1,860 research outputs found
The Secular Bar-Mode Instability in Rapidly Rotating Stars Revisited
Uniformly rotating, homogeneous, incompressible Maclaurin spheroids that spin
sufficiently rapidly are secularly unstable to nonaxisymmetric, bar-mode
perturbations when viscosity is present. The intuitive explanation is that
energy dissipation by viscosity can drive an unstable spheroid to a stable,
triaxial configuration of lower energy - a Jacobi ellipsoid. But what about
rapidly rotating compressible stars? Unlike incompressible stars, which contain
no internal energy and therefore immediately liberate all the energy dissipated
by viscosity, compressible stars have internal energy and can retain the
dissipated energy as internal heat. Now compressible stars that rotate
sufficiently rapidly and also manage to liberate this dissipated energy very
quickly are known to be unstable to bar-mode perturbations, like their
incompressible counterparts. But what is the situation for rapidly rotating
compressible stars that have very long cooling timescales, so that all the
energy dissipated by viscosity is retained as heat, whereby the total energy of
the star remains constant on a secular (viscous) evolution timescale? Are such
stars also unstable to the nonlinear growth of bar modes, or is the viscous
heating sufficient to cause them to expand, drive down the ratio of rotational
kinetic to gravitational potential energy T/|W| ~ 1/R, where R is the
equatorial radius, and turn off the instability before it gets underway? If the
instability still arises in such stars, at what rotation rate do they become
unstable, and to what final state do they evolve? We provide answers to these
questions in the context of the compressible ellipsoid model for rotating
stars. The results should serve as useful guides for numerical simulations in
3+1 dimensions for rotating stars containing viscosity.Comment: Accepted for publication in ApJ 613, 1213-1220, 200
Accurate object reconstruction by statistical moments
Statistical moments can offer a powerful means for object description in object sequences. Moments used in this way provide a description of the changing shape of the object with time. Using these descriptions to predict temporal views of the object requires efficient and accurate reconstruction of the object from a limited set of moments, but accurate reconstruction from moments has as yet received only limited attention. We show how we can improve accuracy not only by consideration of formulation, but also by a new adaptive thresholding technique that removes one parameter needed in reconstruction. Both approaches are equally applicable for Legendre and other orthogonal moments to improve accuracy in reconstruction
The new SAAO infrared photometric standards based on the E regions
Since 1974, when the Glass standards were published, four other infrared groups have been set up at the other main southern observatories, i.e. the two Australian observatories, Mount Stromlo and Siding Spring Observatory CMSO) and Anglo-Australian Observatory (AAO) and the two observatories in Chile, European Southern Observatory (ESO) and the Cerro Tololo Inter-American Observatory (which uses the California Institute of Technology standard system) (CIT). Unfortunately there has been a serious lack of conformity. Each group has used a different set of filters and has different zero points. No standards are common to all the groups and only a few are in common between any of the systems. Several transformat1ons between the various systems have recently been published (Elias et al 1983 and Glass 1983) but the large errors in the transformation equations show the need for more accurate standard stars. Therefore, on the surface, the introduction by us of a new set of standards would appear in danger of adding confusion to the issue, but this is not, in fact, the case. Firstly, the new set is basically the same system as tha Glass standards but with greatly increased accuracy. Secondly, the work involved has shown up several anomalies and inaccuracies in other systems. Thirdly, this work has shown why such large errors are apparent when comparing the various sets of standards and where the causes probably lie. The observations for these new standards were started in August 1979. The standards were introduced at the SAAO at the beginning of March 1984. The set is likely to evolve with time, increase in number of stars and improve in accuracy
Quasiequilibrium sequences of black-hole--neutron-star binaries in general relativity
We construct quasiequilibrium sequences of black hole-neutron star binaries
for arbitrary mass ratios by solving the constraint equations of general
relativity in the conformal thin-sandwich decomposition. We model the neutron
star as a stationary polytrope satisfying the relativistic equations of
hydrodynamics, and account for the black hole by imposing equilibrium boundary
conditions on the surface of an excised sphere (the apparent horizon). In this
paper we focus on irrotational configurations, meaning that both the neutron
star and the black hole are approximately nonspinning in an inertial frame. We
present results for a binary with polytropic index n=1, mass ratio
M_{irr}^{BH}/M_{B}^{NS}=5 and neutron star compaction
M_{ADM,0}^{NS}/R_0=0.0879, where M_{irr}^{BH} is the irreducible mass of the
black hole, M_{B}^{NS} the neutron star baryon rest-mass, and M_{ADM,0}^{NS}
and R_0 the neutron star Arnowitt-Deser-Misner mass and areal radius in
isolation, respectively. Our models represent valid solutions to Einstein's
constraint equations and may therefore be employed as initial data for
dynamical simulations of black hole-neutron star binaries.Comment: 5 pages, 1 figure, revtex4, published in Phys.Rev.
Quasiequilibrium black hole-neutron star binaries in general relativity
We construct quasiequilibrium sequences of black hole-neutron star binaries
in general relativity. We solve Einstein's constraint equations in the
conformal thin-sandwich formalism, subject to black hole boundary conditions
imposed on the surface of an excised sphere, together with the relativistic
equations of hydrostatic equilibrium. In contrast to our previous calculations
we adopt a flat spatial background geometry and do not assume extreme mass
ratios. We adopt a Gamma=2 polytropic equation of state and focus on
irrotational neutron star configurations as well as approximately nonspinning
black holes. We present numerical results for ratios of the black hole's
irreducible mass to the neutron star's ADM mass in isolation of
M_{irr}^{BH}/M_{ADM,0}^{NS} = 1, 2, 3, 5, and 10. We consider neutron stars of
baryon rest mass M_B^{NS}/M_B^{max} = 83% and 56%, where M_B^{max} is the
maximum allowed rest mass of a spherical star in isolation for our equation of
state. For these sequences, we locate the onset of tidal disruption and, in
cases with sufficiently large mass ratios and neutron star compactions, the
innermost stable circular orbit. We compare with previous results for black
hole-neutron star binaries and find excellent agreement with third-order
post-Newtonian results, especially for large binary separations. We also use
our results to estimate the energy spectrum of the outgoing gravitational
radiation emitted during the inspiral phase for these binaries.Comment: 17 pages, 15 figures, published in Phys. Rev.
Immersive 4D Interactive Visualization of Large-Scale Simulations
In dense clusters a bewildering variety of interactions between stars can be
observed, ranging from simple encounters to collisions and other mass-transfer
encounters. With faster and special-purpose computers like GRAPE, the amount of
data per simulation is now exceeding 1TB. Visualization of such data has now
become a complex 4D data-mining problem, combining space and time, and finding
interesting events in these large datasets. We have recently starting using the
virtual reality simulator, installed in the Hayden Planetarium in the American
Museum for Natural History, to tackle some of these problem. This work
(http://www.astro.umd.edu/nemo/amnh/) reports on our first ``observations'',
modifications needed for our specific experiments, and perhaps field ideas for
other fields in science which can benefit from such immersion. We also discuss
how our normal analysis programs can be interfaced with this kind of
visualization.Comment: 4 pages, 1 figure, ADASS-X conference proceeding
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