7,784 research outputs found
Coasting cosmologies with time dependent cosmological constant
The effect of a time dependent cosmological constant is considered in a
family of scalar tensor theories. Friedmann-Robertson-Walker cosmological
models for vacumm and perfect fluid matter are found. They have a linear
expansion factor, the so called coasting cosmology, the gravitational
"constant" decreace inversely with time; this model satisfy the Dirac
hipotesis. The cosmological "constant" decreace inversely with the square of
time, therefore we can have a very small value for it at present time.Comment: 7 pages, latex file (ijmpal macro), accepted for publication in Int.
Mod. Phys.
Disorder from order among anisotropic next-nearest-neighbor Ising spin chains in SrHoO
We describe why Ising spin chains with competing interactions in segregate into ordered and disordered ensembles at low temperatures
(). Using elastic neutron scattering, magnetization, and specific heat
measurements, the two distinct spin chains are inferred to have N\'eel
() and double-N\'eel
() ground states respectively. Below
~K, the N\'eel chains develop three dimensional (3D) long
range order (LRO), which arrests further thermal equilibration of the
double-N\'eel chains so they remain in a disordered incommensurate state for
below ~K. distills an important
feature of incommensurate low dimensional magnetism: kinetically trapped
topological defects in a quasidimensional spin system can preclude order
in dimensions.Comment: 10 pages, 10 figure
Dengue una enfermedad emergente y re-emergente en América
El Dengue es una enfermedad infecciosa, producida por un arbovirus cuyo único reservorio es el hombre. El virusutiliza como vector biológico al mosquito Aedes aegypti o al mosquito Aedes albopictus. La sintomatologÃa se presentahabitualmente como un cuadro febril denominado dengue clásico, que se caracteriza por fiebre alta de presentaciónaguda, de duración limitada (2 a 7 dÃas), con intenso malestar general, acompañado de erupción cutánea. Puedepresentar sÃntomas hemorrágicos de escasa intensidad, como petequias y sangramiento gingival. El tratamiento essintomático y el paciente mejora completamente en aproximadamente 7 dÃas. Esta forma de dengue no producemortalidad. Sin embargo, existen otras presentaciones de la enfermedad que pueden llegar a manifestaciones graves deltipo hemorrágicas con muerte, lo que se presenta en el 5% de los enfermos. El dengue es un problema creciente desalud pública, que afecta a más de 100 paÃses en el mundo, con más de 50 millones de casos informados cada año. Loscuatro tipos de dengue, están circulando en América, donde los casos aumentaron en los últimos años en formaexplosiva. Si bien, en Chile continental no se ha documentado la existencia del mosquito vector del dengue de formaendémica, si se ha hecho en Chile Insular, más especÃficamente en Isla de Pascua, donde desde el año 1999, seincorporó a la lista de enfermedades de declaración obligatoria (D.S. Nº 158) estableciendo su vigilancia. Los factoresque han llevado a la emergencia de esta enfermedad son principalmente el cambio climático, que ha modificado elnicho ecológico de los mosquitos de la familia Aedes. También la urbanización, la falta de control del vector, las fallasen infraestructura básica y el pobre saneamiento ambiental. La intervención primaria de salud pública ha ido por la lÃneadel uso de insecticidas para el control del vector y la detección temprana de casos. No existe aún una vacuna efectiva,sin embargo, actualmente se están realizando esfuerzos en esta materia
Extreme Quiescent Variability of the Transient Neutron Star Low-mass X-ray Binary EXO 1745-248 in Terzan 5
EXO 1745-248 is a transient neutron-star low-mass X-ray binary that resides
in the globular cluster Terzan 5. We studied the transient during its quiescent
state using 18 Chandra observations of the cluster acquired between 2003 and
2016. We found an extremely variable source, with a luminosity variation in the
0.5-10 keV energy range of orders of magnitude (between
erg s and erg s) on timescales
from years down to only a few days. Using an absorbed power-law model to fit
its quiescent spectra, we obtained a typical photon index of ,
indicating that the source is even harder than during outburst and much harder
than typical quiescent neutron stars if their quiescent X-ray spectra are also
described by a single power-law model. This indicates that EXO 1745-248 is very
hard throughout the entire observed X-ray luminosity range. At the highest
luminosity, the spectrum fits better when an additional (soft) component is
added to the model. All these quiescent properties are likely related to strong
variability in the low-level accretion rate in the system. However, its extreme
variable behavior is strikingly different from the one observed for other
neutron star transients that are thought to still accrete in quiescence. We
compare our results to these systems. We also discuss similarities and
differences between our target and the transitional millisecond pulsar IGR
J18245-2452, which also has hard spectra and strong variability during
quiescence.Comment: Accepted for publication in MNRA
New Cataclysmic Variables and other Exotic Binaries in the Globular Cluster 47 Tucanae
We present 22 new (+3 confirmed) cataclysmic variables (CVs) in the non
core-collapsed globular cluster 47 Tucanae (47 Tuc). The total number of CVs in
the cluster is now 43, the largest sample in any globular cluster so far. For
the identifications we used near-ultraviolet (NUV) and optical images from the
Hubble Space Telescope, in combination with X-ray results from the Chandra
X-ray Observatory. This allowed us to build the deepest NUV CV luminosity
function of the cluster to date. We found that the CVs in 47 Tuc are more
concentrated towards the cluster center than the main sequence turnoff stars.
