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## Baroclinic Instability in Stellar Radiation Zones

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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Surfaces of constant pressure and constant density do not coincide in
differentially rotating stars. Stellar radiation zones with baroclinic
stratification can be unstable. Instabilities in radiation zones are of crucial
importance for angular momentum transport, mixing of chemical species and,
possibly, for magnetic field generation. This paper performs linear analysis of
baroclinic instability in differentially rotating stars. Linear stability
equations are formulated for differential rotation of arbitrary shape and then
solved numerically for rotation non-uniform in radius. As the differential
rotation increases, r- and g-modes of initially stable global oscillations
transform smoothly into growing modes of baroclinic instability. The
instability can therefore be interpreted as stability loss to r- and g-modes
excitation. Regions of stellar parameters where r- or g-modes are
preferentially excited are defined. Baroclinic instability onsets at a very
small differential rotation of below 1%. The characteristic time of instability
growth is about one thousand rotation periods. Growing disturbances possess
kinetic helicity. Magnetic field generation by the turbulence resulting from
baroclinic instability in differentially rotating radiation zones is...

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## Intrinsic Instability of Coronal Streamers

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 31/05/2009

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Plasma blobs are observed to be weak density enhancements as radially
stretched structures emerging from the cusps of quiescent coronal streamers. In
this paper, it is suggested that the formation of blobs is a consequence of an
intrinsic instability of coronal streamers occurring at a very localized region
around the cusp. The evolutionary process of the instability, as revealed in
our calculations, can be described as follows: (1) through the localized cusp
region where the field is too weak to sustain the confinement, plasmas expand
and stretch the closed field lines radially outward as a result of the
freezing-in effect of plasma-magnetic field coupling; the expansion brings a
strong velocity gradient into the slow wind regime providing the free energy
necessary for the onset of a subsequent magnetohydrodynamic instability; (2)
the instability manifests itself mainly as mixed streaming sausage-kink modes,
the former results in pinches of elongated magnetic loops to provoke
reconnections at one or many locations to form blobs. Then, the streamer system
returns to the configuration with a lower cusp point, subject to another cycle
of streamer instability. Although the instability is intrinsic, it does not
lead to the loss of the closed magnetic flux...

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## The Gravitational instability of solids assisted by gas drag: slowing by turbulent mass diffusivity

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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The Goldreich and Ward (1973) (axisymmetric) gravitational instability of a
razor thin particle layer occurs when the Toomre parameter $Q_T \equiv c_p
\Omega_0 / \pi G \Sigma_p < 1$ ($c_p$ being the particle dispersion velocity).
Ward(1976,2000) extended this analysis by adding the effect of gas drag upon
particles and found that even when $Q_T > 1$, sufficiently long waves were
always unstable. Youdin (2005a,b) carried out a detailed analysis and showed
that the instability allows chondrule-sized ($\sim 1 $ mm) particles to undergo
radial clumping with reasonable growth times even in the presence of a moderate
amount of turbulent stirring. The analysis of Youdin includes the role of
turbulence in setting the thickness of the dust layer and in creating a
turbulent particle pressure in the momentum equation. However, he ignores the
effect of turbulent mass diffusivity on the disturbance wave. Here we show that
including this effect reduces the growth-rate significantly, by an amount that
depends on the level of turbulence, and reduces the maximum intensity of
turbulence the instability can withstand by 1 to 3 orders of magnitude. The
instability is viable only when turbulence is extremely weak and the solid to
gas surface density of the particle layer is considerably enhanced over
minimum-mass-nebula values. A simple mechanistic explanation of the instability
shows how the azimuthal component of drag promotes instability while the radial
component hinders it. A gravito-diffusive overstability is also possible but
never realized in the nebula models.; Comment: Accepted...

