381 research outputs found
Models of the formation of the planets in the 47 UMa system
Formation of planets in the 47 UMa system is followed in an evolving
protoplanetary disk composed of gas and solids. The evolution of the disk is
calculated from an early stage, when all solids, assumed to be high-temperature
silicates, are in the dust form, to the stage when most solids are locked in
planetesimals. The simulation of planetary evolution starts with a solid embryo
of ~1 Earth mass, and proceeds according to the core accretion -- gas capture
model. Orbital parameters are kept constant, and it is assumed that the
environment of each planet is not perturbed by the second planet. It is found
that conditions suitable for both planets to form within several Myr are easily
created, and maintained throughout the formation time, in disks with . In such disks, a planet of 2.6 Jupiter masses (the minimum for
the inner planet of the 47 UMa system) may be formed at 2.1 AU from the star in
\~3 Myr, while a planet of 0.89 Jupiter masses (the minimum for the outer
planet) may be formed at 3.95 AU from the star in about the same time. The
formation of planets is possible as a result of a significant enhancement of
the surface density of solids between 1.0 and 4.0 AU, which results from the
evolution of a disk with an initially uniform gas-to-dust ratio of 167 and an
initial radius of 40 AU.Comment: Accepted for publication in A&A. 10 pages, 10 figure
The Clustering Dipole of the Local Universe from the Two Micron All Sky Survey
The unprecedented sky coverage and photometric uniformity of the Two Micron
All Sky Survey (2MASS) provides a rich resource for investigating the galaxies
populating the local Universe. A full characterization of the large-scale
clustering distribution is important for theoretical studies of structure
formation. 2MASS offers an all-sky view of the local galaxy population at 2.15
micron, unbiased by young stellar light and minimally affected by dust. We use
2MASS to map the local distribution of galaxies, identifying the largest
structures in the nearby universe. The inhomogeneity of these structures causes
an acceleration on the Local Group of galaxies, which can be seen in the dipole
of the Cosmic Microwave Background (CMB). We find that the direction of the
2MASS clustering dipole is 11 degrees from the CMB dipole, confirming that the
local galaxy distribution accelerates the Local Group. From the magnitude of
the dipole we find a value of the linear bias parameter b=1.37 +/- 0.3 in the
K_s-band. The 2MASS clustering dipole is 19 degrees from the latest measurement
of the dipole using galaxies detected by the Infrared Astronomical Satellite
(IRAS) suggesting that bias may be non-linear in some wavebands.Comment: 7 pages, 4 figures, submitted to ApJ Letters, a version of the paper
with full resolution figures can be found here
http://daisy.astro.umass.edu/~ari
Vortex generation in protoplanetary disks with an embedded giant planet
Vortices in protoplanetary disks can capture solid particles and form
planetary cores within shorter timescales than those involved in the standard
core-accretion model. We investigate vortex generation in thin unmagnetized
protoplanetary disks with an embedded giant planet with planet to star mass
ratio and . Two-dimensional hydrodynamical simulations of a
protoplanetary disk with a planet are performed using two different numerical
methods. The results of the non-linear simulations are compared with a
time-resolved modal analysis of the azimuthally averaged surface density
profiles using linear perturbation theory. Finite-difference methods
implemented in polar coordinates generate vortices moving along the gap created
by Neptune-mass to Jupiter-mass planets. The modal analysis shows that unstable
modes are generated with growth rate of order for azimuthal
numbers m=4,5,6, where is the local Keplerian frequency.
Shock-capturing Cartesian-grid codes do not generate very much vorticity around
a giant planet in a standard protoplanetary disk. Modal calculations confirm
that the obtained radial profiles of density are less susceptible to the growth
of linear modes on timescales of several hundreds of orbital periods.
Navier-Stokes viscosity of the order (in units of )
is found to have a stabilizing effect and prevents the formation of vortices.
