227 research outputs found
Dynamical instability of differentially rotating stars
We study the dynamical instability against bar-mode deformation of
differentially rotating stars. We performed numerical simulation and linear
perturbation analysis adopting polytropic equations of state with the
polytropic index . It is found that rotating stars of a high degree of
differential rotation are dynamically unstable even for the ratio of the
kinetic energy to the gravitational potential energy of .
Gravitational waves from the final nonaxisymmetric quasistationary states are
calculated in the quadrupole formula. For rotating stars of mass
and radius several 10 km, gravitational waves have frequency several 100 Hz and
effective amplitude at a distance of Mpc.Comment: 5 pages, 7 figures, accepted for publication in MNRA
Dynamical bar-mode instability of differentially rotating stars: Effects of equations of state and velocity profiles
As an extension of our previous work, we investigate the dynamical
instability against nonaxisymmetric bar-mode deformations of differentially
rotating stars in Newtonian gravity varying the equations of state and velocity
profiles. We performed the numerical simulation and the followup linear
stability analysis adopting polytropic equations of state with the polytropic
indices n=1, 3/2, and 5/2 and with two types of angular velocity profiles (the
so-called j-constant-like and Kepler-like laws). It is confirmed that rotating
stars of a high degree of differential rotation are dynamically unstable
against the bar-mode deformation, even for the ratio of the kinetic energy to
the gravitational potential energy of order 0.01. The criterion for
onset of the bar-mode dynamical instability depends weakly on the polytropic
index n and the angular velocity profile as long as the degree of differential
rotation is high. Gravitational waves from the final nonaxisymmetric
quasi-stationary states are calculated in the quadrupole formula. For
proto-neutron stars of mass , radius km and \beta \alt
0.1, such gravitational waves have the frequency of 600--1,400 Hz, and
the effective amplitude is larger than at a distance of about 100
Mpc irrespective of n and the angular velocity profile.Comment: 9 pages, 14 figures, accepted to MNRA
Meissner effect in honeycomb arrays of multi-walled carbon nanotubes
We report Meissner effect for type-II superconductors with a maximum Tc of 19
K, which is the highest value among those in new-carbon related
superconductors, found in the honeycomb arrays of multi-walled CNTs (MWNTs).
Drastic reduction of ferromagnetic catalyst and efficient growth of MWNTs by
deoxidization of catalyst make the finding possible. The weak magnetic
anisotropy, superconductive coherence length (- 7 nm), and disappearance of the
Meissner effect after dissolving array structure indicate that the graphite
structure of an MWNT and those intertube coupling in the honeycomb array are
dominant factors for the mechanism.Comment: 6 page
A numerical study of the r-mode instability of rapidly rotating nascent neutron stars
The first results of numerical analysis of classical r-modes of {\it rapidly}
rotating compressible stellar models are reported. The full set of linear
perturbation equations of rotating stars in Newtonian gravity are numerically
solved without the slow rotation approximation. A critical curve of
gravitational wave emission induced instability which restricts the rotational
frequencies of hot young neutron stars is obtained. Taking the standard cooling
mechanisms of neutron stars into account, we also show the `evolutionary
curves' along which neutron stars are supposed to evolve as cooling and
spinning-down proceed. Rotational frequencies of stars suffering
from this instability decrease to around 100Hz when the standard cooling
mechanism of neutron stars is employed. This result confirms the results of
other authors who adopted the slow rotation approximation.Comment: 4 pages, 2 figures; MNRAS,316,L1(2000
Toward inertial confinement fusion energy based on heavy ion beam
Heavy ion inertial fusion (HIF) energy would be one of promising energy
resources securing our future energy in order to sustain our human life for
centuries and beyond. The heavy ion beam (HIB) has remarkable preferable
features to release the fusion energy in inertial confinement fusion: in
particle accelerators HIBs are generated with a high driver efficiency of ~
30-40%, and the HIB ions deposit their energy inside of materials. Therefore, a
requirement for the fusion target energy gain is relatively low, that would be
~50-70 to operate a HIF fusion reactor with the standard energy output of 1GW
of electricity. The HIF reactor operation frequency would be ~10~15 Hz or so.
Several-MJ HIBs illuminate a fusion fuel target, and the fuel target is
imploded to about a thousand times of the solid density. Then the DT fuel is
ignited and burned. The HIB ion deposition range would be ~0.5-1 mm or so
depending on the material. Therefore, a relatively large density-scale length
appears in the fuel target material. The large density-gradient-scale length
helps to reduce the Rayleigh-Taylor (R-T) growth rate. The key merits in HIF
physics are presented in the article toward our bright future energy resource.Comment: 17 pages. arXiv admin note: substantial text overlap with
arXiv:1511.06508, arXiv:1608.0106
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