7,668 research outputs found
Semi-Empirical Cepheid Period-Luminosity Relations in Sloan Magnitudes
In this paper we derive semi-empirical Cepheid period-luminosity (P-L)
relations in the Sloan ugriz magnitudes by combining the observed BVI mean
magnitudes from the Large Magellanic Cloud Cepheids (LMC) and theoretical
bolometric corrections. We also constructed empirical gr band P-L relations,
using the publicly available Johnson-Sloan photometric transformations, to be
compared with our semi-empirical P-L relations. These two sets of P-L relations
are consistent with each other.Comment: 4 pages, 2 tables and 2 figures, ApJ accepte
Modeling the IDV emissions of the BL Lac Objects with a Langevin type stochastic differential equation
In this paper, we introduce a simplified model for explaining the
observations of the optical intraday variability (IDV) of the BL Lac Objects.
We assume that the source of the IDV are the stochastic oscillations of an
accretion disk around a supermassive black hole. The Stochastic Fluctuations on
the vertical direction of the accretion disk are described by using a Langevin
type equation with a damping term and a random, white noise type force.
Furthermore, the preliminary numerical simulation results are presented, which
are based on the numerical analysis of the Langevin stochastic differential
equation.Comment: 4 pages, 4 figures, accepted for publication in J. Astrophys. Ast
Spin-lattice coupling in the ferrimagnetic semiconductor FeCr2S4 probed by surface acoustic waves
Using surface acoustic waves, the elastomagnetic coupling could be studied in
thin single crystalline plates of the ferrimagnetic semiconductor FeCr2S4 by
measuring the attenuation and the frequency tracking in the temperature range
4.2 K to 200 K. The data clearly display the anomalies found in low-field
magnetization measurements.Comment: 15 pages, 3 figures. To appear in J. Appl. Phys., 99 (2006
Crystallization dynamics of a single layer complex plasma
We report a series of complex (dusty) plasma experiments, aimed at the study
of the detailed time evolution of the re-crystallisation process following a
rapid quench of a two dimensional dust liquid. The experiments were accompanied
by large-scale (million particle) molecular dynamics simulations, assuming
Yukawa type inter-particle interaction. Both experiment and simulation show a
(power law) dependence of the linear crystallite domain size
as measured by the bond-order correlation length, translational correlation
length, dislocation (defect) density, and a direct size measurement algorithm.
The results show two stages of order formation: on short time-scales individual
particle motion dominates; this is a fast process characterized by
. At longer time-scales, small crystallites undergo
collective rearrangement, merging into bigger ones, resulting in a smaller
exponent .Comment: 4 pages, 3 figures. Submitted to PR
Clinical impact of double protease inhibitor boosting with Lopinavir/Ritonavir and Amprenavir as part of salvage antiretroviral therapy
Purpose: Double protease inhibitor (PI) boosting is being explored as a new strategy in salvage antiretroviral (ARV) therapy. However, if a negative drug interaction leads to decreased drug levels of either or both PIs, double PI boosting could lead to decreased virologic response. A negative drug interaction has been described between amprenavir (APV) and lopinavir/ritonavir (LPV/r). This observational cohort study assessed the virologic impact of the addition of APV to a salvage ARV regimen, which also contains LPV/r, compared to a regimen containing LPV/r alone. Method: Patients initiated on a salvage ARV regimen that included LPV/r obtained from the expanded access program in Toronto, Canada, were evaluated. APV (600-1,200 mg bid) was added at the discretion of the treating physician. Results: Using multivariate Cox proportional hazards models, we found that the addition of APV to a LPV/r-containing salvage regimen was not significantly associated with time to virologic suppression (< 50 copies/mL; adjusted hazard ratio [HR] = 0.75, p = .12) or with time to virologic rebound (adjusted HR = 1.46, p = .34). Those patients who received higher doses of APV had an increased chance of virologic suppression (p = .03). In a subset of 27 patients, the median LPV Ctrough was significantly lower in patients receiving APV (p = .04), and the median APV Ctrough was reduced compared to reported controls. Conclusion: Our data do not support an additional benefit in virologic reduction of double boosting with APV and LPV/r relative to LPV/r alone in salvage ARV therapy. Our study's limitations include its retrospective nature and the imbalance between the two groups potentially confounding the results. Although these factors were adjusted for in the multivariate analysis, a prospective randomized controlled trial is warranted to confirm our findings
Output functions and fractal dimensions in dynamical systems
