they can be reduced to a
Floquet problem. We solve this problem numerically in the astrophysical regime
and we present several analytical limits for which the mechanism responsible
for the instability can be explained. Finally, we compare the results of the
linear analysis to some high resolution simulations.
Results: We show that most anticyclonic vortices are unstable due to a
resonance between the turnover time and the local epicyclic oscillation period.
In addition, we demonstrate that a strong vertical stratification does not
create any additional stable domain of aspect ratio, but it significantly
reduces growth rates for relatively weak (and therefore elongated) vortices.
Conclusions: Elliptical vortices are always unstable, whatever the horizontal
or vertical aspect-ratio is. The instability can however be weak and is often
found at small scales, making it difficult to detect in low-order
finite-difference simulations.Comment: 13 pages, 14 figures, accepted for publication by A&A. Animations
available on author's websit
FU Orionis disk outburst: evidence for a gravitational instability scenario triggered in a magnetically dead zone
Context: FUors outbursts are a crucial stage of accretion in young stars.
However a complete mechanism at the origin of the outburst still remains
missing. Aims: We aim at constraining the instability mechanism in FU Orionis
star itself, by directly probing the size and the evolution in time of the
outburst region with near-infrared interferometry, and to confront it to
physical models of this region. Methods: FU Orionis has been a regular target
of near-infrared interferometry. In this paper, we analyze more than 20 years
of interferometric observations to perform a temporal monitoring of the region
of the outburst, and compare it to the spatial structure deduced from 1D MHD
simulations. Results: We measure from the interferometric observations that the
size variation of the outburst region is compatible with a constant or slightly
decreasing size over time in the H and K band. The temporal variation and the
mean sizes are consistently reproduced by our 1D MHD simulations. We find that
the most compatible scenario is a model of an outburst occurring in a
magnetically layered disk, where a Magneto-Rotational Instability (MRI) is
triggered by a Gravitational Instability (GI) at the outer edge of a dead-zone.
The scenario of a pure Thermal Instability (TI) fails to reproduce our
interferometric sizes since it can only be sustained in a very compact zone of
the disk <0.1 AU. The scenario of MRI-GI could be compatible with an external
perturbation enhancing the GI, such as tidal interactions with a stellar
companion, or a planet at the outer edge of the dead-zone. Conclusions: The
layered disk model driven by MRI turbulence is favored to interpret the spatial
structure and temporal evolution of FU Orionis outburst region. Understanding
this phase gives a crucial link between the early phase of disk evolution and
the process of planet formation in the first inner AUs.Comment: Accepted for publication in A&
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