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## Instability of bound states of a nonlinear Schr\"odinger equation with a Dirac potential

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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#Nonlinear Sciences - Pattern Formation and Solitons#Nonlinear Sciences - Exactly Solvable and Integrable Systems

We study analytically and numerically the stability of the standing waves for
a nonlinear Schr\"odinger equation with a point defect and a power type
nonlinearity. A main difficulty is to compute the number of negative
eigenvalues of the linearized operator around the standing waves, and it is
overcome by a perturbation method and continuation arguments. Among others, in
the case of a repulsive defect, we show that the standing wave solution is
stable in $\hurad$ and unstable in $\hu$ under subcritical nonlinearity.
Further we investigate the nature of instability: under critical or
supercritical nonlinear interaction, we prove the instability by blowup in the
repulsive case by showing a virial theorem and using a minimization method
involving two constraints. In the subcritical radial case, unstable bound
states cannot collapse, but rather narrow down until they reach the stable
regime (a {\em finite-width instability}). In the non-radial repulsive case,
all bound states are unstable, and the instability is manifested by a lateral
drift away from the defect, sometimes in combination with a finite-width
instability or a blowup instability.

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## Global Aspects of Elliptical Instability in Tidally Distorted Accretion Disks

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 04/09/1995

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Tidally distorted accretion disks in binary star systems are subject to a
local hydrodynamic instability which excites $m=1$ internal waves. This
instability is three dimensional and approximately incompressible. We study the
global aspects of this local instability using equations derived under the
shearing sheet approximation, where the effects of the azimuthal variation
along distorted orbital trajectories are included in source terms which
oscillate with local orbital phase. Linear analyses show that the excitation of
the instability is essentially local, i.e. insensitive to radial boundary
conditions. The region of rapid growth feeds waves into the region of slow or
negligible growth, allowing the instability to become global. The global growth
rate depends the maximum local growth rate, the size of the rapid growth
region, and the local group velocity. We present an empirical expression for
the global growth rate. We note that the local nature of the instability allows
the excitation of waves with $m\ne 1$ when the local growth rate is large.; Comment: ApJ submitted, 13 pages with 6 figures, uuencoded, gzipped, tarred
postscript files, or available upon request to ryu@sirius.chungnam.ac.kr

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## 3D Relativistic Magnetohydrodynamic Simulations of Current-Driven Instability. I. Instability of a static column

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 30/03/2009

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#Astrophysics - High Energy Astrophysical Phenomena#Astrophysics - Cosmology and Nongalactic Astrophysics

We have investigated the development of current-driven (CD) kink instability
through three-dimensional relativistic MHD simulations. A static force-free
equilibrium helical magnetic configuration is considered in order to study the
influence of the initial configuration on the linear and nonlinear evolution of
the instability. We found that the initial configuration is strongly distorted
but not disrupted by the kink instability. The instability develops as
predicted by linear theory. In the non-linear regime the kink amplitude
continues to increase up to the terminal simulation time, albeit at different
rates, for all but one simulation. The growth rate and nonlinear evolution of
the CD kink instability depends moderately on the density profile and strongly
on the magnetic pitch profile. The growth rate of the kink mode is reduced in
the linear regime by an increase in the magnetic pitch with radius and the
non-linear regime is reached at a later time than for constant helical pitch.
On the other hand, the growth rate of the kink mode is increased in the linear
regime by a decrease in the magnetic pitch with radius and reaches the
non-linear regime sooner than the case with constant magnetic pitch. Kink
amplitude growth in the non-linear regime for decreasing magnetic pitch leads
to a slender helically twisted column wrapped by magnetic field. On the other
hand...

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## Eddy viscosity and turbulent Schmidt number by kink-type instability of strong toroidal magnetic fields

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 01/04/2009

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The potential of the nonaxisymmetric magnetic instability to transport
angular momentum and to mix chemicals is probed considering the stability of a
nearly uniform toroidal field between conducting cylinders with different
rotation rates. The fluid between the cylinders is assumed as incompressible
and to be of uniform density. With a linear theory the neutral-stability maps
for m=1 are computed. Rigid rotation must be subAlfvenic to allow instability
while for differential rotation with negative shear also an unstable domain
with superAlfvenic rotation exists. The rotational quenching of the magnetic
instability is strongest for magnetic Prandtl number Pm=1 and becomes much
weaker for Pm unequal 1. The effective angular momentum transport by the
instability is directed outwards(inwards) for subrotation(superrotation). The
resulting magnetic-induced eddy viscosities exceed the microscopic values by
factors of 10-100. This is only true for superAlfvenic flows; in the
strong-field limit the values remain much smaller. The same instability also
quenches concentration gradients of chemicals by its nonmagnetic fluctuations.
The corresponding diffusion coefficient remains always smaller than the
magnetic-generated eddy viscosity. A Schmidt number of order 30 is found as the
ratio of the effective viscosity and the diffusion coefficient. The magnetic
instability transports much more angular momentum than that it mixes chemicals.; Comment: 9 pages...

