186,001 research outputs found
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
Einstein-Cartan theory as a theory of defects in space-time
The Einstein-Cartan theory of gravitation and the classical theory of defects
in an elastic medium are presented and compared. The former is an extension of
general relativity and refers to four-dimensional space-time, while we
introduce the latter as a description of the equilibrium state of a
three-dimensional continuum. Despite these important differences, an analogy is
built on their common geometrical foundations, and it is shown that a
space-time with curvature and torsion can be considered as a state of a
four-dimensional continuum containing defects. This formal analogy is useful
for illustrating the geometrical concept of torsion by applying it to concrete
physical problems. Moreover, the presentation of these theories using a common
geometrical basis allows a deeper understanding of their foundations.Comment: 18 pages, 7 EPS figures, RevTeX4, to appear in the American Journal
of Physics, revised version with typos correcte
Electronic Phase Separation Transition as the Origin of the Superconductivity and the Pseudogap Phase of Cuprates
We propose a new phase of matter, an electronic phase separation transition
that starts near the upper pseudogap and segregates the holes into high and low
density domains. The Cahn-Hilliard approach is used to follow quantitatively
this second order transition. The resulting grain boundary potential confines
the charge in domains and favors the development of intragrain superconducting
amplitudes. The zero resistivity transition arises only when the intergrain
Josephson coupling is of the order of the thermal energy and phase
locking among the superconducting grains takes place. We show that this
approach explains the pseudogap and superconducting phases in a natural way and
reproduces some recent scanning tunneling microscopy dataComment: 4 pages and 5 eps fig
Energy density and pressure of long wavelength gravitational waves
Inflation leads us to expect a spectrum of gravitational waves (tensor
perturbations) extending to wavelengths much bigger than the present observable
horizon. Although these gravity waves are not directly observable, the energy
density that they contribute grows in importance during the radiation- and
dust-dominated ages of the universe. We show that the back reaction of tensor
perturbations during matter domination is limited from above, since
gravitational waves of wavelength have a share of the total energy
density during matter domination that is at most
equal to the share of the total energy density that they had when the mode
exited the Hubble radius during inflation. This work is to
be contrasted to that of Sahni, who analyzed the energy density of gravity
waves only insofar as their wavelengths are smaller than . Such a
cut-off in the spectral energy of gravity waves leads to the breakdown of
energy conservation, and we show that this anomaly is eliminated simply by
taking into account the energy density and pressure of long wavelength
gravitational waves as well as short wavelength ones.Comment: Updated one reference; 17 pages, no figure
Lattice Dynamics in the Half-Space, II. Energy Transport Equation
We consider the lattice dynamics in the half-space. The initial data are
random according to a probability measure which enforces slow spatial variation
on the linear scale . We establish two time regimes. For
times of order , , locally the measure
converges to a Gaussian measure which is time stationary with a covariance
inherited from the initial measure (non-Gaussian, in general). For times of
order , this covariance changes in time and is governed by a
semiclassical transport equation.Comment: 35 page
Exploiting the full power of temporal gene expression profiling through a new statistical test: Application to the analysis of muscular dystrophy data
Background: The identification of biologically interesting genes in a temporal expression profiling
dataset is challenging and complicated by high levels of experimental noise. Most statistical methods
used in the literature do not fully exploit the temporal ordering in the dataset and are not suited
to the case where temporal profiles are measured for a number of different biological conditions.
We present a statistical test that makes explicit use of the temporal order in the data by fitting
polynomial functions to the temporal profile of each gene and for each biological condition. A
Hotelling T2-statistic is derived to detect the genes for which the parameters of these polynomials
are significantly different from each other.
Results: We validate the temporal Hotelling T2-test on muscular gene expression data from four
mouse strains which were profiled at different ages: dystrophin-, beta-sarcoglycan and gammasarcoglycan
deficient mice, and wild-type mice. The first three are animal models for different
muscular dystrophies. Extensive biological validation shows that the method is capable of finding
genes with temporal profiles significantly different across the four strains, as well as identifying
potential biomarkers for each form of the disease. The added value of the temporal test compared
to an identical test which does not make use of temporal ordering is demonstrated via a simulation
study, and through confirmation of the expression profiles from selected genes by quantitative PCR
experiments. The proposed method maximises the detection of the biologically interesting genes,
whilst minimising false detections.
Conclusion: The temporal Hotelling T2-test is capable of finding relatively small and robust sets
of genes that display different temporal profiles between the conditions of interest. The test is
simple, it can be used on gene expression data generated from any experimental design and for any
number of conditions, and it allows fast interpretation of the temporal behaviour of genes. The R
code is available from V.V. The microarray data have been submitted to GEO under series
GSE1574 and GSE3523
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