61,043 research outputs found
A spatial scan statistic for zero-inflated Poisson process
The scan statistic is widely used in spatial cluster detection applications
of inhomogeneous Poisson processes. However, real data may present substantial
departure from the underlying Poisson process. One of the possible departures
has to do with zero excess. Some studies point out that when applied to data
with excess zeros, the spatial scan statistic may produce biased inferences. In
this work, we develop a closed-form scan statistic for cluster detection of
spatial zero-inflated count data. We apply our methodology to simulated and
real data. Our simulations revealed that the Scan-Poisson statistic steadily
deteriorates as the number of zeros increases, producing biased inferences. On
the other hand, our proposed Scan-ZIP and Scan-ZIP+EM statistics are, most of
the time, either superior or comparable to the Scan-Poisson statistic
Categorical Aspects of the Double Structure of a Module
In this work we develop some categorical aspects of the double structure of a
module
Coherent phonon transport in short-period two-dimensional superlattices of graphene and boron nitride
Promoting coherent transport of phonons at material interfaces is a promising strategy for controlling thermal transport in nanostructures and an alternative to traditional methods based on structural defects. Coherent transport is particularly relevant in short-period heterostructures with smooth interfaces and long-wavelength heat-carrying phonons, such as two-dimensional superlattices of graphene and boron nitride. In this work, we predict phonon properties and thermal conductivities in these superlattices using a normal mode decomposition approach. We study the variation of the frequency dependence of these properties with the periodicity and interface configuration (zigzag and armchair) for superlattices with period lengths within the coherent regime. Our results showed that the thermal conductivity decreases significantly from the first period length (0.44 nm) to the second period length (0.87 nm), 13% across the interfaces and 16% along the interfaces. For greater periods, the conductivity across the interfaces continues decreasing at a smaller rate of 11 W/mK per period length increase (0.43 nm), driven by changes in the phonon group velocities (coherent effects). In contrast, the conductivity along the interfaces slightly recovers at a rate of 2 W/mK per period, driven by changes in the phonon relaxation times (diffusive effects). By changing the interface configuration from armchair to zigzag, the conductivities for all period lengths increase by approximately 7% across the interfaces and 19% along the interfaces
Dynamical Evolution of an Unstable Gravastar with Zero Mass
Using the conventional gravastar model, that is, an object constituted by two
components where one of them is a massive infinitely thin shell and the other
one is a de Sitter interior spacetime, we physically interpret a solution
characterized by a zero Schwarzschild mass. No stable gravastar is formed and
it collapses without forming an event horizon, originating what we call a
massive non-gravitational object. The most surprise here is that the collapse
occurs with an exterior de Sitter vacuum spacetime. This creates an object
which does not interact gravitationally with an outside test particle and it
may evolve to a point-like topological defect.Comment: 8 pages, 10 figures, to appear in Astrophysics and Space Scienc
A photometric and spectroscopic survey of solar twin stars within 50 parsecs of the Sun: I. Atmospheric parameters and color similarity to the Sun
Solar twins and analogs are fundamental in the characterization of the Sun's
place in the context of stellar measurements, as they are in understanding how
typical the solar properties are in its neighborhood. They are also important
for representing sunlight observable in the night sky for diverse photometric
and spectroscopic tasks, besides being natural candidates for harboring
planetary systems similar to ours and possibly even life-bearing environments.
We report a photometric and spectroscopic survey of solar twin stars within 50
pc of the Sun. Hipparcos absolute magnitudes and (B-V)_Tycho colors were used
to define a 2 sigma box around the solar values, where 133 stars were
considered. Additional stars resembling the solar UBV colors in a broad sense,
plus stars present in the lists of Hardorp, were also selected. All objects
were ranked by a color-similarity index with respect to the Sun, defined by
uvby and BV photometry. Moderately high-resolution, high-S/N spectra were used
for a subsample of equatorial-southern stars to derive Teff, log g, and [Fe/H]
with average internal errors better than 50 K, 0.20 dex, and 0.08 dex,
respectively. Ages and masses were estimated from theoretical HR diagrams. The
color-similarity index proved very successful. We identify and rank new
excellent solar analogs, which are fit to represent the Sun in the night sky.
Some of them are faint enough to be of interest for moderately large
telescopes. We also identify two stars with near-UV spectra indistinguishable
from the Sun's. We present five new "probable" solar twin stars, besides five
new "possible" twins. Masses and ages for the best solar twin candidates lie
very close to the solar values, but chromospheric activity levels range
somewhat. We propose that the solar twins be emphasized in the ongoing searches
for extra-solar planets and SETI searches.Comment: 25 pages, 15 figures, 14 table
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