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## Magnetic effects on the low-T/|W| instability in differentially rotating neutron stars

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 09/05/2014

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Dynamical instabilities in protoneutron stars may produce gravitational waves
whose observation could shed light on the physics of core-collapse supernovae.
When born with sufficient differential rotation, these stars are susceptible to
a shear instability (the "low-T/|W| instability"), but such rotation can also
amplify magnetic fields to strengths where they have a considerable impact on
the dynamics of the stellar matter. Using a new magnetohydrodynamics module for
the Spectral Einstein Code, we have simulated a differentially-rotating neutron
star in full 3D to study the effects of magnetic fields on this instability.
Though strong toroidal fields were predicted to suppress the low-T/|W|
instability, we find that they do so only in a small range of field strengths.
Below 4e13 G, poloidal seed fields do not wind up fast enough to have an effect
before the instability saturates, while above 5e14 G, magnetic instabilities
can actually amplify a global quadrupole mode (this threshold may be even lower
in reality, as small-scale magnetic instabilities remain difficult to resolve
numerically). Thus, the prospects for observing gravitational waves from such
systems are not in fact diminished over most of the magnetic parameter space.
Additionally...

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## Kelvin-Helmholtz instability driven by coronal mass ejections in the turbulent corona

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 11/08/2014

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Recent high resolution AIA/SDO images show evidence of the development of the
Kelvin-Helmholtz instability, as coronal mass ejections (CMEs) expand in the
ambient corona. A large-scale magnetic field mostly tangential to the interface
is observed, both on the CME and on the background sides. However, this
magnetic field is not intense enough to quench the instability. There is also
observational evidence that the ambient corona is in a turbulent regime, and
therefore the development of the instability can differ significantly from the
laminar case.
To study the evolution of the Kelvin-Helmholtz instability with a turbulent
background, we perform three-dimensional simulations of the magnetohydrodynamic
equations. The instability is driven by a velocity profile tangential to the
CME-corona interface, which we simulate through a hyperbolic tangent profile.
The turbulent background is obtained by the application of a stationary
stirring force. The main result from these simulations is the computation of
the instability growth-rate for different values of the intensity of the
turbulence.

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## Screw instability of the magnetic field connecting a rotating black hole with its surrounding disk

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 28/09/2003

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Screw instability of the magnetic field connecting a rotating black hole (BH)
with its surrounding disk is discussed based on the model of the coexistence of
the Blandford-Znajek (BZ) process and the magnetic coupling (MC) process
(CEBZMC). A criterion for the screw instability with the state of CEBZMC is
derived based on the calculations of the poloidal and toroidal components of
the magnetic field on the disk. It is shown by the criterion that the screw
instability will occur, if the BH spin and the power-law index for the
variation of the magnetic field on the disk are greater than some critical
values. It turns out that the instability occurs outside some critical radii on
the disk. It is argued that the state of CEBZMC always accompanies the screw
instability. In addtition, we show that the screw instability contributes only
a small fraction of magnetic extraction of energy from a rotating BH.; Comment: 18 pages, 13 figures; Accepted by ApJ

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## Instability criterion for oblique modes in stratified circular Couette flow

