356 research outputs found
Gaussianity of Cosmic Velocity Fields and Linearity of the Velocity-Gravity Relation
We present a numerical study of the relation between the cosmic peculiar
velocity field and the gravitational acceleration field. We show that on mildly
non-linear scales (4-10 Mpc Gaussian smoothing), the distribution of the
Cartesian coordinates of each of these fields is well approximated by a
Gaussian. In particular, their kurtoses and negentropies are small compared to
those of the velocity divergence and density fields. We find that at these
scales the relation between the velocity and gravity field follows linear
theory to good accuracy. Specifically, the systematic errors in
velocity-velocity comparisons due to assuming the linear model do not exceed 6%
in beta. To correct for them, we test various nonlinear estimators of velocity
from density. We show that a slight modification of the alpha-formula proposed
by Kudlicki et al. yields an estimator which is essentially unbiased and has a
small variance.Comment: 11 pages, 15 figures; matches the version accepted for publication in
MNRA
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&
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