3,951 research outputs found
Non-linear axisymmetric pulsations of rotating relativistic stars in the conformal flatness approximation
We study non-linear axisymmetric pulsations of rotating relativistic stars
using a general relativistic hydrodynamics code under the assumption of a
conformal flatness. We compare our results to previous simulations where the
spacetime dynamics was neglected. The pulsations are studied along various
sequences of both uniformly and differentially rotating relativistic polytropes
with index N = 1. We identify several modes, including the lowest-order l = 0,
2, and 4 axisymmetric modes, as well as several axisymmetric inertial modes.
Differential rotation significantly lowers mode frequencies, increasing
prospects for detection by current gravitational wave interferometers. We
observe an extended avoided crossing between the l = 0 and l = 4 first
overtones, which is important for correctly identifying mode frequencies in
case of detection. For uniformly rotating stars near the mass-shedding limit,
we confirm the existence of the mass-shedding-induced damping of pulsations,
though the effect is not as strong as in the Cowling approximation. We also
investigate non-linear harmonics of the linear modes and notice that rotation
changes the pulsation frequencies in a way that would allow for various
parametric instabilities between two or three modes to take place. We assess
the detectability of each obtained mode by current gravitational wave detectors
and outline how the empirical relations that have been constructed for
gravitational wave asteroseismology could be extended to include the effects of
rotation.Comment: 24 pages, 20 figures; minor corrections, added extended discussion
and one figure in one subsectio
Wearing a single DNA molecule with an AFM tip
While the fundamental limit on the resolution achieved in an atomic force
microscope (AFM) is clearly related to the tip radius, the fact that the tip
can creep and/or wear during an experiment is often ignored. This is mainly due
to the difficulty in characterizing the tip, and in particular a lack of
reliable methods that can achieve this in situ. Here, we provide an in situ
method to characterize the tip radius and monitor tip creep and/or wear and
biomolecular sample wear in ambient dynamic AFM. This is achieved by monitoring
the dynamics of the cantilever and the critical free amplitude to observe a
switch from the attractive to the repulsive regime. The method is exemplified
on the mechanically heterogeneous sample of single DNA molecules bound to mica
mineral surfaces. Simultaneous monitoring of apparent height and width of
single DNA molecules while detecting variations in the tip radius R as small as
one nanometer are demonstrated. The yield stress can be readily exceeded for
sharp tips (R10nm). The ability to
know the AFM tip radius in situ and in real-time opens up the future for
quantitative nanoscale materials properties determination at the highest
possible spatial resolution.Comment: 26 pages, 6 figure
Nonlinear r-modes in Rapidly Rotating Relativistic Stars
The r-mode instability in rotating relativistic stars has been shown recently
to have important astrophysical implications (including the emission of
detectable gravitational radiation, the explanation of the initial spins of
young neutron stars and the spin-distribution of millisecond pulsars and the
explanation of one type of gamma-ray bursts), provided that r-modes are not
saturated at low amplitudes by nonlinear effects or by dissipative mechanisms.
Here, we present the first study of nonlinear r-modes in isentropic, rapidly
rotating relativistic stars, via 3-D general-relativistic hydrodynamical
evolutions. Our numerical simulations show that (1) on dynamical timescales,
there is no strong nonlinear coupling of r-modes to other modes at amplitudes
of order one -- unless nonlinear saturation occurs on longer timescales, the
maximum r-mode amplitude is of order unity (i.e., the velocity perturbation is
of the same order as the rotational velocity at the equator). An absolute upper
limit on the amplitude (relevant, perhaps, for the most rapidly rotating stars)
is set by causality. (2) r-modes and inertial modes in isentropic stars are
predominantly discrete modes and possible associated continuous parts were not
identified in our simulations. (3) In addition, the kinematical drift
associated with r-modes, recently found by Rezzolla, Lamb and Shapiro (2000),
appears to be present in our simulations, but an unambiguous confirmation
requires more precise initial data. We discuss the implications of our findings
for the detectability of gravitational waves from the r-mode instability.Comment: 4 pages, 4 eps figures, accepted in Physical Review Letter
Cantilever dynamics in amplitude modulation AFM: continuous and discontinuous transitions
Transitions between the attractive and the repulsive force regimes for amplitude modulation atomic force microscopy (AFM) can be either discontinuous, with a corresponding jump in amplitude and phase, or continuous and smooth. During the transitions, peak repulsive and average forces can be up to an order of magnitude higher when these are discrete. Under certain circumstances, for example, when the tip radius is relatively large (e.g. R > 20–30 nm) and for high cantilever free amplitudes (e.g. A0 > 40–50 nm), the L state can be reached with relatively low set-points only (e.g. Asp/A0 < 0.30). We find that these cases do not generally lead to higher resolution but increase the background noise instead. This is despite the fact that the imaging can be non-contact under these conditions. The appearance of background noise is linked to increasing cantilever mean deflection and tip–surface proximity with increasing free amplitude in the L state.