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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An analytical approach is carried out that provides an inviscid stability
criterion for the strato-rotational instability (in short SRI) occurring in a
Taylor-Couette system. The control parameters of the problem are the rotation
ratio $\mu$ and the radius ratio $\eta$. The study is motivated by recent
experimental \cite{legal} and numerical \cite{rudi, rudi2} results reporting
the existence of unstable modes beyond the Rayleigh line for centrifugal
instability ($\mu =\eta^2$). The modified Rayleigh criterion for stably
stratified flows provides the instability condition, $\mu<1$, while in
experiments unstable modes were never found beyond the line $\mu=\eta$. Taking
into account finite gap effects, we consider non axisymmetric perturbations
with azimuthal wavenumber $l$ in the limit $l Fr<<1$, where $Fr$ is the Froude
number. We derive a necessary condition for instability : $\mu < \mu^*$ where
$\mu^*$, a function of $\eta$, takes the asymptotic values, $\mu^* \to 1$ in
the narrow gap limit, and $\mu^* \to 2\eta^2(1+\eta)$ in the wide gap limit, in
agreement with recent numerical findings. A stronger condition, $\mu<\eta$, is
found when $\eta>0.38$, in agreement with experimental results obtained for
$\eta=0.8$. Whatever the gap size...

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## Current driven "plasmonic boom" instability in three-dimensional gated periodic ballistic nanostructures

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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A new approach of using distributed transmission line analogy for solving
transport equations for ballistic nanostructures is applied for solving the
three dimensional problem of the electron transport in gated ballistic
nanostructures with periodically changing width. The structures with the
varying width allow for modulation of the electron drift velocity while keeping
the plasma velocity constant. We predict that in such structures biased by a
constant current, a periodic modulation of the electron drift velocity due the
varying width results in the instability of the plasma waves if the electron
drift velocity to plasma wave velocity ratio changes from below to above unity.
The physics of such instability is similar to that of the sonic boom, but, in
the periodically modulated structures, this analog of the sonic boom is
repeated many times leading to a larger increment of the instability. The
constant plasma velocity in the sections of different width leads to the
resonant excitation of the unstable plasma modes with the varying bias current.
This effect (that we refer to as the super plasmonic boom condition) results in
a strong enhancement of the instability. The predicted instability involves the
oscillating dipole charge carried by the plasma waves. The plasmons can be
efficiently coupled to the terahertz (THz) electromagnetic radiation due to the
periodic geometry of the gated structure. Our estimates show that the analyzed
instability should enable powerful tunable terahertz electronic sources.; Comment: 12 pages...

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## Energy Gradient Theory of Hydrodynamic Instability

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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A new universal theory for flow instability and turbulent transition is
proposed in this study. Flow instability and turbulence transition have been
challenging subjects for fluid dynamics for a century. The critical condition
of turbulent transition from theory and experiments differs largely from each
other for Poiseuille flows. In this paper, a new mechanism of flow instability
and turbulence transition is presented for parallel shear flows and the energy
gradient theory of hydrodynamic instability is proposed. It is stated that the
total energy gradient in the transverse direction and that in the streamwise
direction of the main flow dominate the disturbance amplification or decay. A
new dimensionless parameter K for characterizing flow instability is proposed
for wall bounded shear flows, which is expressed as the ratio of the energy
gradients in the two directions. It is thought that flow instability should
first occur at the position of Kmax which may be the most dangerous position.
This speculation is confirmed by Nishioka et al's experimental data. Comparison
with experimental data for plane Poiseuille flow and pipe Poiseuille flow
indicates that the proposed idea is really valid. It is found that the
turbulence transition takes place at a critical value of Kmax of about 385 for
both plane Poiseuille flow and pipe Poiseuille flow...

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## The magnetron instability in a pulsar's cylindrical electrosphere

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/11/2007

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(abridged) The physics of the pulsar magnetosphere remains poorly constrained
by observations. Little is known about their emission mechanism. Large vacuum
gaps probably exist, and a non-neutral plasma partially fills the neutron star
surroundings to form an electrosphere. We showed that the differentially
rotating equatorial disk in the pulsar's electrosphere is diocotron unstable
and that it tends to stabilise when relativistic effects are included. However,
when approaching the light cylinder, particle inertia becomes significant and
the electric drift approximation is violated. In this paper, we study the most
general instability, i.e. by including particle inertia effects, as well as
relativistic motions. This general non-neutral plasma instability is called the
magnetron instability. We linearise the coupled relativistic cold-fluid and
Maxwell equations. The non-linear eigenvalue problem for the perturbed
azimuthal electric field component is solved numerically. The spectrum of the
magnetron instability in a non-neutral plasma column confined between two
cylindrically conducting walls is computed for several cylindrical
configurations. For a pulsar electrosphere, no outer wall exists. In this case,
we allow for electromagnetic wave emission propagating to infinity. When the
self-field induced by the plasma becomes significant...

