4,122 research outputs found
Relativistic precession around rotating neutron stars: Effects due to frame-dragging and stellar oblateness
General relativity predicts that a rotating body produces a frame-dragging
(or Lense-Thirring) effect: the orbital plane of a test particle in a
non-equatorial orbit precesses about the body's symmetry axis. In this paper we
compute the precession frequencies of circular orbits around rapidly rotating
neutron stars for a variety of masses and equations of state. The precession
frequencies computed are expressed as numerical functions of the orbital
frequency observed at infinity. The post-Newtonian expansion of the exact
precession formula is examined to identify the relative magnitudes of the
precession caused by the Lense-Thirring effect, the usual Newtonian quadrupole
effect and relativistic corrections. The first post-Newtonian correction to the
Newtonian quadrupole precession is derived in the limit of slow rotation. We
show that the post-Newtonian precession formula is a good approximation to the
exact precession close to the neutron star in the slow rotation limit (up to
\sim 400 Hz in the present context).
The results are applied to recent RXTE observations of neutron star low-mass
X-ray binaries, which display kHz quasi-periodic oscillations and, within the
framework of beat frequency models, allow the measurement of both the neutron
star spin frequency and the Keplerian frequency of the innermost ring of matter
in the accretion disk around it. For a wide range of realistic equations of
state, we find that the predicted precession frequency of this ring is close to
one half of the low-frequency (\sim 20 - 35 Hz) quasi-periodic oscillations
seen in several Atoll sources.Comment: 35 pages including 10 figures and 6 tables. To appear in the
Astrophysical Journa
Steady-state analysis of shortest expected delay routing
We consider a queueing system consisting of two non-identical exponential
servers, where each server has its own dedicated queue and serves the customers
in that queue FCFS. Customers arrive according to a Poisson process and join
the queue promising the shortest expected delay, which is a natural and
near-optimal policy for systems with non-identical servers. This system can be
modeled as an inhomogeneous random walk in the quadrant. By stretching the
boundaries of the compensation approach we prove that the equilibrium
distribution of this random walk can be expressed as a series of product-forms
that can be determined recursively. The resulting series expression is directly
amenable for numerical calculations and it also provides insight in the
asymptotic behavior of the equilibrium probabilities as one of the state
coordinates tends to infinity.Comment: 41 pages, 13 figure
Anomalous scaling due to correlations: Limit theorems and self-similar processes
We derive theorems which outline explicit mechanisms by which anomalous
scaling for the probability density function of the sum of many correlated
random variables asymptotically prevails. The results characterize general
anomalous scaling forms, justify their universal character, and specify
universality domains in the spaces of joint probability density functions of
the summand variables. These density functions are assumed to be invariant
under arbitrary permutations of their arguments. Examples from the theory of
critical phenomena are discussed. The novel notion of stability implied by the
limit theorems also allows us to define sequences of random variables whose sum
satisfies anomalous scaling for any finite number of summands. If regarded as
developing in time, the stochastic processes described by these variables are
non-Markovian generalizations of Gaussian processes with uncorrelated
increments, and provide, e.g., explicit realizations of a recently proposed
model of index evolution in finance.Comment: Through text revision. 15 pages, 3 figure
Spectroscopic Binaries in Southern Open Clusters
Aportamos un informe del trabajo sobre binarias que venimos desarrollando en cúmulos del hemisferio sur. Elpropósito es contribuir a comprender la formación y evolución de binarias espectroscópicas, proporcionandocondiciones de contorno que permitan verificar algunas de las teorías actuales sobre la formación de binariasen cúmulos abiertos.This is a report on an ongoing program about binaries in southern open clusters. The long-term purpose ofthis project is to contribute to understanding the formation and evolution of spectroscopic binaries, providingobservational constraints that will permit tests of some of the current theories on binary formation in openclusters
Spectroscopic Binaries in Southern Open Clusters
Aportamos un informe del trabajo sobre binarias que venimos desarrollando en cúmulos del hemisferio sur. El propósito es contribuir a comprender la formación y evolución de binarias espectroscópicas, proporcionando condiciones de contorno que permitan verificar algunas de las teorías actuales sobre la formación de binarias en cúmulos abiertos.This is a report on an ongoing program about binaries in southern open clusters. The long-term purpose of this project is to contribute to understanding the formation and evolution of spectroscopic binaries, providing observational constraints that will permit tests of some of the current theories on binary formation in open clusters.Facultad de Ciencias Astronómicas y Geofísica
Prodrugs: Do They Have Advantages in Clinical Practice?
