938 research outputs found
Vertical shear instability in accretion disc models with radiation transport
The origin of turbulence in accretion discs is still not fully understood.
While the magneto-rotational instability is considered to operate in
sufficiently ionized discs, its role in the poorly ionized protoplanetary disc
is questionable. Recently, the vertical shear instability (VSI) has been
suggested as a possible alternative. Our goal is to study the characteristics
of this instability and the efficiency of angular momentum transport, in
extended discs, under the influence of radiative transport and irradiation from
the central star. We use multi-dimensional hydrodynamic simulations to model a
larger section of an accretion disc. First we study inviscid and weakly viscous
discs using a fixed radial temperature profile in two and three spatial
dimensions. The simulations are then extended to include radiative transport
and irradiation from the central star. In agreement with previous studies we
find for the isothermal disc a sustained unstable state with a weak positive
angular momentum transport of the order of . Under the
inclusion of radiative transport the disc cools off and the turbulence
terminates. For discs irradiated from the central star we find again a
persistent instability with a similar value as for the isothermal
case. We find that the VSI can indeed generate sustained turbulence in discs
albeit at a relatively low level with about few times Comment: 12 pages, 24 figures, accepted for publication in Astronomy &
Astrophysic
The Dynamical Origin of the Multi-Planetary System HD45364
The recently discovered planetary system HD45364 which consists of a Jupiter
and Saturn mass planet is very likely in a 3:2 mean motion resonance. The
standard scenario to form planetary commensurabilities is convergent migration
of two planets embedded in a protoplanetary disc. When the planets are
initially separated by a period ratio larger than two, convergent migration
will most likely lead to a very stable 2:1 resonance for moderate migration
rates. To avoid this fate, formation of the planets close enough to prevent
this resonance may be proposed. However, such a simultaneous formation of the
planets within a small annulus, seems to be very unlikely.
Rapid type III migration of the outer planet crossing the 2:1 resonance is
one possible way around this problem. In this paper, we investigate this idea
in detail. We present an estimate for the required convergent migration rate
and confirm this with N-body and hydrodynamical simulations. If the dynamical
history of the planetary system had a phase of rapid inward migration that
forms a resonant configuration, we predict that the orbital parameters of the
two planets are always very similar and hence should show evidence of that.
We use the orbital parameters from our simulation to calculate a radial
velocity curve and compare it to observations. Our model can explain the
observational data as good as the previously reported fit. The eccentricities
of both planets are considerably smaller and the libration pattern is
different. Within a few years, it will be possible to observe the planet-planet
interaction directly and thus distinguish between these different dynamical
states.Comment: 9 pages, 9 figures - accepted for publication in Astronomy and
Astrophysic
Modeling the resonant planetary system GJ876
The two planets about the star GJ 876 appear to have undergone extensive
migration from their point of origin in the protoplanetary disk -- both because
of their close proximity to the star (30 and 60 day orbital periods) and
because of their occupying three stable orbital resonances at the 2:1
mean-motion commensurability. The resonances were most likely established by
converging differential migration of the planets leading to capture into the
resonances. A problem with this scenario is that continued migration of the
system while it is trapped in the resonances leads to orbital eccentricities
that rapidly exceed the observational upper limits of e_1 = 0.31 and e_2 =
0.05. As seen in forced 3-body simulations, lower eccentricities would persist
during migration only for an applied eccentricity damping.
Here we explore the evolution of the GJ 876 system using two-dimensional
hydrodynamical simulations that include viscous heating and radiative effects.
We find that a hydrodynamic evolution within the resonance, where only the
outer planet interacts with the disk, always rapidly leads to large values of
eccentricities that exceed those observed.
Only if mass is removed from the disk on a time scale of the order of the
migration time scale (before there has been extensive migration after capture),
as might occur for photoevaporation in the late phases of planet formation, can
we end up with eccentricities that are consistent with the observations.Comment: Paper accepted by A&A, 17 Pages, 17 Figure
The GSFC Mark-2 three band hand-held radiometer
A self-contained, portable, hand-radiometer designed for field usage was constructed and tested. The device, consisting of a hand-held probe containing three sensors and a strap supported electronic module, weighs 4 1/2 kilograms. It is powered by flashlight and transistor radio batteries, utilizes two silicon and one lead sulfide detectors, has three liquid crystal displays, sample and hold radiometric sampling, and its spectral configuration corresponds to LANDSAT-D's thematic mapper bands. The device was designed to support thematic mapper ground-truth data collection efforts and to facilitate 'in situ' ground-based remote sensing studies of natural materials. Prototype instruments were extensively tested under laboratory and field conditions with excellent results
Spiral structures in gravito-turbulent gaseous disks
CONTEXT. Gravitational instabilities can drive small-scale turbulence and large-scale spiral arms in massive gaseous disks under conditions of slow radiative cooling. These motions affect the observed disk morphology, its mass accretion rate and variability, and could control the process of planet formation via dust grain concentration, processing, and collisional fragmentation.
