2,613 research outputs found
KH15D: a star eclipsed by a large scale dusty vortex?
We propose that the large photometric variations of KH15D are due to an
eclipsing swarm of solid particles trapped in giant gaseous vortex rotating at
\~0.2 AU from the star. The efficiency of the capture-in-vortex mechanism
easily explains the observed large optical depth. The weaker opacity at
mid-eclipse is consistent with a size segregation of the particles toward the
center of the vortex. This dusty structure must extend over ~1/3 of an orbit to
account for the long eclipse duration. The estimated size of the trapped
particles is found to range from 1 to 10cm, consistent with the gray extinction
of the star. The observations of KH15D support the idea that giant vortices can
grow in circumstellar disks and play a central role in planet formation.Comment: Accepted in ApJ Letters - 4 pages - 2 figure
Conversational leadership, organizational identification, and social motivation : a field descriptive study in two organizations
Dissertation (Ph.D.)--University of Kansas, Communication Studies, 1985
The Formation and Role of Vortices in Protoplanetary Disks
We carry out a two-dimensional, compressible, simulation of a disk, including
dust particles, to study the formation and role of vortices in protoplanetary
disks. We find that anticyclonic vortices can form out of an initial random
perturbation of the vorticity field. Vortices have a typical decay time of the
order of 50 orbital periods (for a viscosity parameter alpha=0.0001 and a disk
aspect ratio of H/r = 0.15). If vorticity is continuously generated at a
constant rate in the flow (e.g. by convection), then a large vortex can form
and be sustained (due to the merger of vortices).
We find that dust concentrates in the cores of vortices within a few orbital
periods, when the drag parameter is of the order of the orbital frequency.
Also, the radial drift of the dust induces a significant increase in the
surface density of dust particles in the inner region of the disk. Thus,
vortices may represent the preferred location for planetesimal formation in
protoplanetary disks.
We show that it is very difficult for vortex mergers to sustain a relatively
coherent outward flux of angular momentum.Comment: Sumitted to the Astrophysical Journal, October 20, 199
CoRoT's first seven planets: An overview
The up to 150 day uninterrupted high-precision photometry of about 100000
stars - provided so far by the exoplanet channel of the CoRoT space telescope -
gave a new perspective on the planet population of our galactic neighbourhood.
The seven planets with very accurate parameters widen the range of known planet
properties in almost any respect. Giant planets have been detected at low
metallicity, rapidly rotating and active, spotted stars. CoRoT-3 populated the
brown dwarf desert and closed the gap of measured physical properties between
standard giant planets and very low mass stars. CoRoT extended the known range
of planet masses down to 5 Earth masses and up to 21 Jupiter masses, the radii
to less than 2 Earth radii and up to the most inflated hot Jupiter found so
far, and the periods of planets discovered by transits to 9 days. Two CoRoT
planets have host stars with the lowest content of heavy elements known to show
a transit hinting towards a different planet-host-star-metallicity relation
then the one found by radial-velocity search programs. Finally the properties
of the CoRoT-7b prove that terrestrial planets with a density close to Earth
exist outside the Solar System. The detection of the secondary transit of
CoRoT-1 at the -level and the very clear detection of the 1.7 Earth
radii of CoRoT-7b at relative flux are promising evidence of
CoRoT being able to detect even smaller, Earth sized planets.Comment: 8 pages, 19 figures and 3 table
A toral diffeomorphism with a non-polygonal rotation set
We construct a diffeomorphism of the two-dimensional torus which is isotopic
to the identity and whose rotation set is not a polygon
Protoplanetary Disk Turbulence Driven by the Streaming Instability: Non-Linear Saturation and Particle Concentration
We present simulations of the non-linear evolution of streaming instabilities
in protoplanetary disks. The two components of the disk, gas treated with grid
hydrodynamics and solids treated as superparticles, are mutually coupled by
drag forces. We find that the initially laminar equilibrium flow spontaneously
develops into turbulence in our unstratified local model. Marginally coupled
solids (that couple to the gas on a Keplerian time-scale) trigger an upward
cascade to large particle clumps with peak overdensities above 100. The clumps
evolve dynamically by losing material downstream to the radial drift flow while
receiving recycled material from upstream. Smaller, more tightly coupled solids
produce weaker turbulence with more transient overdensities on smaller length
scales. The net inward radial drift is decreased for marginally coupled
particles, whereas the tightly coupled particles migrate faster in the
saturated turbulent state. The turbulent diffusion of solid particles, measured
by their random walk, depends strongly on their stopping time and on the
solids-to-gas ratio of the background state, but diffusion is generally modest,
particularly for tightly coupled solids. Angular momentum transport is too weak
and of the wrong sign to influence stellar accretion. Self-gravity and
collisions will be needed to determine the relevance of particle overdensities
for planetesimal formation.Comment: Accepted for publication in ApJ (17 pages). Movies of the simulations
can be downloaded at http://www.mpia.de/~johansen/research_en.ph
Formation of Giant Planets by Concurrent Accretion of Solids and Gas inside an Anti-Cyclonic Vortex
We study the formation of a giant gas planet by the core--accretion
gas--capture process, with numerical simulations, under the assumption that the
planetary core forms in the center of an anti-cyclonic vortex. The presence of
the vortex concentrates particles of centimeter to meter size from the
surrounding disk, and speeds up the core formation process. Assuming that a
planet of Jupiter mass is forming at 5 AU from the star, the vortex enhancement
results in considerably shorter formation times than are found in standard
core--accretion gas--capture simulations. Also, formation of a gas giant is
possible in a disk with mass comparable to that of the minimum mass solar
nebula.Comment: 27 pages, 4 figures, ApJ in pres
A Conley index study of the evolution of the Lorenz strange set
In this paper we study the Lorenz equations using the perspective of the
Conley index theory. More specifically, we examine the evolution of the strange
set that these equations posses throughout the different values of the
parameter. We also analyze some natural Morse decompositions of the global
attractor of the system and the role of the strange set in these
decompositions. We calculate the corresponding Morse equations and study their
change along the successive bifurcations. In addition, we formulate and prove
some theorems which are applicable in more general situations. These theorems
refer to Poincar\'{e}-Andronov-Hopf bifurcations of arbitrary codimension,
bifurcations with two homoclinic loops and a study of the role of the
travelling repellers in the transformation of repeller-attractor pairs into
attractor-repeller ones.Comment: 22 pages, 1 figur
Vortices in Thin, Compressible, Unmagnetized Disks
We consider the formation and evolution of vortices in a hydrodynamic
shearing-sheet model. The evolution is done numerically using a version of the
ZEUS code. Consistent with earlier results, an injected vorticity field evolves
into a set of long-lived vortices, each of which has a radial extent comparable
to the local scale height. But we also find that the resulting velocity field
has a positive shear stress, . This effect appears
only at high resolution. The transport, which decays with time as t^-1/2,
arises primarily because the vortices drive compressive motions. This result
suggests a possible mechanism for angular momentum transport in low-ionization
disks, with two important caveats: a mechanism must be found to inject
vorticity into the disk, and the vortices must not decay rapidly due to
three-dimensional instabilities.Comment: 8 pages, 10 figures (high resolution figures available in ApJ
electronic edition
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