This is the published version, also available from the publisher at http://adisonline.com/drugs/Abstract/1985/29050/Prodrugs__Do_They_Have_Advantages_in_Clinical.2.aspxProdrugs are pharmacologically inactive chemical derivatives of a drug molecule that require a transformation within the body in order to release the active drug. They are designed to overcome pharmaceutical and/or pharmacokinetically based problems associated with the parent drug molecule that would otherwise limit the clinical usefulness of
the drug.
The scientific rationale, based on clinical pharmaceutical and chemical experience,
for the design of various currently used prodrugs is presented in this review. The examples presented are by no means comprehensive, but are representative of the different ways in which the prodrug approach has been used to enhance the clinical efficacy of various drug molecules
Probing the Evaporation Dynamics of Ethanol/Gasoline Biofuel Blends Using Single Droplet Manipulation Techniques
Peer reviewedPublisher PD
Branching Processes and Evolution at the Ends of a Food Chain
In a critically self--organized model of punctuated equilibrium, boundaries
determine peculiar scaling of the size distribution of evolutionary avalanches.
This is derived by an inhomogeneous generalization of standard branching
processes, extending previous mean field descriptions and yielding
together with , as distribution exponent of avalanches starting from
species at the ends of a food chain. For the nearest neighbor chain one obtains
numerically , and for the
first return times of activity, again distinct from bulk exponents.Comment: REVTex file, 12 pages, 2 figures in eps-files uuencoded, psfig.st
Adsorption-like Collapse of Diblock Copolymers
A linear copolymer made of two reciprocally attracting N-monomer blocks
collapses to a compact phase through a novel transition, whose exponents are
determined with extensive MC simulations in two and three dimensions. In the
former case, an identification with the statistical geometry of suitable
percolation paths allows to predict that the number of contacts between the
blocks grows like . In the compact phase the blocks are mixed and, in
two dimensions, also zipped, in such a way to form a spiral, double chain
structure.Comment: 4 pages, 5 Postscript figure
Anomalous thermal expansion in 1D transition-metal cyanides: what makes the novel trimetallic cyanide Cu1/3Ag1/3Au1/3CN behave differently?
The structural dynamics of a quasi-one-dimensional (1D) mixed-metal cyanide, Cu1/3Ag1/3Au1/3CN, with intriguing thermal properties is explored. All the current known related compounds with straight-chain structures, such as group 11 cyanides CuCN, AgCN, AuCN and bimetallic cyanides MxM’1-xCN (M, M’ = Cu, Ag, Au), exhibit 1D negative thermal expansion (NTE) along the chains and positive thermal expansion (PTE) perpendicular to them. Cu1/3Ag1/3Au1/3CN exhibits similar PTE perpendicular to the chains, however PTE, rather than NTE, is also observed along the chains. In order to understand the origin of this unexpected behavior, inelastic neutron scattering (INS) measurements were carried out, underpinned by lattice-dynamical density-functional-theory (DFT) calculations. Synchrotron-based pair-distribution-function (PDF) analysis and 13C solid-state nuclear-magnetic-resonance (SSNMR) measurements were also performed to build an input structural model for the lattice dynamical study. The results indicate that transverse motions of the metal ions are responsible for the PTE perpendicular to the chains, as is the case for the related group 11 cyanides. However NTE along the chain due to the tension effect of these transverse motions is not observed. As there are different metal-to-cyanide bond lengths in Cu1/3Ag1/3Au1/3CN, the metals in neighboring chains cannot all be truly co-planar in a straight-chain model. For this system, DFT-based phonon calculations predict small PTE along the chain due to low-energy chain-slipping modes induced by a bond-rotation effect on the weak metallophilic bonds. However the observed PTE is greater than that predicted with the straight-chain model. Small bends in the chain to accommodate truly co-planar metals provide an alternative explanation for thermal behavior. These would mitigate the tension effect induced by the transverse motions of the metals and, as temperature increases and the chains move further apart, a straightening could occur resulting in the observed PTE. This hypothesis is further supported by unusual evolution in the phonon spectra, which suggest small changes in local symmetry with temperature
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