937 research outputs found
Short Gamma-Ray Bursts and Binary Mergers in Spiral and Elliptical Galaxies: Redshift Distribution and Hosts
To test whether the short GRB rates, redshift distribution and host galaxies
are consistent with current theoretical predictions, we use avery large
database of population synthesis calculations to examine BH-NS and NS-NS merger
rates in the universe, factoring in (i) the star formation history of the
universe, (ii) a heterogeneous population of star-forming galaxies, including
spirals and ellipticals, and (iii) a simple flux-limited selection model for
short GRB detection. When we require our models reproduce the known short GRB
rates and redshift measurements (and, for NS-NS, the merger rates extrapolated
from binary pulsars in the Galaxy), a small fraction of models reproduce all
observations, both when we assume a NS-NS and a BH-NS origin for bursts. Most
commonly models produce mergers preferentially in spiral galaxies if short GRBs
arise from NS-NS mergers alone. Model universes where present-day binary
mergers occur preferentially in elliptical galaxies necessarily include a
significant fraction of binaries with long delay times between birth and merger
(often ). Though long delays occur, almost all of our models
predict that a higher proportion of short GRBs should occur at moderate to high
redshift (e.g., ) than has presently been observed, in agreement with
recent observations which suggest a selection bias towards successful follow-up
of low-redshift short GRBs. Finally, if only a fraction of BH-NS mergers have
the right combination of masses and spins to make GRBs, then at best only a
small fraction of BH-NS models could be consistent with all {\em current}
available data. (Abridged)Comment: 14 figures, using bitmapped fonts (via eps2eps) to fit in archive
space restrictions; better resolution figures are available from the author.
Accepted for publication in ApJ. v3 updates reference
Constraining population synthesis models via the binary neutron star population
The observed sample of double neutron-star (NS-NS) binaries presents a
challenge to population-synthesis models of compact object formation: the
parameters entering into these models must be carefully chosen so as to match
(i) the observed star formation rate and (ii) the formation rate of NS-NS
binaries, which can be estimated from the observed sample and the selection
effects related to the discoveries with radio-pulsar surveys. In this paper, we
select from an extremely broad family of possible population synthesis models
those few (2%) which are consistent with the observed sample of NS-NS binaries.
To further sharpen the constraints the observed NS-NS population places upon
our understanding of compact-object formation processes, we separate the
observed NS-NS population into two channels: (i) merging NS-NS binaries, which
will inspiral and merge through the action of gravitational waves within
Gyr, and (ii) wide NS-NS binaries, consisting of all the rest. With the subset
of astrophysically consistent models, we explore the implications for the rates
at which double black hole (BH-BH), black hole-neutron star (BH-NS), and NS-NS
binaries will merge through the emission of gravitational waves.Comment: (v1) Submitted to ApJ. Uses emulateapj.cls. 8 pages, 7 figures. (v2)
Minor textual changes in response to referee queries. Substantial additions
in appendicies, including a detailed discussion of sample multidimensional
population synthesis fit
Identifying Advantages and Disadvantages of Variable Rate Irrigation – An Updated Review
Variable rate irrigation (VRI) sprinklers on mechanical move irrigation systems (center pivot or lateral move) have been commercially available since 2004. Although the number of VRI, zone or individual sprinkler, systems adopted to date is lower than expected there is a continued interest to harness this technology, especially when climate variability, regulatory nutrient management, water conservation policies, and declining water for agriculture compound the challenges involved for irrigated crop production. This article reviews the potential advantages and potential disadvantages of VRI technology for moving sprinklers, provides updated examples on such aspects, suggests a protocol for designing and implementing VRI technology and reports on the recent advancements. The advantages of VRI technology are demonstrated in the areas of agronomic improvement, greater economic returns, environmental protection and risk management, while the main drawbacks to VRI technology include the complexity to successfully implement the technology and the lack of evidence that it assures better performance in net profit or water savings. Although advances have been made in VRI technologies, its penetration into the market will continue to depend on tangible and perceived benefits by producers
Non-Equilibrium in Adsorbed Polymer Layers
High molecular weight polymer solutions have a powerful tendency to deposit
adsorbed layers when exposed to even mildly attractive surfaces. The
equilibrium properties of these dense interfacial layers have been extensively
studied theoretically. A large body of experimental evidence, however,
indicates that non-equilibrium effects are dominant whenever monomer-surface
sticking energies are somewhat larger than kT, a common case. Polymer
relaxation kinetics within the layer are then severely retarded, leading to
non-equilibrium layers whose structure and dynamics depend on adsorption
kinetics and layer ageing. Here we review experimental and theoretical work
exploring these non-equilibrium effects, with emphasis on recent developments.
