618 research outputs found
Units of relativistic time scales and associated quantities
This note suggests nomenclature for dealing with the units of various
astronomical quantities that are used with the relativistic time scales TT,
TDB, TCB and TCG. It is suggested to avoid wordings like "TDB units" and "TT
units" and avoid contrasting them to "SI units". The quantities intended for
use with TCG, TCB, TT or TDB should be called "TCG-compatible",
"TCB-compatible", "TT-compatible" or "TDB-compatible", respectively. The names
of the units second and meter for numerical values of all these quantities
should be used with out any adjectives. This suggestion comes from a special
discussion forum created within IAU Commission 52 "Relativity in Fundamental
Astronomy"
Hereditary sensory and autonomic neuropathy type IV and orthopaedic complications
SummaryHereditary sensory and autonomic neuropathy type IV (HSAN-IV) is a very rare autosomal recessive disorder characterized by recurrent episodes of unexplained fever, extensive anhidrosis, total insensitivity to pain, hypotonia, and mental retardation. The most frequent complications of this disease are corneal scarring, multiple fractures, joint deformities, osteomyelitis, and disabling self-mutilations. We reported the case of a 12-year-old boy. The goal was to discuss our decision-making and compare this case with cases described in the literature
GPS observables in general relativity
I present a complete set of gauge invariant observables, in the context of
general relativity coupled with a minimal amount of realistic matter (four
particles). These observables have a straightforward and realistic physical
interpretation. In fact, the technology to measure them is realized by the
Global Positioning System: they are defined by the physical reference system
determined by GPS readings. The components of the metric tensor in this
physical reference system are gauge invariant quantities and, remarkably, their
evolution equations are local.Comment: 6 pages, 1 figure, references adde
Aquatic pollution may favor the success of the invasive species A. franciscana
The genus Artemia consists of several bisexual and parthenogenetic sibling species. One of them, A. franciscana, originally restricted to the New World, becomes invasive when introduced into ecosystems out of its natural range of distribution. Invasiveness is anthropically favored by the use of cryptobiotic eggs in the aquaculture and pet trade. The mechanisms of out-competition of the autochthonous Artemia by the invader are still poorly understood. Ecological fitness may play a pivotal role, but other underlying biotic and abiotic factors may contribute. Since the presence of toxicants in hypersaline aquatic ecosystems has been documented, our aim here is to study the potential role of an organophosphate pesticide, chlorpyrifos, in a congeneric mechanism of competition between the bisexual A. franciscana (AF), and one of the Old World parthenogenetic siblings, A. parthenogenetica (PD). For this purpose we carried out life table experiments with both species, under different concentrations of the toxicant (0.1, 1 and 5 μg/l), and analyzed the cholinesterase inhibition at different developmental stages. The results evidence that both, AF and PD, showed an elevated tolerance to high ranges of chlorpyrifos, but AF survived better and its fecundity was less affected by the exposure to the pesticide than that of PD. The higher fecundity of AF is a selective advantage in colonization processes leading to its establishment as NIS. Besides, under the potential selective pressure of abiotic factors, such as the presence of toxicants, its higher resistance in terms of survival and biological fitness also indicates out-competitive advantages.Spanish Ministry for Science and Innovation project, CGL2005-02306, CGL2008-04737-E
Molecular Systematics of the Deep-Sea Hydrothermal Vent Endemic Brachyuran Family Bythograeidae: A Comparison of Three Bayesian Species Tree Methods
Brachyuran crabs of the family Bythograeidae are endemic to deep-sea hydrothermal vents and represent one of the most successful groups of macroinvertebrates that have colonized this extreme environment. Occurring worldwide, the family includes six genera (Allograea, Austinograea, Bythograea, Cyanagraea, Gandalfus, and Segonzacia) and fourteen formally described species. To investigate their evolutionary relationships, we conducted Maximum Likelihood and Bayesian molecular phylogenetic analyses, based on DNA sequences from fragments of three mitochondrial genes (16S rDNA, Cytochrome oxidase I, and Cytochrome b) and three nuclear genes (28S rDNA, the sodium–potassium ATPase a-subunit ‘NaK’, and Histone H3A). We employed traditional concatenated (i.e., supermatrix) phylogenetic methods, as well as three recently developed Bayesian multilocus methods aimed at inferring species trees from potentially discordant gene trees. We found strong support for two main clades within Bythograeidae: one comprising the members of the genus Bythograea; and the other comprising the remaining genera. Relationships within each of these two clades were partially resolved. We compare our results with an earlier hypothesis on the phylogenetic relationships among bythograeid genera based on morphology. We also discuss the biogeography of the family in the light of our results. Our species tree analyses reveal differences in how each of the three methods weighs conflicting phylogenetic signal from different gene partitions and how limits on the number of outgroup taxa may affect the results
