25,300 research outputs found
Is there a propeller neutron star in Cas?
Cas is the prototype of a small population of B0-B1.5 III-V
classical Be (cBe) stars that emit anomalous and hard X-rays with a unique
array of properties. Cas is known to host, like other cBe stars, a
decretion disk and also a low mass companion. Recently Postnov et al. have
posited that this companion is a magnetized rapidly spinning neutron star that
deflects direct gravitational accretion from a stellar/disk wind via the
"propeller mechanism." These authors state that the key X-ray observations are
"remarkably well produced" in this scenario. We reexamine this mechanism in
detail and conclude that there are a number of fatal objections in its
application to the Cas case. Among other considerations these issues
include the prediction under the propeller scenario of a much smaller
population of Cas stars than is observed and the lack of allowance for
observed correlations of X-ray and UV and/or optical properties over a variety
of timescales.Comment: 9 page
MCMC Bayesian Estimation in FIEGARCH Models
Bayesian inference for fractionally integrated exponential generalized
autoregressive conditional heteroskedastic (FIEGARCH) models using Markov Chain
Monte Carlo (MCMC) methods is described. A simulation study is presented to
access the performance of the procedure, under the presence of long-memory in
the volatility. Samples from FIEGARCH processes are obtained upon considering
the generalized error distribution (GED) for the innovation process. Different
values for the tail-thickness parameter \nu are considered covering both
scenarios, innovation processes with lighter (\nu2) tails
than the Gaussian distribution (\nu=2). A sensitivity analysis is performed by
considering different prior density functions and by integrating (or not) the
knowledge on the true parameter values to select the hyperparameter values
Contribuições de Gaston Bachelard ao ensino de ciências
Starting from an analysis on Gaston Bachelard's epistemological work, a survey is made of his conceptions on the teaching-learning process in the physical sciences, with special emphasis on Chemistry, which is so frequently rejected by the students, for it is presented as a set of superposed facts devoid of rational coherence. Accomplising the criticism of continuous course of scientific history and structuring the psychoanalytical basis of the objective knowledge, Bachelard offers subsidies to the inquiries into the teacher, the student and the school roles, into teaching methods and into didatic books
Nambu monopoles interacting with lattice defects in two-dimensional artificial square spin ice
The interactions between an excitation (similar to a pair of Nambu monopoles)
and a lattice defect are studied in an artificial two-dimensional square spin
ice. This is done by considering a square array of islands containing only one
island different from all others. This difference is incorporated in the
magnetic moment (spin) of the "imperfect" island and several cases are studied,
including the special situation in which this distinct spin is zero (vacancy).
We have shown that the two extreme points of a malformed island behave like two
opposite magnetic charges. Then, the effective interaction between a pair of
Nambu monopoles with the deformed island is a problem involving four magnetic
charges (two pairs of opposite poles) and a string. We also sketch the
configuration of the field lines of these four charges to confirm this picture.
The influence of the string on this interaction decays rapidly with the string
distance from the defect.Comment: 7 pages, 13 figure
The Digitized Second Palomar Observatory Sky Survey (DPOSS) II: Photometric Calibration
We present the photometric calibration technique for the Digitized Second
Palomar Observatory Sky Survey (DPOSS), used to create seamless catalogs of
calibrated objects over large sky areas. After applying a correction for
telescope vignetting, the extensive plate overlap regions are used to transform
sets of plates onto a common instrumental photometric system. Photometric
transformations to the Gunn gri system for each plate, for stars and galaxies,
are derived using these contiguous stitched areas and an extensive CCD imaging
library obtained for this purpose. We discuss the resulting photometric
accuracy, survey depth, and possible systematic errors.Comment: 25 pages, 13 figures. Accepted to AJ. Some figures shrunk or missing
to limit file size; the full paper is available at
http://www.sdss.jhu.edu/~rrg/science/papers/photometrypaper.ps.g
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