The discussion addresses the structure and dynamics in non-equilibrium polymer
layers adsorbed from dilute polymer solutions and from polymer melts and more
concentrated solutions. Two distinct classes of behaviour arise, depending on
whether physisorption or chemisorption is involved. A given adsorbed chain
belonging to the layer has a certain fraction of its monomers bound to the
surface, f, and the remainder belonging to loops making bulk excursions. A
natural classification scheme for layers adsorbed from solution is the
distribution of single chain f values, P(f), which may hold the key to
quantifying the degree of irreversibility in adsorbed polymer layers. Here we
calculate P(f) for equilibrium layers; we find its form is very different to
the theoretical P(f) for non-equilibrium layers which are predicted to have
infinitely many statistical classes of chain. Experimental measurements of P(f)
are compared to these theoretical predictions.Comment: 29 pages, Submitted to J. Phys.: Condens. Matte
What Governs Attitudes Toward Artifcial Intelligence Adoption and Governance?
Designing effective and inclusive governance and public communication strategies for artificial intelligence (AI) requires understanding how stakeholders reason about its use and governance. We examine underlying factors and mechanisms that drive attitudes toward the use and governance of AI across six policy-relevant applications using structural equation modeling and surveys of both US adults (N = 3,524) and tech- nology workers enrolled in an online computer science master’s degree program (N = 425). We find that the cultural values of individualism, egalitarianism, general risk aversion, and techno-skepticism are important drivers of AI attitudes. Perceived benefit drives attitudes toward AI use but not its governance. Experts hold more nuanced views than the public and are more supportive of AI use but not its regulation. Drawing on these findings, we discuss challenges and opportunities for participatory AI governance, and we recommend that trustworthy AI governance be emphasized as strongly as trustworthy AI
Nonlinear equation for anomalous diffusion: unified power-law and stretched exponential exact solution
The nonlinear diffusion equation is analyzed here, where , and , and are real parameters.
This equation unifies the anomalous diffusion equation on fractals ()
and the spherical anomalous diffusion for porous media (). Exact
point-source solution is obtained, enabling us to describe a large class of
subdiffusion (), normal diffusion () and
superdiffusion (). Furthermore, a thermostatistical basis
for this solution is given from the maximum entropic principle applied to the
Tsallis entropy.Comment: 3 pages, 2 eps figure
Nonlinear anomalous diffusion equation and fractal dimension: Exact generalized gaussian solution
In this work we incorporate, in a unified way, two anomalous behaviors, the
power law and stretched exponential ones, by considering the radial dependence
of the -dimensional nonlinear diffusion equation where , ,
, and are real parameters and is a time-dependent
source. This equation unifies the O'Shaugnessy-Procaccia anomalous diffusion
equation on fractals () and the spherical anomalous diffusion for
porous media (). An exact spherical symmetric solution of this
nonlinear Fokker-Planck equation is obtained, leading to a large class of
anomalous behaviors. Stationary solutions for this Fokker-Planck-like equation
are also discussed by introducing an effective potential.Comment: Latex, 6 pages. To appear in Phys. Rev.
