76,124 research outputs found
Ricci dark energy in Chern-Simons modified gravity
In this work, we have considered the Ricci dark energy model, where the
energy density of the universe is proportional to the Ricci scalar curvature,
in the dynamic Chern-Simons modified gravity. We show that in this context the
evolution of the scale factor is similar to that displayed by the modified
Chaplygin gas.Comment: 7 pages; to appear in EPJ
A coordinate-dependent superspace deformation from string theory
Starting from a type II superstring model defined on in
a linear graviphoton background, we derive a coordinate dependent -deformed
, superspace. The chiral fermionic coordinates
satisfy a Clifford algebra, while the other coordinate algebra remains
unchanged. We find a linear relation between the graviphoton field strength and
the deformation parameter. The null coordinate dependence of the graviphoton
background allows to extend the results to all orders in .Comment: 14 pages, reference added, accepted for publication in JHE
Simulation of Chua's Circuit by Means of Interval Analysis
The Chua's circuit is a paradigm for nonlinear scientific studies. It is
usually simulated by means of numerical methods under IEEE 754-2008 standard.
Although the error propagation problem is well known, little attention has been
given to the relationship between this error and inequalities presented in
Chua's circuit model. Taking the average of round mode towards and
, we showed a qualitative change on the dynamics of Chua's circuit.Comment: 6th International Conference on Nonlinear Science and Complexity -
S\~ao Jos\'e dos Campos, 2016, p. 1-
Are z>2 Herschel galaxies proto-spheroids?
We present a backward approach for the interpretation of the evolution of the
near-infrared and the far-infrared luminosity functions across the redshift
range 0<z<3. In our method, late-type galaxies are treated by means of a
parametric phenomenological method based on PEP/HerMES data up to z~4, whereas
spheroids are described by means of a physically motivated backward model. The
spectral evolution of spheroids is modelled by means of a single-mass model,
associated to a present-day elliptical with K-band luminosity comparable to the
break of the local early-type luminosity function. The formation of
proto-spheroids is assumed to occurr across the redshift range 1< z < 5. The
key parameter is represented by the redshift z_0.5 at which half
proto-spheroids are already formed. A statistical study indicates for this
parameter values between z_0.5=1.5 and z_0.5=3. We assume as fiducial value
z_0.5~2, and show that this assumption allows us to describe accourately the
redshift distributions and the source counts. By assuming z_0.5 ~ 2 at the
far-IR flux limit of the PEP-COSMOS survey, the PEP-selected sources observed
at z>2 can be explained as progenitors of local spheroids caught during their
formation. We also test the effects of mass downsizing by dividing the
spheroids into three populations of different present-day stellar masses. The
results obtained in this case confirm the validity of our approach, i.e. that
the bulk of proto-spheroids can be modelled by means of a single model which
describes the evolution of galaxies at the break of the present-day early type
K-band LF.Comment: Accepted for publication in ApJ; 26 pages; 13 figure
Spin dependent transport in organic light-emitting diodes
Electrically Detected Magnetic Resonance (EDMR) was used to study a series of
multilayer organic devices based on aluminum (III) 8-hydroxyquinoline. These
devices were designed to identify the micoscopic origin of different spin
dependent process, i.e. hopping and exciton formation. EDMR is demonstrated to
probe molecular orbitals of charge, and thus indirectly explore interfaces,
exciton formation, charge accumalation and electric fields in operating organic
based devices
On FRW Model in Conformal Teleparallel Gravity
In this paper we use the conformal teleparallel gravity to study an isotropic
and homogeneous Universe which is settled by the FRW metric. We solve the field
equations and we obtain the behavior of some cosmological parameters such as
scale factor, deceleration parameter and the energy density of the perfect
fluid which is the matter field of our model. The field equations, that we
called modified Friedmann equations, allow us to define a dark fluid, with dark
energy density and dark pressure, responsible for the acceleration in the
Universe.Comment: Accepted in EPJ
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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