7,972 research outputs found
Global axisymmetric stability analysis for a composite system of two gravitationally coupled scale-free discs
In a composite system of gravitationally coupled stellar and gaseous discs,
we perform linear stability analysis for axisymmetric coplanar perturbations
using the two-fluid formalism. The background stellar and gaseous discs are
taken to be scale-free with all physical variables varying as powers of
cylindrical radius with compatible exponents. The unstable modes set in as
neutral modes or stationary perturbation configurations with angular frequency
.Comment: 7 pages using AAS styl
A Model for the Moving `Wisps' in the Crab Nebula
I propose that the moving `wisps' near the center of the Crab Nebula result
from nonlinear Kelvin-Helmholtz instabilities in the equatorial plane of the
shocked pulsar wind. Recent observations suggest that the wisps trace out
circular wavefronts in this plane, expanding radially at speeds approximately
less than c/3. Instabilities could develop if there is sufficient velocity
shear between a faster-moving equatorial zone and a slower moving shocked
pulsar wind at higher latitudes. The development of shear could be related to
the existence of a neutral sheet -- with weak magnetic field -- in the
equatorial zone, and could also be related to a recent suggestion by Begelman
that the magnetic field in the Crab pulsar wind is much stronger than had been
thought. I show that plausible conditions could lead to the growth of
instabilities at the radii and speeds observed, and that their nonlinear
development could lead to the appearance of sharp wisplike features.Comment: 7 pages; 3 postscript figures; LaTex, uses emulateapj.sty; to Appear
in the Astrophysical Journal, Feb. 20, 1999, Vol. 51
Topological phase transition in a narrow bandgap semiconductor nanolayer
Narrow bandgap semiconductor nanostructures have been explored for
realization of topological superconducting quantum devices in which Majorana
states can be created and employed for constructing topological qubits.
However, a prerequisite to achieve the topological phase transition in these
nanostructures is application of a magnetic field, which could complicate the
technology development towards topological quantum computing. Here we
demonstrate that a topological phase transition can be achieved in a narrow
bandgap semiconductor nanolayer under application of a perpendicular electric
field. Based on full band structure calculations, it is shown that the
topological phase transition occurs at an electric-field induced band inversion
and is accompanied by a sharp change of the invariant at the
critical field. We also demonstrate that the nontrivial topological phase is
manifested by the quantum spin Hall edge states in a band-inverted nanolayer
Hall-bar structure. We present the phase diagram of the nanolayer in the space
of layer thickness and electric field strength, and discuss the optimal
conditions to achieve a large topological bandgap in the electric-field induced
topological phase of a semiconductor nanolayer.Comment: 6 pages, 5 figure
Recommended from our members
Electron quantum interference in epitaxial antiferromagnetic NiO thin films
The electron reflectivity from NiO thin films grown on Ag(001) has been systematically studied as a function of film thickness and electron energy. A strong electron quantum interference effect was observed from the NiO film, which is used to derive the unoccupied band dispersion above the Fermi surface along the Γ-X direction using the phase accumulation model. The experimental bands agree well with first-principles calculations. A weaker electron quantum interference effect was also observed from the CoO film
X-ray triple rings around the M87 jets in the central Virgo cluster
The Chandra X-ray data of the central Virgo cluster are re-examined to reveal
a triple-ring structure around the galaxy M87, reminiscent of the spectacular
triple-ring pattern of the SN1987A in the Large Magellanic Cloud (LMC). In the
sky plane, the two apparent smaller ellipses are roughly aligned along the M87
jets; the larger ring centers at the M87 nucleus and is likely a circle roughly
perpendicular to the M87 jet. Certain similarities of these two triple-ring
structures might hint at similar processes that operate in these two systems
with entirely different sizes and mass scales. We suspect that a major merging
event of two galaxies with nuclear supermassive black holes (SMBHs) might
create such a triple-ring structure and drove acoustic and internal gravity
waves far and near. The M87 jets are perhaps powered by a spinning SMBH
resulting from this catastrophic merging event.Comment: accepted by ApJ
MHD tidal waves on a spinning magnetic compact star
In an X-ray binary system, the companion star feeds the compact neutron star
with plasma materials via accretions. The spinning neutron star is likely
covered with a thin "magnetized ocean" and may support {\it magnetohydrodynamic
(MHD) tidal waves}. While modulating the thermal properties of the ocean, MHD
tidal waves periodically shake the base of the stellar magnetosphere that traps
energetic particles, including radiating relativistic electrons. For a radio
pulsar, MHD tidal waves in the stellar surface layer may modulate radio
emission processes and leave indelible signatures on timescales different from
the spin period. Accretion activities are capable of exciting these waves but
may also obstruct or obscure their detections meanwhile. Under fortuitous
conditions, MHD tidal waves might be detectable and offer valuable means to
probe properties of the underlying neutron star. Similar situations may also
occur for a cataclysmic variable -- an accretion binary system that contains a
rotating magnetic white dwarf. This Letter presents the theory for MHD tidal
waves in the magnetized ocean of a rotating degenerate star and emphasizes
their potential diagnostics in X-ray and radio emissions.Comment: ApJ Letter paper already publishe
Dynamic Evolution of a Quasi-Spherical General Polytropic Magnetofluid with Self-Gravity
In various astrophysical contexts, we analyze self-similar behaviours of
magnetohydrodynamic (MHD) evolution of a quasi-spherical polytropic magnetized
gas under self-gravity with the specific entropy conserved along streamlines.
In particular, this MHD model analysis frees the scaling parameter in the
conventional polytropic self-similar transformation from the constraint of
with being the polytropic index and therefore
substantially generalizes earlier analysis results on polytropic gas dynamics
that has a constant specific entropy everywhere in space at all time. On the
basis of the self-similar nonlinear MHD ordinary differential equations, we
examine behaviours of the magnetosonic critical curves, the MHD shock
conditions, and various asymptotic solutions. We then construct global
semi-complete self-similar MHD solutions using a combination of analytical and
numerical means and indicate plausible astrophysical applications of these
magnetized flow solutions with or without MHD shocks.Comment: 21 pages, 7 figures, accepted for publication in APS
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