Revision of the Cretaceous shark Protoxynotus (Chondrichthyes, Squaliformes) and early evolution of somniosid sharks
Due to the peculiar combination of dental features characteristic for different squaliform families, the position of the Late Cretaceous genera Protoxynotus and Paraphorosoides within Squaliformes has long been controversial. In this study, we revise these genera based on previously known fossil teeth and new dental material. The phylogenetic placement of Protoxynotus and Paraphorosoides among other extant and extinct squaliforms is discussed based on morphological characters combined with DNA sequence data of extant species. Our results suggest that Protoxynotus and Paraphorosoides should be included in the Somniosidae and that Paraphorosoides is a junior synonym of Protoxynotus. New dental material from the Campanian of Germany and the Maastrichtian of Austria enabled the description of a new species Protoxynotus mayrmelnhofi sp. nov. In addition, the evolution and origin of the characteristic squaliform tooth morphology are discussed, indicating that the elongated lower jaw teeth with erected cusp and distinct dignathic heterodonty of Protoxynotus represents a novel functional adaptation in its cutting-clutching type dentition among early squaliform sharks. Furthermore, the depositional environment of the tooth bearing horizons allows for an interpretation of the preferred habitat of this extinct dogfish shark, which exclusively occupied shelf environments of the Boreal- and northern Tethyan realms during the Late Cretaceous.publishedVersio
On the folding and deployment of tape springs: a large displacements and large rotations rod model with highly flexible thin-walled cross-section
International audienceIn the framework of deployable structures, we focus on the modeling of tape springs, i.e. rod-like elastic bodies with thin-walled cross-section which develop localized folds due to a flattening of the cross-section. A rod model with highly deformable cross-section and few kinematics parameters is derived from a complete shell model, for large displacements, large rotations and dynamics. The simplicity of the model is achieved by introducing an elastica kinematics to describe the changes in the cross-section shape. This model is able to handle the formation of localized folds which can move along the rod line, merge or split, allowing to simulate complex scenarios of folding and deployment
European regulatory agenices should employ full time statisticians
No abstract available
Limits on the Stochastic Gravitational Wave Background from the North American Nanohertz Observatory for Gravitational Waves
We present an analysis of high-precision pulsar timing data taken as part of
the North American Nanohertz Observatory for Gravitational waves (NANOGrav)
project. We have observed 17 pulsars for a span of roughly five years using the
Green Bank and Arecibo radio telescopes. We analyze these data using standard
pulsar timing models, with the addition of time-variable dispersion measure and
frequency-variable pulse shape terms. Sub-microsecond timing residuals are
obtained in nearly all cases, and the best root-mean-square timing residuals in
this set are ~30-50 ns. We present methods for analyzing post-fit timing
residuals for the presence of a gravitational wave signal with a specified
spectral shape. These optimally take into account the timing fluctuation power
removed by the model fit, and can be applied to either data from a single
pulsar, or to a set of pulsars to detect a correlated signal. We apply these
methods to our dataset to set an upper limit on the strength of the
nHz-frequency stochastic supermassive black hole gravitational wave background
of h_c (1 yr^-1) < 7x10^-15 (95%). This result is dominated by the timing of
the two best pulsars in the set, PSRs J1713+0747 and J1909-3744.Comment: To be submitted to Ap
Shapiro Effect as a Possible Cause of the Low-Frequency Pulsar Timing Noise in Globular Clusters
A prolonged timing of millisecond pulsars has revealed low-frequency
uncorrelated noise, presumably of astrophysical origin, in the pulse arrival
time (PAT) residuals for some of them. In most cases, pulsars in globular
clusters show a low-frequency modulation of their rotational phase and spin
rate. The relativistic time delay of the pulsar signal in the curved space time
of randomly distributed and moving globular cluster stars (the Shapiro effect)
is suggested as a possible cause of this modulation.
Given the smallness of the aberration corrections that arise from the
nonstationarity of the gravitational field of the randomly distributed ensemble
of stars under consideration, a formula is derived for the Shapiro effect for a
pulsar in a globular cluster. The derived formula is used to calculate the
autocorrelation function of the low-frequency pulsar noise, the slope of its
power spectrum, and the behavior of the statistic that characterizes
the spectral properties of this noise in the form of a time function. The
Shapiro effect under discussion is shown to manifest itself for large impact
parameters as a low-frequency noise of the pulsar spin rate with a spectral
index of n=-1.8 that depends weakly on the specific model distribution of stars
in the globular cluster. For small impact parameters, the spectral index of the
noise is n=-1.5.Comment: 23 pages, 6 figure
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