113 research outputs found
Gravitational Waves from Compact Sources
We review sources of high-frequency gravitational waves, summarizing our
current understanding of emission mechanisms, expected amplitudes and event
rates. The most promising sources are gravitational collapse (formation of
black holes or neutron stars) and subsequent ringing of the compact star,
secular or dynamical rotational instabilities and high-mass compact objects
formed through the merger of binary neutron stars. Significant and unique
information for the various stages of the collapse, the structure of
protoneutron stars and the high density equation of state of compact objects
can be drawn from careful study of gravitational wave signals.Comment: 22 pages, Proceedings of the 5th International Workshop "New Worlds
in Astroparticle Physics", Faro, Portugal, 8-10 January 200
Collapse of differentially rotating neutron stars and cosmic censorship
We present new results on the dynamics and gravitational-wave emission from
the collapse of differentially rotating neutron stars. We have considered a
number of polytropic stellar models having different values of the
dimensionless angular momentum J/M^2, where J and M are the asymptotic angular
momentum and mass of the star, respectively. For neutron stars with J/M^2<1,
i.e., "sub-Kerr" models, we were able to find models that are dynamically
unstable and that collapse promptly to a rotating black hole. Both the dynamics
of the collapse and the consequent emission of gravitational waves resemble the
one seen for uniformly rotating stars, although with an overall decrease in the
efficiency of gravitational-wave emission. For stellar models with J/M^2>1,
i.e., "supra-Kerr" models, on the other hand, we were not able to find models
that are dynamically unstable and all of the computed supra-Kerr models were
found to be far from the stability threshold. For these models a gravitational
collapse is possible only after a very severe and artificial reduction of the
pressure, which then leads to a torus developing nonaxisymmetric instabilities
and eventually contracting to a stable axisymmetric stellar configuration.
While this does not exclude the possibility that a naked singularity can be
produced by the collapse of a differentially rotating star, it also suggests
that cosmic censorship is not violated and that generic conditions for a
supra-Kerr progenitor do not lead to a naked singularity.Comment: 15 pages, 15 figures. Minor changes to the text and to the
references. In press on Phys. Rev.
Exploring properties of high-density matter through remnants of neutron-star mergers
Remnants of neutron-star mergers are essentially massive, hot, differentially
rotating neutron stars, which are initially strongly oscillating. They
represent a unique probe for high-density matter because the oscillations are
detectable via gravitational-wave measurements and are strongly dependent on
the equation of state. The impact of the equation of state is apparent in the
frequency of the dominant oscillation mode of the remnant. For a fixed total
binary mass a tight relation between the dominant postmerger frequency and the
radii of nonrotating neutron stars exists. Inferring observationally the
dominant postmerger frequency thus determines neutron star radii with high
accuracy of the order of a few hundred meters. By considering symmetric and
asymmetric binaries of the same chirp mass, we show that the knowledge of the
binary mass ratio is not critical for this kind of radius measurements. We
summarize different possibilities to deduce the maximum mass of nonrotating
neutron stars. We clarify the nature of the three most prominent features of
the postmerger gravitational-wave spectrum and argue that the merger remnant
can be considered to be a single, isolated, self-gravitating object that can be
described by concepts of asteroseismology. The understanding of the different
mechanisms shaping the gravitational-wave signal yields a physically motivated
analytic model of the gravitational-wave emission, which may form the basis for
template-based gravitational-wave data analysis. We explore the observational
consequences of a scenario of two families of compact stars including hadronic
and quark stars. We find that this scenario leaves a distinctive imprint on the
postmerger gravitational-wave signal. In particular, a strong discontinuity in
the dominant postmerger frequency as function of the total mass will be a
strong indication for two families of compact stars. (abridged)Comment: 22 pages, 17 figures; accepted for publication in EPJ
An Analysis of the Structural Causes of Poverty in New Mexico
A review of the professional literature on poverty in America shows that poverty is a particularly severe problem in New Mexico. Data from the Census of Population indicate that New Mexico during the past and until the present has been ranked among the top five states with high poverty rates. There is strong evidence that New Mexico with the desert land and its colorful population never had the opportunity to follow the rest of the nation and to enjoy completely the fruits of American prosperity
Quasi-normal modes of rotating relativistic stars - neutral modes for realistic equations of state
We compute zero-frequency (neutral) quasi-normal f-modes of fully
relativistic and rapidly rotating neutron stars, using several realistic
equations of state (EOSs) for neutron star matter. The zero-frequency modes
signal the onset of the gravitational radiation-driven instability. We find
that the l=m=2 (bar) f-mode is unstable for stars with gravitational mass as
low as 1.0 - 1.2 M_\odot, depending on the EOS. For 1.4 M_\odot neutron stars,
the bar mode becomes unstable at 83 % - 93 % of the maximum allowed rotation
rate. For a wide range of EOSs, the bar mode becomes unstable at a ratio of
rotational to gravitational energies T/W \sim 0.07-0.09 for 1.4 M_\odot stars
and T/W \sim 0.06 for maximum mass stars. This is to be contrasted with the
Newtonian value of T/W \sim 0.14. We construct the following empirical formula
for the critical value of T/W for the bar mode,
(T/W)_2 = 0.115 - 0.048 M / M_{max}^{sph}, which is insensitive to the EOS to
within 4 - 6 %. This formula yields an estimate for the neutral mode sequence
of the bar mode as a function only of the star's mass, M, given the maximum
allowed mass, M_{max}^{sph}, of a nonrotating neutron star. The recent
discovery of the fast millisecond pulsar in the supernova remnant N157B,
supports the suggestion that a fraction of proto-neutron stars are born in a
supernova collapse with very large initial angular momentum. Thus, in a
fraction of newly born neutron stars the instability is a promising source of
continuous gravitational waves. It could also play a major role in the
rotational evolution (through the emission of angular momentum) of merged
binary neutron stars, if their post-merger angular momentum exceeds the maximum
allowed to form a Kerr black hole.Comment: 12 pages, 5 figures, submitted to Ap
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