58,127 research outputs found
Using survey participants to estimate the impact of nonparticipation
The authors evaluate the effectiveness of two models often used to measure the extent of nonparticipation bias in survey estimates. The first model establishes a "continuum of resistance" to being surveyed, placing people who were interviewed after one phone call on one end and nonparticipants on the other. The second assumes that there are "classes" of nonparticipants and that similar classes can be found among participants; it identifies groups of participants thought to be like nonparticipants and uses them as "proxies" to estimate the characteristics of nonparticipants. The authors use these models to examine how accurately they estimate the characteristics of nonparticipants and the impact of nonparticipation on survey estimates of means of child support awards and payments in Wisconsin. They find that neither model detects the true extent of nonparticipation bias.
Dissociation Transition of a Composite Lattice of Magnetic Vortices in the Flux-Flow Regime of Two-Band Superconductors
In multiband superconductors, each superconducting condensate supports
vortices with fractional quantum flux. In the ground state, vortices in
different bands are spatially bounded together to form a composite vortex,
carrying one quantum flux \Phi_0. Here we predict dissociation of the composite
vortices lattice in the flux flow state due to the disparity of the vortex
viscosity and flux of the vortex in different bands. For a small driving
current, composite vortices start to deform, but the constituting vortices in
different bands move with the same velocity. For a large current, composite
vortices dissociate and vortices in different bands move with different
velocities. The dissociation transition shows up as an increase of flux flow
resistivity. In the dissociated phase, Shapiro steps are developed when an ac
current is superimposed with a dc current.Comment: 4.5 pages, 3 figure
Toward a Deterministic Model of Planetary Formation IV: Effects of Type-I Migration
In a further development of a deterministic planet-formation model (Ida & Lin
2004), we consider the effect of type-I migration of protoplanetary embryos due
to their tidal interaction with their nascent disks. During the early embedded
phase of protostellar disks, although embryos rapidly emerge in regions
interior to the ice line, uninhibited type-I migration leads to their efficient
self-clearing. But, embryos continue to form from residual planetesimals at
increasingly large radii, repeatedly migrate inward, and provide a main channel
of heavy element accretion onto their host stars. During the advanced stages of
disk evolution (a few Myr), the gas surface density declines to values
comparable to or smaller than that of the minimum mass nebula model and type-I
migration is no longer an effective disruption mechanism for mars-mass embryos.
Over wide ranges of initial disk surface densities and type-I migration
efficiency, the surviving population of embryos interior to the ice line has a
total mass several times that of the Earth. With this reservoir, there is an
adequate inventory of residual embryos to subsequently assemble into rocky
planets similar to those around the Sun. But, the onset of efficient gas
accretion requires the emergence and retention of cores, more massive than a
few M_earth, prior to the severe depletion of the disk gas. The formation
probability of gas giant planets and hence the predicted mass and semimajor
axis distributions of extrasolar gas giants are sensitively determined by the
strength of type-I migration. We suggest that the observed fraction of
solar-type stars with gas giant planets can be reproduced only if the actual
type-I migration time scale is an order of magnitude longer than that deduced
from linear theories.Comment: 32 pages, 8 figures, 1 table, accepted for publication in Ap
An alternative model for the origin of gaps in circumstellar disks
Motivated by recent observational and numerical studies suggesting that
collapsing protostellar cores may be replenished from the local environment, we
explore the evolution of protostellar cores submerged in the external
counter-rotating environment. These models predict the formation of
counter-rotating disks with a deep gap in the gas surface density separating
the inner disk (corotating with the star) and the outer counter-rotating disk.
The properties of these gaps are compared to those of planet-bearing gaps that
form in disks hosting giant planets. We employ numerical hydrodynamics
simulations of collapsing cores that are replenished from the local
counter-rotating environment, as well as numerical hydrodynamic simulations of
isolated disks hosting giant planets, to derive the properties of the gaps that
form in both cases. Our numerical simulations demonstrate that counter-rotating
disks can form for a wide range of mass and angular momentum available in the
local environment. The gap that separates both disks has a depletion factor
smaller than 1%, can be located at a distance from ten to over a hundred AU
from the star, and can propagate inward with velocity ranging from 1 AU/Myr to
>100 AU/Myr. Unlike our previous conclusion, the gap can therefore be a
long-lived phenomenon, comparable in some cases to the lifetime of the disk
itself. For a proper choice of the planetary mass, the viscous \alpha-parameter
and the disk mass, the planet-bearing gaps and the gaps in counter-rotating
disks may show a remarkable similarity in the gas density profile and depletion
factor, which may complicate their observational differentiation.Comment: 13 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
-Particle Spectrum in the Reaction p+B
Using a simple phenomenological parametrization of the reaction amplitude we
calculated -particle spectrum in the reaction p+B at the resonance proton energy 675 KeV. The parametrization
includes Breit-Wigner factor with an energy dependent width for intermediate
state and the Coulomb and the centrifugal factors in -particle
emission vertexes. The shape of the spectrum consists of a well defined peak
corresponding to emission of the primary and a flat shoulder going
down to very low energy. We found that below 1.5 MeV there are 17.5% of
's and below 1 MeV there are 11% of them.Comment: 6 pages, 3 figure
Measuring spectrum of spin wave using vortex dynamics
We propose to measure the spectrum of magnetic excitation in magnetic
materials using motion of vortex lattice driven by both ac and dc current in
superconductors. When the motion of vortex lattice is resonant with oscillation
of magnetic moments, the voltage decreases at a given current. From transport
measurement, one can obtain frequency of the magnetic excitation with the wave
number determined by vortex lattice constant. By changing the lattice constant
through applied magnetic fields, one can obtains the spectrum of the magnetic
excitation up to a wave vector of order .Comment: 4 pages, 2 figure
Measurements of quasi-particle tunneling in the nu = 5/2 fractional quantum Hall state
Some models of the 5/2 fractional quantum Hall state predict that the
quasi-particles, which carry the charge, have non-Abelian statistics: exchange
of two quasi-particles changes the wave function more dramatically than just
the usual change of phase factor. Such non-Abelian statistics would make the
system less sensitive to decoherence, making it a candidate for implementation
of topological quantum computation. We measure quasi-particle tunneling as a
function of temperature and DC bias between counter-propagating edge states.