We present a novel method for the calculation of the fractal dimension of
boundaries in dynamical systems, which is in many cases many orders of
magnitude more efficient than the uncertainty method. We call it the Output
Function Evaluation (OFE) method. The OFE method is based on an efficient
scheme for computing output functions, such as the escape time, on a
one-dimensional portion of the phase space. We show analytically that the OFE
method is much more efficient than the uncertainty method for boundaries with
, where is the dimension of the intersection of the boundary with a
one-dimensional manifold. We apply the OFE method to a scattering system, and
compare it to the uncertainty method. We use the OFE method to study the
behavior of the fractal dimension as the system's dynamics undergoes a
topological transition.Comment: Uses REVTEX; to be published in Phys. Rev. Let
Investigation of Prediction Accuracy, Sensitivity, and Parameter Stability of Large-Scale Propagation Path Loss Models for 5G Wireless Communications
This paper compares three candidate large-scale propagation path loss models
for use over the entire microwave and millimeter-wave (mmWave) radio spectrum:
the alpha-beta-gamma (ABG) model, the close-in (CI) free space reference
distance model, and the CI model with a frequency-weighted path loss exponent
(CIF). Each of these models have been recently studied for use in standards
bodies such as 3GPP, and for use in the design of fifth generation (5G)
wireless systems in urban macrocell, urban microcell, and indoor office and
shopping mall scenarios. Here we compare the accuracy and sensitivity of these
models using measured data from 30 propagation measurement datasets from 2 GHz
to 73 GHz over distances ranging from 4 m to 1238 m. A series of sensitivity
analyses of the three models show that the physically-based two-parameter CI
model and three-parameter CIF model offer computational simplicity, have very
similar goodness of fit (i.e., the shadow fading standard deviation), exhibit
more stable model parameter behavior across frequencies and distances, and
yield smaller prediction error in sensitivity testing across distances and
frequencies, when compared to the four-parameter ABG model. Results show the CI
model with a 1 m close-in reference distance is suitable for outdoor
environments, while the CIF model is more appropriate for indoor modeling. The
CI and CIF models are easily implemented in existing 3GPP models by making a
very subtle modification -- by replacing a floating non-physically based
constant with a frequency-dependent constant that represents free space path
loss in the first meter of propagation.Comment: Open access available at:
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=743465
A multi-color and Fourier study of RR Lyrae variables in the globular cluster NGC 5272 (M3)
We have performed a detailed study of the pulsational and evolutionary
characteristics of 133 RR Lyrae stars in the globular cluster NGC5272 (M3)
using highly accurate BVI data taken on 5 separate epochs. M3 seems to contain
no less than ~32% of Blazhko stars, and the occurrence and characteristics of
the Blazhko effect have been analyzed in detail. We have identified a good
number (~ 14%) of overluminous RR Lyrae stars that are likely in a more
advanced evolutionary stage off the Zero Age Horizontal Branch (ZAHB). Physical
parameters (i.e. temperature, luminosity, mass) have been derived from (B--V)
colors and accurate color-temperature calibration, and compared with Horizontal
Branch evolutionary models and with the requirements of stellar pulsation
theory. Additional analysis by means of Fourier decomposition of the V light
curves confirms, as expected, that no metallicity spread is present in M3.
Evolution off the ZAHB does not affect [Fe/H] determinations, whereas Blazhko
stars at low amplitude phase do affect [Fe/H] distributions as they appear more
metal-rich. Absolute magnitudes derived from Fourier coefficients might provide
useful average estimates for groups of stars, if applicable, but do not give
reliable {\em individual} values. Intrinsic colors derived from Fourier
coefficients show significant discrepancies with the observed ones, hence the
resulting temperatures and temperature-related parameters are unreliable.Comment: 86 pages, 19 figures, 13 tables, in press A
J-band variability of M dwarfs in the WFCAM Transit Survey
We present an analysis of the photometric variability of M dwarfs in the Wide Field Camera (WFCAM) Transit Survey. Although periodic light-curve variability in low mass stars is generally dominated by photospheric star spot activity, M dwarf variability in the J band has not been as thoroughly investigated as at visible wavelengths. Spectral type estimates for a sample of over 200 000 objects are made using spectral type-colour relations, and over 9600 dwarfs (J 0.2 mag flaring event from an M4V star in our sample.Peer reviewe
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