79 research outputs found
Angular momentum transport and large eddy simulations in magnetorotational turbulence: the small Pm limit
Angular momentum transport in accretion discs is often believed to be due to
magnetohydrodynamic turbulence mediated by the magnetorotational instability.
Despite an abundant literature on the MRI, the parameters governing the
saturation amplitude of the turbulence are poorly understood and the existence
of an asymptotic behavior in the Ohmic diffusion regime is not clearly
established. We investigate the properties of the turbulent state in the small
magnetic Prandtl number limit. Since this is extremely computationally
expensive, we also study the relevance and range of applicability of the most
common subgrid scale models for this problem. Unstratified shearing boxes
simulations are performed both in the compressible and incompressible limits,
with a resolution up to 800 cells per disc scale height. The latter constitutes
the largest resolution ever attained for a simulation of MRI turbulence. In the
presence of a mean magnetic field threading the domain, angular momentum
transport converges to a finite value in the small Pm limit. When the mean
vertical field amplitude is such that {\beta}, the ratio between the thermal
and magnetic pressure, equals 1000, we find {\alpha}~0.032 when Pm approaches
zero. In the case of a mean toroidal field for which {\beta}=100, we find
{\alpha}~0.018 in the same limit. Both implicit LES and Chollet-Lesieur closure
model reproduces these results for the {\alpha} parameter and the power
spectra. A reduction in computational cost of a factor at least 16 (and up to
256) is achieved when using such methods. MRI turbulence operates efficiently
in the small Pm limit provided there is a mean magnetic field. Implicit LES
offers a practical and efficient mean of investigation of this regime but
should be used with care, particularly in the case of a vertical field.
Chollet-Lesieur closure model is perfectly suited for simulations done with a
spectral code.Comment: Accepted for publication in A&
Multiple spiral patterns in the transitional disk of HD 100546
Protoplanetary disks around young stars harbor many structures related to
planetary formation. Of particular interest, spiral patterns were discovered
among several of these disks and are expected to be the sign of gravitational
instabilities leading to giant planets formation or gravitational perturbations
caused by already existing planets. In this context, the star HD100546 presents
some specific characteristics with a complex gas and dusty disk including
spirals as well as a possible planet in formation. The objective of this study
is to analyze high contrast and high angular resolution images of this
emblematic system to shed light on critical steps of the planet formation. We
retrieved archival images obtained at Gemini in the near IR (Ks band) with the
instrument NICI and processed the data using advanced high contrast imaging
technique taking advantage of the angular differential imaging. These new
images reveal the spiral pattern previously identified with HST with an
unprecedented resolution, while the large-scale structure of the disk is mostly
erased by the data processing. The single pattern at the southeast in HST
images is now resolved into a multi-armed spiral pattern. Using two models of a
gravitational perturber orbiting in a gaseous disk we attempted to bring
constraints on the characteristics of this perturber assuming each spiral being
independent and we derived qualitative conclusions. The non-detection of the
northeast spiral pattern observed in HST allows to put a lower limit on the
intensity ratio between the two sides of the disk, which if interpreted as
forward scattering yields a larger anisotropic scattering than derived in the
visible. Also, we found that the spirals are likely spatially resolved with a
thickness of about 5-10AU. Finally, we did not detect the candidate forming
planet recently discovered in the Lp band, with a mass upper limit of 16-18 MJ.Comment: Accepted for publication in Astronomy and Astrophysics, 10 pages, 8
figure
Warping modes in discs around accreting neutron stars
The origin and stability of a thin sheet of plasma in the magnetosphere of an
accreting neutron star is investigated. First the radial extension of such a
magnetospheric disc is explored. Then a mechanism for magnetospheric accretion
is proposed, reconsidering the bending wave explored by Agapitou, Papaloizou &
Terquem (1997), that was found to be stable in ideal MHD. We show that this
warping becomes unstable and can reach high amplitudes, in a variant of
Pringle's radiation-driven model for the warping of AGN accretion discs
(Pringle (1996)). Finally we discuss how this mechanism might give a clue to
explain the observed X-ray kHz QPO of neutron star binaries.Comment: Accepted for publication in MNRA
The multiple spirals in the disk of HD100546
We report the detection of multi-armed spirals in the environment of HD 100546 using NICI at Gemini South in the Ks band. These data feature a better angular resolution and higher contrast than previous HST images, which allows to resolve the former known spiral into a multiple pattern. An analytic model with a gravitational perturber is used to fit the spiral pattern. We derived limit of detections which set constraints on the discovered forming plane
Relation de 5 mélanoblastomes : 2 chez le bœuf, 3 chez le chien
Lombard Charles, Cazieux André, Bourge M., Meheut-Ferron Pierre. Relation de 5 mélanoblastomes : 2 chez le Bœuf, 3 chez le Chien. In: Bulletin de l'Académie Vétérinaire de France tome 117 n°4, 1964. pp. 181-188
Rossby Wave Instability and three-dimensional vortices in accretion disks
Context. The formation of vortices in accretion disks is of high interest in
various astrophysical contexts, in particular for planet formation or in the
disks of compact objects. But despite numerous attempts it has thus far not
been possible to produce strong vortices in fully three-dimensional simulations
of disks. Aims. The aim of this paper is to present the first 3D simulation of
a strong vortex, established across the vertically stratified structure of a
disk by the Rossby Wave Instability. Methods. Using the Versatile Advection
Code (VAC), we set up a fully 3D cylindrical stratified disk potentially prone
to the Rossby Wave Instability. Results. The simulation confirms the basic
expectations obtained from previous 2D analytic and numerical works. The
simulation exhibits a strong vortex that grows rapidly and saturates at a
finite amplitude. On the other hand the third dimension shows unexpected
additional behaviours that could be of strong importance in the astrophysical
roles that such vortices can play.Comment: Accepted by Astronomy and Astrophysic
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