This result holds at high resolution runs and using different types of boundary
conditions. Giant protoplanets of Neptune-mass to Jupiter-mass can excite the
Rossby wave instability and generate vortices in thin disks. The presence of
vortices in protoplanetary disks has implications for planet formation, orbital
migration, and angular momentum transport in disks.Comment: 14 pages, 15 figures, accepted for publication in A&
A comparative study of disc-planet interaction
We perform numerical simulations of a disc-planet system using various
grid-based and smoothed particle hydrodynamics (SPH) codes. The tests are run
for a simple setup where Jupiter and Neptune mass planets on a circular orbit
open a gap in a protoplanetary disc during a few hundred orbital periods. We
compare the surface density contours, potential vorticity and smoothed radial
profiles at several times. The disc mass and gravitational torque time
evolution are analyzed with high temporal resolution. There is overall
consistency between the codes. The density profiles agree within about 5% for
the Eulerian simulations while the SPH results predict the correct shape of the
gap although have less resolution in the low density regions and weaker
planetary wakes. The disc masses after 200 orbital periods agree within 10%.
The spread is larger in the tidal torques acting on the planet which agree
within a factor 2 at the end of the simulation. In the Neptune case the
dispersion in the torques is greater than for Jupiter, possibly owing to the
contribution from the not completely cleared region close to the planet.Comment: 32 pages, accepted for publication in MNRA
Planet formation in Alpha Centauri A revisited: not so accretion-friendly after all
We numerically explore planet formation around alpha Cen A by focusing on the
crucial planetesimals-to-embryos phase. Our code computes the relative velocity
distribution, and thus the accretion vs. fragmentation trend, of planetesimal
populations having any given size distribution. This is a critical aspect of
planet formation in binaries since the pericenter alignment of planetesimal
orbits due to the gravitational perturbations of the companion star and to gas
friction strongly depends on size. We find that, for the nominal case of a MMSN
gas disc, the region beyond 0.5AU from the primary is hostile to planetesimal
accretion. In this area, impact velocities between different-size bodies are
increased, by the differential orbital phasing, to values too high to allow
mutual accretion. For any realistic size distribution for the planetesimal
population, this accretion-inhibiting effect is the dominant collision outcome
and the accretion process is halted. Results are robust with respect to the
profile and density of the gas disc: except for an unrealistic almost gas-free
case, the inner accretion safe area never extends beyond 0.75AU. We conclude
that planet formation is very difficult in the terrestrial region around alpha
Cen A, unless it started from fast-formed very large (>30km) planetesimals.
Notwithstanding these unlikely initial conditions, the only possible
explanation for the presence of planets around 1 AU from the star would be the
hypothetical outward migration of planets formed closer to the star or a
different orbital configuration in the binary's early history. Our conclusions
differ from those of several studies focusing on the later embryos-to-planets
stage, confirming that the planetesimals-to-embryos phase is more affected by
binary perturbations.Comment: accepted for publication in MNRAS (Note: abstract truncated. Full
abstract in the pdf file
Algorithmic comparisons of decaying, isothermal, supersonic turbulence
Contradicting results have been reported in the literature with respect to
the performance of the numerical techniques employed for the study of
supersonic turbulence. We aim at characterising the performance of different
particle-based and grid-based techniques on the modelling of decaying
supersonic turbulence. Four different grid codes (ENZO, FLASH, TVD, ZEUS) and
three different SPH codes (GADGET, PHANTOM, VINE) are compared. We additionally
analysed two calculations denoted as PHANTOM A and PHANTOM B using two
different implementations of artificial viscosity. Our analysis indicates that
grid codes tend to be less dissipative than SPH codes, though details of the
techniques used can make large differences in both cases. For example, the
Morris & Monaghan viscosity implementation for SPH results in less dissipation
(PHANTOM B and VINE versus GADGET and PHANTOM A). For grid codes, using a
smaller diffusion parameter leads to less dissipation, but results in a larger
bottleneck effect (our ENZO versus FLASH runs). As a general result, we find
that by using a similar number of resolution elements N for each spatial
direction means that all codes (both grid-based and particle-based) show
encouraging similarity of all statistical quantities for isotropic supersonic
turbulence on spatial scales k<N/32 (all scales resolved by more than 32 grid
cells), while scales smaller than that are significantly affected by the
specific implementation of the algorithm for solving the equations of
hydrodynamics. At comparable numerical resolution, the SPH runs were on average
about ten times more computationally intensive than the grid runs, although
with variations of up to a factor of ten between the different SPH runs and
between the different grid runs. (abridged)Comment: accepted by A&A, 22 pages, 14 figure
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