AIMS. We study gravito-turbulence and its associated spiral structure in thin gaseous disks subject to a prescribed cooling law. We characterize the morphology, coherence, and propagation of the spirals and examine when the flow deviates from viscous disk models.
METHODS. We used the finite-volume code PLUTO to integrate the equations of self-gravitating hydrodynamics in three-dimensional spherical geometry. The gas was cooled over longer-than-orbital timescales to trigger the gravitational instability and sustain turbulence. We ran models for various disk masses and cooling rates.
RESULTS. In all cases considered, the turbulent gravitational stress transports angular momentum outward at a rate compatible with viscous disk theory. The dissipation of orbital energy happens via shocks in spiral density wakes, heating the disk back to a marginally stable thermal equilibrium. These wakes drive vertical motions and contribute to mix material from the disk with its corona. They are formed and destroyed intermittently, and they nearly corotate with the gas at every radius. As a consequence, large-scale spiral arms exhibit no long-term global coherence, and energy thermalization is an essentially local process.
CONCLUSIONS. In the absence of radial substructures or tidal forcing, and provided a local cooling law, gravito-turbulence reduces to a local phenomenon in thin gaseous disks
3D-radiation hydro simulations of disk-planet interactions: I. Numerical algorithm and test cases
We study the evolution of an embedded protoplanet in a circumstellar disk
using the 3D-Radiation Hydro code TRAMP, and treat the thermodynamics of the
gas properly in three dimensions. The primary interest of this work lies in the
demonstration and testing of the numerical method. We show how far numerical
parameters can influence the simulations of gap opening. We study a standard
reference model under various numerical approximations. Then we compare the
commonly used locally isothermal approximation to the radiation hydro
simulation using an equation for the internal energy. Models with different
treatments of the mass accretion process are compared. Often mass accumulates
in the Roche lobe of the planet creating a hydrostatic atmosphere around the
planet. The gravitational torques induced by the spiral pattern of the disk
onto the planet are not strongly affected in the average magnitude, but the
short time scale fluctuations are stronger in the radiation hydro models.
An interesting result of this work lies in the analysis of the temperature
structure around the planet. The most striking effect of treating the
thermodynamics properly is the formation of a hot pressure--supported bubble
around the planet with a pressure scale height of H/R ~ 0.5 rather than a thin
Keplerian circumplanetary accretion disk. We also observe an outflow of gas
above and below the planet during the gap opening phase.Comment: 14 pages, 15 figures, A&A in pres
Three-dimensional Calculations of High and Low-mass Planets Embedded in Protoplanetary Discs
We analyse the non-linear, three-dimensional response of a gaseous, viscous
protoplanetary disc to the presence of a planet of mass ranging from one Earth
mass (1 M) to one Jupiter mass (1 M) by using the ZEUS hydrodynamics
code. We determine the gas flow pattern, and the accretion and migration rates
of the planet. The planet is assumed to be in a fixed circular orbit about the
central star. It is also assumed to be able to accrete gas without expansion on
the scale of its Roche radius. Only planets with masses M \gsim 0.1 M
produce significant perturbations in the disc's surface density. The flow
within the Roche lobe of the planet is fully three-dimensional. Gas streams
generally enter the Roche lobe close to the disc midplane, but produce much
weaker shocks than the streams in two-dimensional models. The streams supply
material to a circumplanetary disc that rotates in the same sense as the
planet's orbit. Much of the mass supply to the circumplanetary disc comes from
non-coplanar flow. The accretion rate peaks with a planet mass of approximately
0.1 M and is highly efficient, occurring at the local viscous rate. The
migration timescales for planets of mass less than 0.1 M, based on torques
from disc material outside the planets' Roche lobes, are in excellent agreement
with the linear theory of Type I (non-gap) migration for three-dimensional
discs. The transition from Type I to Type II (gap) migration is smooth, with
changes in migration times of about a factor of 2. Starting with a core which
can undergo runaway growth, a planet can gain up to a few M with little
migration. Planets with final masses of order 10 M would undergo large
migration, which makes formation and survival difficult.Comment: Accepted by MNRAS, 18 pages, 13 figures (6 degraded resolution).