Fits to theory give e*, the quasi-particle effective charge, close to the
expected value of e/4 and g, the strength of the interaction between
quasi-particles, close to 3/8. Fits corresponding to the various proposed wave
functions, along with qualitative features of the data, strongly favor the
Abelian 331 state
Three-dimensional waves generated at Lindblad resonances in thermally stratified disks
We analyze the linear, 3D response to tidal forcing of a disk that is thin
and thermally stratified in the direction normal to the disk plane. We model
the vertical disk structure locally as a polytrope which represents a disk of
high optical depth. We solve the 3D gas-dynamic equations semi-analytically in
the neighborhood of a Lindblad resonance. These solutions match asymptotically
on to those valid away from resonances and provide solutions valid at all
radii. We obtain the following results. 1) A variety of waves are launched at
resonance. However, the f mode carries more than 95% of the torque exerted at
the resonance. 2) These 3D waves collectively transport exactly the amount of
angular momentum predicted by the 2D torque formula. 3) Near resonance, the f
mode occupies the full vertical extent of the disk. Away from resonance, the f
mode becomes confined near the surface of the disk, and, in the absence of
other dissipation mechanisms, damps via shocks. The radial length scale for
this process is roughly r_L/m (for resonant radius r_L and azimuthal wavenumber
m), independent of the disk thickness H. This wave channeling process is due to
the variations of physical quantities in r and is not due to wave refraction.
4) However, the inwardly propagating f mode launched from an m=2 inner Lindblad
resonance experiences relatively minor channeling.
We conclude that for binary stars, tidally generated waves in highly
optically thick circumbinary disks are subject to strong nonlinear damping by
the channeling mechanism, while those in circumstellar accretion disks are
subject to weaker nonlinear effects. We also apply our results to waves excited
by young planets for which m is approximately r/H and conclude that the waves
are damped on the scale of a few H.Comment: 15 pages, 3 figures, 2 colour plates, to be published in the
Astrophysical Journa
Critical Protoplanetary Core Masses in Protoplanetary Disks and the Formation of Short-Period Giant Planets
We study a solid protoplanetary core of 1-10 earth masses migrating through a
disk. We suppose the core luminosity is generated as a result of planetesimal
accretion and calculate the structure of the gaseous envelope assuming
equilibrium. This is a good approximation when the core mass is less than the
critical value, M_{crit}, above which rapid gas accretion begins. We model the
structure of the protoplanetary nebula as an accretion disk with constant
\alpha. We present analytic fits for the steady state relation between disk
surface density and mass accretion rate as a function of radius r. We calculate
M_{crit} as a function of r, gas accretion rate through the disk, and
planetesimal accretion rate onto the core \dot{M}. For a fixed \dot{M},
M_{crit} increases inwards, and it decreases with \dot{M}. We find that \dot{M}
onto cores migrating inwards in a time 10^3-10^5 yr at 1 AU is sufficient to
prevent the attainment of M_{crit} during the migration process. Only at small
radii where planetesimals no longer exist can M_{crit} be attained. At small
radii, the runaway gas accretion phase may become longer than the disk lifetime
if the core mass is too small. However, massive cores can be built-up through
the merger of additional incoming cores on a timescale shorter than for in situ
formation. Therefore, feeding zone depletion in the neighborhood of a fixed
orbit may be avoided. Accordingly, we suggest that giant planets may begin to
form early in the life of the protostellar disk at small radii, on a timescale
that may be significantly shorter than for in situ formation. (abridged)Comment: 24 pages (including 9 figures), LaTeX, uses emulateapj.sty, to be
published in ApJ, also available at http://www.ucolick.org/~ct/home.htm
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