Cantilever dynamics in amplitude modulation AFM: Continuous and discontinuous transitions (PDF Download Available). Available from: https://www.researchgate.net/publication/231025693_Cantilever_dynamics_in_amplitude_modulation_AFM_Continuous_and_discontinuous_transitions [accessed Mar 27, 2017].Peer ReviewedPostprint (author's final draft
Relativistic simulations of rotational core collapse. I. Methods, initial models, and code tests
We describe an axisymmetric general relativistic code for rotational core
collapse. The code evolves the coupled system of metric and fluid equations
using the ADM 3+1 formalism and a conformally flat metric approximation of the
Einstein equations. The relativistic hydrodynamics equations are formulated as
a first-order flux-conservative hyperbolic system and are integrated using
high-resolution shock-capturing schemes based on Riemann solvers. We assess the
quality of the conformally flat metric approximation for relativistic core
collapse and present a comprehensive set of tests which the code successfully
passed. The tests include relativistic shock tubes, the preservation of the
rotation profile and of the equilibrium of rapidly and differentially rotating
neutron stars (approximated as rotating polytropes), spherical relativistic
core collapse, and the conservation of rest-mass and angular momentum in
dynamic spacetimes. The application of the code to relativistic rotational core
collapse, with emphasis on the gravitational waveform signature, is presented
in an accompanying paper.Comment: 18 pages, 12 figure
Relativistic gravitational collapse in comoving coordinates: The post-quasistatic approximation
A general iterative method proposed some years ago for the description of
relativistic collapse, is presented here in comoving coordinates. For doing
that we redefine the basic concepts required for the implementation of the
method for comoving coordinates. In particular the definition of the
post-quasistatic approximation in comoving coordinates is given. We write the
field equations, the boundary conditions and a set of ordinary differential
equations (the surface equations) which play a fundamental role in the
algorithm. As an illustration of the method, we show how to build up a model
inspired in the well known Schwarzschild interior solution. Both, the adiabatic
and non adiabatic, cases are considered.Comment: 14 pages, 11 figures; updated version to appear in Int. J. Modern
Phys.
Scalar field induced oscillations of neutron stars and gravitational collapse
We study the interaction of massless scalar fields with self-gravitating
neutron stars by means of fully dynamic numerical simulations of the
Einstein-Klein-Gordon perfect fluid system. Our investigation is restricted to
spherical symmetry and the neutron stars are approximated by relativistic
polytropes. Studying the nonlinear dynamics of isolated neutron stars is very
effectively performed within the characteristic formulation of general
relativity, in which the spacetime is foliated by a family of outgoing light
cones. We are able to compactify the entire spacetime on a computational grid
and simultaneously impose natural radiative boundary conditions and extract
accurate radiative signals. We study the transfer of energy from the scalar
field to the fluid star. We find, in particular, that depending on the
compactness of the neutron star model, the scalar wave forces the neutron star
either to oscillate in its radial modes of pulsation or to undergo
gravitational collapse to a black hole on a dynamical timescale. The radiative
signal, read off at future null infinity, shows quasi-normal oscillations
before the setting of a late time power-law tail.Comment: 12 pages, 13 figures, submitted to Phys. Rev.
Gravitational waves from relativistic rotational core collapse
We present results from simulations of axisymmetric relativistic rotational
core collapse. The general relativistic hydrodynamic equations are formulated
in flux-conservative form and solved using a high-resolution shock-capturing
scheme. The Einstein equations are approximated with a conformally flat
3-metric. We use the quadrupole formula to extract waveforms of the
gravitational radiation emitted during the collapse. A comparison of our
results with those of Newtonian simulations shows that the wave amplitudes
agree within 30%. Surprisingly, in some cases, relativistic effects actually
diminish the amplitude of the gravitational wave signal. We further find that
the parameter range of models suffering multiple coherent bounces due to
centrifugal forces is considerably smaller than in Newtonian simulations.Comment: 4 pages, 3 figure
Couplings in Asymmetric Orbifolds and Grand Unified String Models
Using the bosonic supercurrent (or covariant lattice) formalism, we review
how to compute scattering amplitudes in asymmetric orbifold string models. This
method is particularly useful for calculating scattering of multiple
asymmetrically twisted string states, where the twisted states are rewritten as
ordinary momentum states. We show how to reconstruct some of the 3-family grand
unified string models in this formalism, and identify the quantum numbers of
the massless states in their spectra. The discrete symmetries of these models
are rather intricate. The superpotentials for the 3-family E_6 model and a
closely related SO(10) model are discussed in some detail. The forms of the
superpotentials of the two 3-family SU(6) models (with asymptotically-free
hidden sectors SU(3) and SU(2) \otimes SU(2)) are also presented.Comment: 54 pages, Revtex 3.0 (to appear in Nucl. Phys. B
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