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## Pair-Instability Supernovae in the Local Universe

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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The discovery of 150 - 300 M$_{\odot}$ stars in the Local Group and
pair-instability supernova candidates at low redshifts has excited interest in
this exotic explosion mechanism. Realistic light curves for pair-instability
supernovae at near-solar metallicities are key to identifying and properly
interpreting these events as more are found. We have modeled pair-instability
supernovae of 150 - 500 M$_{\odot}$ Z $\sim$ 0.1 - 0.4 Z$_{\odot}$ stars. These
stars lose up to 80% of their mass to strong line-driven winds and explode as
bare He cores. We find that their light curves and spectra are quite different
from those of Population III pair-instability explosions, which therefore
cannot be used as templates for low-redshift events. Although non-zero
metallicity pair-instability supernovae are generally dimmer than their
Population III counterparts, in some cases they will be bright enough to be
detected at the earliest epochs at which they can occur, the formation of the
first galaxies at $z \sim$ 10 - 15. Others can masquerade as dim, short
duration supernovae that are only visible in the local universe and that under
the right conditions could be hidden in a wide variety of supernova classes. We
also report for the first time that some pair-instability explosions can create
black holes with masses of $\sim$ 100 M$_{\odot}$.; Comment: 16 pages...

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## Plasma effect in the longitudinal space charge induced microbunching instability for low energy electron beams

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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The microbunching instability usually exists in the LINAC of a free electron
laser (FEL) facility. In many cases, the longitudinal space charge (LSC) is a
dominant factor that generates the instability. For the highly bright electron
beams, the plasma effect is found to be non-trivial in the development of the
instability. In this paper, starting from the Vlasov and Poisson equations in
the multiple-dimensional phase space, we perform the straightforward analysis
of the microbunching instability based on the explicit formula of the
longitudinal electric field introduced by the density perturbation in the
longitudinal direction, in such a way to be highly comparable to the
well-developed method for higher energy beams. This method generally applies in
both the cases with and without acceleration and independent of lattice
components. The results show that for a electron beam with small transverse
emittance at low energies, which is always the case in the injector of a free
electron laser device, the plasma effect results in the oscillation of the
longitudinal electric field in the modified plasma frequency that depends on
the transverse size of the beam, and the Landau damping effect in the
longitudinal electric field due to the uncorrelated longitudinal velocity
spread during the beam transportation. These two effects both play important
roles in the development of the instability. As the result...

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## Constraints for the aperiodic O-mode streaming instability

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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In plasmas where the thermal energy density exceeds the magnetic energy
density ($\beta_\parallel > 1$), the aperiodic ordinary mode (O-mode)
instability is driven by an excess of parallel temperature $A = T_\perp
/T_\parallel < 1$ (where $\parallel$ and $\perp$ denote directions relative to
the uniform magnetic field). When stimulated by parallel plasma streams the
instability conditions extend to low beta states, i.e., $\beta_\parallel <1$,
and recent studies have proven the existence of a new regime, where the
anisotropy threshold decreases steeply with lowering $\beta_\parallel \to 0$ if
the streaming velocity is sufficiently high. However, the occurrence of this
instability is questionable especially in the low-beta plasmas, where the
electrostatic two-stream instabilities are expected to develop much faster in
the process of relaxation of the counterstreams. It is therefore proposed here
to identify the instability conditions for the O-mode below those required for
the onset of the electrostatic instability. An hierarchy of these two
instabilities is established for both the low $\beta_\parallel <1$ and large
$\beta_\parallel > 1$ plasmas. The conditions where the O-mode instability can
operate efficiently are markedly constrained by the electrostatic instabilities
especially in the low-beta plasmas.; Comment: 9 pages...