Transition from inspiral to plunge for eccentric equatorial Kerr orbits
Ori and Thorne have discussed the duration and observability (with LISA) of
the transition from circular, equatorial inspiral to plunge for stellar-mass
objects into supermassive () Kerr black holes. We
extend their computation to eccentric Kerr equatorial orbits. Even with orbital
parameters near-exactly determined, we find that there is no universal length
for the transition; rather, the length of the transition depends sensitively --
essentially randomly -- on initial conditions. Still, Ori and Thorne's
zero-eccentricity results are essentially an upper bound on the length of
eccentric transitions involving similar bodies (e.g., fixed). Hence the
implications for observations are no better: if the massive body is
, the captured body has mass , and the process occurs at
distance from LISA, then , with the precise constant depending on
the black hole spin. For low-mass bodies () for which the
event rate is at least vaguely understood, we expect little chance (probably
[much] less than 10%, depending strongly on the astrophysical assumptions) of
LISA detecting a transition event with during its run; however, even a
small infusion of higher-mass bodies or a slight improvement in LISA's noise
curve could potentially produce transition events during LISA's
lifetime.Comment: Submitted to PR
Random walk through fractal environments
We analyze random walk through fractal environments, embedded in
3-dimensional, permeable space. Particles travel freely and are scattered off
into random directions when they hit the fractal. The statistical distribution
of the flight increments (i.e. of the displacements between two consecutive
hittings) is analytically derived from a common, practical definition of
fractal dimension, and it turns out to approximate quite well a power-law in
the case where the dimension D of the fractal is less than 2, there is though
always a finite rate of unaffected escape. Random walks through fractal sets
with D less or equal 2 can thus be considered as defective Levy walks. The
distribution of jump increments for D > 2 is decaying exponentially. The
diffusive behavior of the random walk is analyzed in the frame of continuous
time random walk, which we generalize to include the case of defective
distributions of walk-increments. It is shown that the particles undergo
anomalous, enhanced diffusion for D_F < 2, the diffusion is dominated by the
finite escape rate. Diffusion for D_F > 2 is normal for large times, enhanced
though for small and intermediate times. In particular, it follows that
fractals generated by a particular class of self-organized criticality (SOC)
models give rise to enhanced diffusion. The analytical results are illustrated
by Monte-Carlo simulations.Comment: 22 pages, 16 figures; in press at Phys. Rev. E, 200
Fracton pairing mechanism for "strange" superconductors: Self-assembling organic polymers and copper-oxide compounds
Self-assembling organic polymers and copper-oxide compounds are two classes
of "strange" superconductors, whose challenging behavior does not comply with
the traditional picture of Bardeen, Cooper, and Schrieffer (BCS)
superconductivity in regular crystals. In this paper, we propose a theoretical
model that accounts for the strange superconducting properties of either class
of the materials. These properties are considered as interconnected
manifestations of the same phenomenon: We argue that superconductivity occurs
in the both cases because the charge carriers (i.e., electrons or holes)
exchange {\it fracton excitations}, quantum oscillations of fractal lattices
that mimic the complex microscopic organization of the strange superconductors.
For the copper oxides, the superconducting transition temperature as
predicted by the fracton mechanism is of the order of K. We suggest
that the marginal ingredient of the high-temperature superconducting phase is
provided by fracton coupled holes that condensate in the conducting
copper-oxygen planes owing to the intrinsic field-effect-transistor
configuration of the cuprate compounds. For the gate-induced superconducting
phase in the electron-doped polymers, we simultaneously find a rather modest
transition temperature of K owing to the limitations imposed by
the electron tunneling processes on a fractal geometry. We speculate that
hole-type superconductivity observes larger onset temperatures when compared to
its electron-type counterpart. This promises an intriguing possibility of the
high-temperature superconducting states in hole-doped complex materials. A
specific prediction of the present study is universality of ac conduction for
.Comment: 12 pages (including separate abstract page), no figure
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