We compared our results to the CV populations of the core-collapsed globular
clusters NGC 6397 and NGC 6752. We found that 47 Tuc has fewer bright CVs per
unit mass than those two other clusters. That suggests that dynamical
interactions in core-collapsed clusters play a major role creating new CVs. In
47 Tuc, the CV population is probably dominated by primordial and old
dynamically formed systems. We estimated that the CVs in 47 Tuc have total
masses of approx. 1.4 M_sun. We also found that the X-ray luminosity function
of the CVs in the three clusters is bimodal. Additionally, we discuss a
possible double degenerate system and an intriguing/unclassified object.
Finally, we present four systems that could be millisecond pulsar companions
given their X-ray and NUV/optical colors. For one of them we present very
strong evidence for being an ablated companion. The other three could be CO- or
He-WDs.Comment: Published on MNRAS. 31 Pages, 23 Figures, 5 Tables. Minor changes
with respect to previous arXiv versio
A Study of the Accuracy of Mass-Radius Relationships for Silicate-Rich and Ice-Rich Planets up to 100 Earth Masses
A mass-radius relationship is proposed for solid planets and solid cores
ranging from 1 to 100 Earth-mass planets. It relies on the assumption that
solid spheres are composed of iron and silicates, around which a variable
amount of water is added. The M-R law has been set up assuming that the
planetary composition is similar to the averaged composition for silicates and
iron obtained from the major elements ratio of 94 stars hosting exoplanets.
Except on Earth for which a tremendous amount of data is available, the
composition of silicate mantles and metallic cores cannot be constrained.
Similarly, thermal profiles are poorly known. In this work, the effect of
compositional parameters and thermal profiles on radii estimates is quantified.
It will be demonstrated that uncertainties related to composition and
temperature are of second order compared to the effect of the water amount. The
Super-Earths family includes four classes of planets: iron-rich, silicate-rich,
water-rich, or with a thick atmosphere. For a given mass, the planetary radius
increases significantly from the ironrich to the atmospheric-rich planet. Even
if some overlaps are likely, M-R measurements could be accurate enough to
ascertain the discovery of an earth-like planet .The present work describes how
the amount of water can be assessed from M-R measurements. Such an estimate
depends on several assumptions including i) the accuracy of the internal
structure model and ii) the accuracy of mass and radius measurements. It is
shown that if the mass and the radius are perfectly known, the standard
deviation on the amount of water is about 4.5 %. This value increases rapidly
with the radius uncertainty but does not strongly depend on the mass
uncertainty.Comment: In press in The Astrophysical Journa
Shower approach in the simulation of ion scattering from solids
An efficient approach for the simulation of ion scattering from solids is
proposed. For every encountered atom, we take multiple samples of its thermal
displacements among those which result in scattering with high probability to
finally reach the detector. As a result, the detector is illuminated by
intensive "showers", where each event of detection must be weighted according
to the actual probability of the atom displacement. The computational cost of
such simulation is orders of magnitude lower than in the direct approach and a
comprehensive analysis of multiple and plural scattering effects becomes
possible. We use the new method for two purposes. First, the accuracy of the
approximate approaches, developed mainly for ion-beam structural analysis, is
verified. Second, the possibility to reproduce a wide class of experimental
conditions is used to analyze some basic features of ion-solid collisions: the
role of double violent collisions in low-energy ion scattering; the origin of
the "surface peak" in scattering from amorphous samples; the low-energy tail in
the energy spectra of scattered medium-energy ions due to plural scattering;
the degradation of blocking patterns in 2D angular distributions with
increasing depth of scattering. As an example of simulation for ions of MeV
energies, we verify the time-reversibility for channeling/blocking of 1 MeV
protons in a W crystal. The possibilities of analysis that our approach offers
may be very useful for various applications in particular for structural
analysis with atomic resolution.Comment: 16 pages, 9 figures. Finally published version; large parts
reformulated, Fig. 9 and references adde
Why Major Programs Need Innovation Support Labs: An Example from the Space Shuttle Launch Program at KSC
For over 30 years the Kennedy Space Center (KSC) has processed the Space Shuttle; handling all hands-on aspects from receiving the Orbiter, External Tanks, Solid Rocket Booster Segments, and Payloads, through certification, check-out, and assembly, and ending with fueling, count-down, and launch. A team of thousands have worked this highly complicated, yet supremely organized, process and have, as a consequence, generated an exceptional amount of technology to solve a host of problems. This paper describes the contributions of one team that formed with the express purpose to help solve some of these diverse Shuttle ground processing problems
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