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## The ballistic transport instability in Saturn's rings I: formalism and linear theory

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 13/09/2012

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Planetary rings sustain a continual bombardment of hypervelocity meteoroids
that erode the surfaces of ring particles on time scales of 10^5 - 10^7 years.
The debris ejected from such impacts re-accretes on to the ring, though often
at a slightly different orbital radius from the point of emission. This
`ballistic transport' leads to a rearrangement of the disk's mass and angular
momentum, and gives rise to a linear instability that generates structure on
relatively large scales. It is likely that the 100-km undulations in Saturn's
inner B-ring and the plateaus and 1000-km waves in Saturn's C-ring are
connected to the nonlinear saturation of the instability. In this paper the
physical problem is reformulated so as to apply to a local patch of disk (the
shearing sheet). This new streamlined model helps facilitate our physical
understanding of the instability, and also makes more tractable the analysis of
its nonlinear dynamics. We concentrate on the linear theory in this paper,
showing that the instability is restricted to a preferred range of intermediate
wavenumbers and optical depths. We subsequently apply these general results to
the inner B-ring and the C-ring and find that in both regions the ballistic
transport instability should be near marginality...

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## Effects of Rotation on the Minimum Mass of Primordial Progenitors of Pair Instability Supernovae

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

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The issue of which stars may reach the conditions of electron/positron pair
formation instability is of importance to understand the final evolution both
of the first stars and of contemporary stars. The criterion to enter the pair
instability regime in density and temperature is basically controlled by the
mass of the oxygen core. The main sequence masses that produce a given oxygen
core mass are, in turn, dependent on metallicity, mass loss, and convective and
rotationally-induced mixing. We examine the evolution of massive stars to
determine the minimum main sequence mass that can encounter pair-instability
effects, either a pulsational pair instability (PPISN) or a full-fledged
pair-instability supernova (PISN). We concentrate on zero-metallicity stars
with no mass loss subject to the Schwarzschild criterion for convective
instability, but also explore solar metallicity and mass loss and the Ledoux
criterion. As expected, for sufficiently strong rotationally-induced mixing,
the minimum main sequence mass is encountered for conditions that induce
effectively homogeneous evolution such that the original mass is converted
almost entirely to helium and then to oxygen. For this case, we find that the
minimum main sequence mass is ~40 Msun to encounter PPISN and ~65 Msun to
encounter a PISN. When mass-loss is taken into account those mass limits become
~50 Msun for PPISN and ~80 Msun for PISN progenitors. The implications of these
results for the first stars and for contemporary supernovae is discussed.; Comment: 23 pages...

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## Is the Weibel instability enhanced by the suprathermal populations, or not?

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 15/04/2010

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The kinetic instabilities of the Weibel-type are presently invoked in a large
variety of astrophysical scenarios because anisotropic plasma structures are
ubiquitous in space. The Weibel instability is driven by a temperature
anisotropy which is commonly modeled by a bi-axis distribution function, such
as a bi-Maxwellian or a generalized bi-Kappa. Previous studies have been
limited to a bi-Kappa distribution and found a suppression of this instability
in the presence of suprathermal tails. In the present paper it is shown that
the Weibel growth rate is rather more sensitive to the shape of the anisotropic
distribution function. In order to illustrate the distinguishing properties of
this instability a \emph{product-bi-Kappa distribution} is introduced, with the
advantage that this distribution function enables the use of different values
of the spectral index in the two directions, $\kappa_{\parallel} \ne
\kappa_{\perp}$. The growth rates and the instability threshold are derived and
contrasted with those for a simple bi-Kappa and a bi-Maxwellian. Thus, while
the maximum growth rates reached at the saturation are found to be higher, the
threshold is drastically reduced making the anisotropic product-bi-Kappa (with
small kappas) highly susceptible to the Weibel instability. This effect could
also rise questions on the temperature or the temperature anisotropy that seems
to be not an exclusive source of free energy for this instability...

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