139 research outputs found

### Evolution of circumbinary planets around eccentric binaries: The case of Kepler-34

The existence of planets orbiting a central binary star system immediately
raises questions regarding their formation and dynamical evolution. Recent
discoveries of circumbinary planets by the Kepler space telescope has shown
that some of these planets reside close to the dynamical stability limit where
it is very difficult to form planets in situ. For binary systems with nearly
circular orbits, such as Kepler-38, the observed proximity of planetary orbits
to the stability limit can be understood by an evolutionary process in which
planets form farther out in the disk and migrate inward to their observed
position. The Kepler-34 system has a high orbital eccentricity of 0.52. Here,
we analyse evolutionary scenarios for the planet observed around this system
using two-dimensional hydrodynamical simulations.
The highly eccentric binary opens a wide inner hole in the disk which is also
eccentric, and displays a slow prograde precession. As a result of the large,
eccentric inner gap, an embedded planet settles in a final equilibrium position
that lies beyond the observed location of Kepler-34 b, but has the correct
eccentricity. In this configuration the planetary orbit is aligned with the
disk in a state of apsidal corotation.To account for the closer orbit of
Kepler-34 b to the central binary, we considered a two-planet scenario and
examined the evolution of the system through joint inward migration and capture
into mean-motion resonances. When the inner planet orbits inside the gap of the
disk, planet-planet scattering ensues. While often one object is thrown into a
large, highly eccentric orbit, at times the system is left with a planet close
to the observed orbit, suggesting that Kepler 34 might have had two
circumbinary planets where one might have been scattered out of the system or
into an orbit where it did not transit the central binary during the operation
of Kepler.Comment: 11 pages, 13 figures, accepted by Astronomy & Astrophysics. arXiv
admin note: text overlap with arXiv:1401.764

### The vertical structure of the boundary layer around compact objects

Context. Mass transfer due to Roche lobe overflow leads to the formation of
an accretion disk around a weakly magnetized white dwarf (WD) in cataclysmic
variables. At the inner edge of the disk, the gas comes upon the surface of the
WD and has to get rid of its excess kinetic energy in order to settle down on
the more slowly rotating outer stellar layers. This region is known as the
boundary layer (BL). Aims. In this work we investigate the vertical structure
of the BL, which is still poorly understood. We shall provide details of the
basic structure of the two-dimensional (2D) BL and how it depends on parameters
such as stellar mass and rotation rate, as well as the mass-accretion rate. We
further investigate the destination of the disk material and compare our
results with previous one-dimensional (1D) simulations. Methods. We solve the
2D equations of radiation hydrodynamics in a spherical geometry using a
parallel grid-based code that employs a Riemann solver. The radiation energy is
considered in the two-temperature approach with a radiative flux given by the
flux-limited diffusion approximation. Results. The BL around a non-rotating WD
is characterized by a steep drop in angular velocity over a width of only 1% of
the stellar radius, a heavy depletion of mass, and a high temperature (500 000
K) as a consequence of the strong shear. Variations in Om_star, M_star, and
M_dot influence the extent of the changes of the variables in the BL but not
the general structure. Depending on Om_star, the disk material travels up to
the poles or is halted at a certain latitude. The extent of mixing with the
stellar material also depends on Om_star. We find that the 1D approximation
matches the 2D data well, apart from an underestimated temperature.Comment: 15 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic

### The accretion of migrating giant planets

Most studies concerning the growth and evolution of massive planets focus
either on their accretion or their migration only. In this work we study both
processes concurrently to investigate how they might mutually affect each
other. We modeled a 2-dimensional disk with a steady accretion flow onto the
central star and embed a Jupiter mass planet at 5.2 au. The disk is locally
isothermal and viscosity is modeled using a constant $\alpha$. The planet is
held on a fixed orbit for a few hundred orbits to allow the disk to adapt and
carve a gap. After this period, the planet is released and free to move
according to the gravitational interaction with the gas disk. The mass
accretion onto the planet is modeled by removing a fraction of gas from the
inner Hill sphere, and the removed mass and momentum can be added to the
planet. Our results show that a fast migrating planet is able to accrete more
gas than a slower migrating planet. Utilizing a tracer fluid we analyzed the
origin of the accreted gas which comes predominantly originating from the inner
disk for a fast migrating planet. In case of slower migration the fraction of
gas from the outer disk increases. We also found that even for very high
accretion rates in some cases gas crosses the planetary gap from the inner to
the outer disk. Our simulations show that the crossing of gas changes during
the migration process as the migration rate slows down. Therefore classical
type II migration where the planet migrates with the viscous drift rate and no
gas crosses the gap is no general process but may only occur for special
parameters and at a certain time during the orbital evolution of the planet.Comment: 9 pages, 14 figures, accepted for publication in A&