Paper with high-resolution figures available at
http://www.astro.ex.ac.uk/people/mbate
3D-MHD simulations of an accretion disk with star-disk boundary layer
We present global 3D MHD simulations of geometrically thin but unstratified
accretion disks in which a near Keplerian disk rotates between two bounding
regions with initial rotation profiles that are stable to the MRI. The inner
region models the boundary layer between the disk and an assumed more slowly
rotating central, non magnetic star. We investigate the dynamical evolution of
this system in response to initial vertical and toroidal fields imposed in a
variety of domains contained within the near Keplerian disk. Cases with both
non zero and zero net magnetic flux are considered and sustained dynamo
activity found in runs for up to fifty orbital periods at the outer boundary of
the near Keplerian disk. Simulations starting from fields with small radial
scale and with zero net flux lead to the lowest levels of turbulence and
smoothest variation of disk mean state variables. For our computational set up,
average values of the Shakura & Sunyaev (1973) parameter in the
Keplerian disk are typically Magnetic field eventually always
diffuses into the boundary layer resulting in the build up of toroidal field
inward angular momentum transport and the accretion of disk material. The mean
radial velocity, while exhibiting large temporal fluctuations is always
subsonic. Simulations starting with net toroidal flux may yield an average
While being characterized by one order of magnitude larger
average , simulations starting from vertical fields with large radial
scale and net flux may lead to the formation of persistent non-homogeneous,
non-axisymmetric magnetically dominated regions of very low density.Comment: Accepted for publication in Ap
Eccentricity Growth Rates of Tidally Distorted Discs
We consider discs that orbit a central object and are tidally perturbed by a
circular orbit companion. Such discs are sometimes subject to an eccentric
instability due to the effects of certain resonances. Eccentric instabilities
may be present in planetary rings perturbed by satellites, protostellar discs
perturbed by planets, and discs in binary star systems. Although the basic
mechanism for eccentric instability is well understood, the detailed response
of a gaseous disc to such an instability is not understood. We apply a linear
eccentricity evolution equation developed by Goodchild and Ogilvie. We explore
how the eccentricity is distributed in such a disc and how the distribution in
turn affects the instability growth rate for a range of disc properties. We
identify a disc mode, termed the superhump mode, that is likely at work in the
superhump binary star case. The mode results from the excitation of the
fundamental free precession mode. We determine an analytic expression for the
fundamental free mode precession rate that is applicable to a sufficiently cool
disc. Depending on the disc sound speed and disc edge location, other eccentric
modes can grow faster than the superhump mode and dominate.Comment: 11 pages, 15 figures to be published on MNRA
Particle accretion onto planets in discs with hydrodynamic turbulence
The growth process of proto-planets can be sped-up by accreting a large
number of solid, pebble-sized objects that are still present in the
protoplanetary disc. It is still an open question on how efficient this process
works in realistic turbulent discs. Here, we investigate the accretion of
pebbles in turbulent discs that are driven by the purely hydrodynamical
vertical shear instability (VSI). For this purpose, we perform global
three-dimensional simulations of locally isothermal, VSI turbulent discs with
embedded protoplanetary cores from 5 to 100 that are placed at 5.2
au distance from the star. In addition, we follow the evolution of a swarm of
embedded pebbles of different size under the action of drag forces between gas
and particles in this turbulent flow. Simultaneously, we perform a set of
comparison simulations for laminar viscous discs where the particles experience
stochastic kicks. For both cases, we measure the accretion rate onto the cores
as a function of core mass and Stokes number () of the particles and
compare it to recent MRI turbulence simulations. Overall the dynamic is very
similar for the particles in the VSI turbulent disc and the laminar case with
stochastic kicks. For the small mass planets (i.e. 5 and 10 ),
well-coupled particles with , which have a size of about one meter
at this location, we find an accretion efficiency (rate of particles accreted
over drifting inward) of about 1.6-3%. For smaller and larger particles this
efficiency is higher. However, the fast inward drift for particles
makes them the most effective for rapid growth, leading to mass doubling times
of about 20,000 yr. For masses between 10 and 30 the core reaches
the pebble isolation mass and the particles are trapped at the pressure maximum
just outside of the planet, shutting off further particle accretion.Comment: 18 pages, accepted to A&
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