### Modelling Circumbinary Planets: The case of Kepler-38

Recently, a number of planets orbiting binary stars have been discovered by
the Kepler space telescope. In a few systems the planets reside close to the
dynamical stability limit. Due to the difficulty of forming planets in such
close orbits, it is believed that they have formed further out in the disk and
migrated to their present locations. Our goal is to construct more realistic
models of planet migration in circumbinary disks, and to determine the final
position of these planets. In our work, we focus on the system Kepler-38. The
evolution of the circumbinary disk is studied using two-dimensional
hydrodynamical simulations. We study locally isothermal disks as well as more
realistic models with viscous heating, radiative cooling from the disk
surfaces, and radiative diffusion in the disk mid plane. After the disk has
been brought into equilibrium, a 115 Earth-mass planet is embedded and its
evolution is followed. In all cases the planets stop inward migration near the
inner edge of the disk. In isothermal disks with a typical disk scale height of
H/r = 0.05, the final outcome agrees very well with the observed location of
planet Kepler-38b. For the radiative models, the disk thickness and location of
the inner edge is determined by the mass in the system. For surface densities
in the order of 3000 g/cm^2 at 1 AU, the inner gap lies close to the binary and
planets stop in the region between the 5:1 and 4:1 mean-motion resonances with
the binary. A model with a disk with approximately a quarter of the mass yields
a final position very close to the observed one. For planets migrating in
circumbinary disks, the final position is dictated by the structure of the
disk. Knowing the observed orbits of circumbinary planets, radiative disk
simulations with embedded planets can provide important information on the
physical state of the system during the final stages of its evolution.Comment: 14 pages, accepted by Astronomy & Astrophysics, Animations available
under: http://www.tat.physik.uni-tuebingen.de/~kley/projects/kep38

### Wave mediated angular momentum transport in astrophysical boundary layers

Context. Disk accretion onto weakly magnetized stars leads to the formation
of a boundary layer (BL) where the gas loses its excess kinetic energy and
settles onto the star. There are still many open questions concerning the BL,
for instance the transport of angular momentum (AM) or the vertical structure.
Aims. It is the aim of this work to investigate the AM transport in the BL
where the magneto-rotational instability (MRI) is not operating owing to the
increasing angular velocity $\Omega(r)$ with radius. We will therefore search
for an appropriate mechanism and examine its efficiency and implications.
Methods. We perform 2D numerical hydrodynamical simulations in a cylindrical
coordinate system $(r, \varphi)$ for a thin, vertically inte- grated accretion
disk around a young star. We employ a realistic equation of state and include
both cooling from the disk surfaces and radiation transport in radial and
azimuthal direction. The viscosity in the disk is treated by the
{\alpha}-model; in the BL there is no viscosity term included. Results. We find
that our setup is unstable to the sonic instability which sets in shortly after
the simulations have been started. Acoustic waves are generated and traverse
the domain, developing weak shocks in the vicinity of the BL. Furthermore, the
system undergoes recurrent outbursts where the activity in the disk increases
strongly. The instability and the waves do not die out for over 2000 orbits.
Conclusions. There is indeed a purely hydrodynamical mechanism that enables AM
transport in the BL. It is efficient and wave mediated; however, this renders
it a non-local transport method, which means that models of a effective local
viscosity like the {\alpha}-viscosity are probably not applicable in the BL. A
variety of further implications of the non-local AM transport are discussed.Comment: 18 pages, 15 figures, accepted for publication in Astronomy &
Astrophysic

### 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 $\alpha \approx 10^{-4}$. 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 $\alpha$ 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 $\alpha$ about few times $10^{-4}$Comment: 12 pages, 24 figures, accepted for publication in Astronomy &
Astrophysic

### Tensile & shear strength of porous dust agglomerates

Context.Within the sequential accretion scenario of planet formation, planets
are build up through a sequence sticking collisions. The outcome of collisions
between porous dust aggregates is very important for the growth from very small
dust particles to planetesimals. In this work we determine the necessary
material properties of dust aggregates as a function the porosity.
Aims: Continuum models such as SPH that are capable of simulating collisions
of macroscopic dust aggregates require a set of material parameters. Some of
them such as the tensile and shear strength are difficult to obtain from
laboratory experiments. The aim of this work is to determine these parameters
from ab-initio molecular dynamics simulations.
Methods: We simulate the behavior of porous dust aggregates using a detailed
micro-physical model of the interaction of spherical grains that includes
adhesion forces, rolling, twisting, and sliding. Using different methods of
preparing the samples we study the strength behavior of our samples with
varying porosity and coordination number of the material.
Results: For the tensile strength, we can reproduce data from laboratory
experiments very well. For the shear strength, there are no experimental data
available. The results from our simulations differ significantly from previous
theoretical models, which indicates that the latter might not be sufficient to
describe porous dust aggregates.
Conclusions: We have provided functional behavior of tensile and shear
strength of porous dust aggregates as a function of the porosity that can be
directly applied in continuum simulations of these objects in planet formation
scenarios.Comment: Accepted for publication in A&

### Erosion of dust aggregates

Aims: The aim of this work is to gain a deeper insight into how much
different aggregate types are affected by erosion. Especially, it is important
to study the influence of the velocity of the impacting projectiles. We also
want to provide models for dust growth in protoplanetary disks with simple
recipes to account for erosion effects.
Methods: To study the erosion of dust aggregates we employed a molecular
dynamics approach that features a detailed micro-physical model of the
interaction of spherical grains. For the first time, the model has been
extended by introducing a new visco-elastic damping force which requires a
proper calibration. Afterwards, different sample generation methods were used
to cover a wide range of aggregate types.
Results: The visco-elastic damping force introduced in this work turns out to
be crucial to reproduce results obtained from laboratory experiments. After
proper calibration, we find that erosion occurs for impact velocities of 5 m/s
and above. Though fractal aggregates as formed during the first growth phase
are most susceptible to erosion, we observe erosion of aggregates with rather
compact surfaces as well.
Conclusions: We find that bombarding a larger target aggregate with small
projectiles results in erosion for impact velocities as low as a few m/s. More
compact aggregates suffer less from erosion. With increasing projectile size
the transition from accretion to erosion is shifted to higher velocities. This
allows larger bodies to grow through high velocity collisions with smaller
aggregates.Comment: accepted for publication in Astronomy & Astrophysic

- …