8,877 research outputs found
From Hipparcos to Gaia
The measurement of the positions, distances, motions and luminosities of
stars represents the foundations of modern astronomical knowledge. Launched at
the end of the eighties, the ESA Hipparcos satellite was the first space
mission dedicated to such measurements. Hipparcos improved position accuracies
by a factor of 100 compared to typical ground-based results and provided
astrometric and photometric multi-epoch observations of 118,000 stars over the
entire sky. The impact of Hipparcos on astrophysics has been extremely valuable
and diverse. Building on this important European success, the ESA Gaia
cornerstone mission promises an even more impressive advance. Compared to
Hipparcos, it will bring a gain of a factor 50 to 100 in position accuracy and
of a factor of 10,000 in star number, collecting photometric,
spectrophotometric and spectroscopic data for one billion celestial objects.
During its 5-year flight, Gaia will measure objects repeatedly, up to a few
hundred times, providing an unprecedented database to study the variability of
all types of celestial objects. Gaia will bring outstanding contributions,
directly or indirectly, to most fields of research in astrophysics, such as the
study of our Galaxy and of its stellar constituents, the search for planets
outside the solar system.Comment: 6 pages. New Horizons in Time Domain Astronomy Proceedings IAU
Symposium No. 285, 2012, E. Griffin, B. Hanisch & R. Seaman, ed
Comparison of Viscosities from the Chapman-Enskog and Relaxation Time Methods
A quantitative comparison between the results of shear viscosities from the
Chapman-Enskog and relaxation time methods is performed for selected test cases
with specified elastic differential cross sections: (i) the non-relativistic,
relativistic and ultra-relativistic hard sphere gas with angle and energy
independent differential cross section, (ii) the Maxwell gas, (iii) chiral
pions and (iv) massive pions. Our quantitative results reveal that the extent
of agreement (or disagreement) depends very sensitively on the energy
dependence of the differential cross sections employed.Comment: Submitted to Cent. Eur. J.Phy
The association between parenting behavior and somatization in adolescents explained by physiological responses in adolescents
Introduction: This study adds to the knowledge on somatization in adolescents by exploring its relation with parenting behavior and the mediating/moderating role of physiological responses in adolescents to parenting behavior. Method: Eighteen adolescents with high and 18 adolescents with low somatization scores and their mothers completed a discussion task, from which observed parenting behavior scores were derived. Skin conductance in adolescents was measured before and during the discussion. Results: For adolescents with high levels of physiological responses, unadaptive parenting was related to a higher chance of high somatization scores. For low physiologically responsive adolescents, the relation between parenting behavior and somatization was not significant. Conclusion: Parenting behavior is not univocally related to somatization in adolescents, but the association depends on physiological responses in adolescents. (C) 2014 Elsevier B.V. All rights reserved
Correlation effects in bistability at the nanoscale: steady state and beyond
The possibility of finding multistability in the density and current of an
interacting nanoscale junction coupled to semi-infinite leads is studied at
various levels of approximation. The system is driven out of equilibrium by an
external bias and the non-equilibrium properties are determined by real-time
propagation using both time-dependent density functional theory (TDDFT) and
many-body perturbation theory (MBPT). In TDDFT the exchange-correlation effects
are described within a recently proposed adiabatic local density approximation
(ALDA). In MBPT the electron-electron interaction is incorporated in a
many-body self-energy which is then approximated at the Hartree-Fock (HF),
second-Born (2B) and GW level. Assuming the existence of a steady-state and
solving directly the steady-state equations we find multiple solutions in the
HF approximation and within the ALDA. In these cases we investigate if and how
these solutions can be reached through time evolution and how to reversibly
switch between them. We further show that for the same cases the inclusion of
dynamical correlation effects suppresses bistability.Comment: 13 pages, 12 figure
Hipparcos open clusters and stellar evolution
By relying on recently improved Hipparcos parallaxes for the Hyades, Pleiades
and Ursa Major clusters we find that stellar models with updated physical
inputs nicely reproduce the location in the color magnitude diagram of main
sequence stars of different metallicities. Stars in the helium burning phase
are also discussed, showing that the luminosity of giants in the Hyades,
Praesepe and Ursa Major clusters appears to be in reasonable agreement with
theoretical predictions. A short discussion concerning the current evolutionary
scenarios closes the paper.Comment: 5 pages, 6 Postscript figures, accepted by MNRA
Recommended from our members
Multiplicative nonâgaussian model error estimation in data assimilation
Abstract: Model uncertainty quantification is an essential component of effective data assimilation. Model errors associated with subâgrid scale processes are often represented through stochastic parameterizations of the unresolved process. Many existing Stochastic Parameterization schemes are only applicable when knowledge of the true subâgrid scale process or full observations of the coarse scale process are available, which is typically not the case in real applications. We present a methodology for estimating the statistics of subâgrid scale processes for the more realistic case that only partial observations of the coarse scale process are available. Model error realizations are estimated over a training period by minimizing their conditional sum of squared deviations given some informative covariates (e.g., state of the system), constrained by available observations and assuming that the observation errors are smaller than the model errors. From these realizations a conditional probability distribution of additive model errors given these covariates is obtained, allowing for complex nonâGaussian error structures. Random draws from this density are then used in actual ensemble data assimilation experiments. We demonstrate the efficacy of the approach through numerical experiments with the multiâscale Lorenz 96 system using both small and large time scale separations between slow (coarse scale) and fast (fine scale) variables. The resulting error estimates and forecasts obtained with this new method are superior to those from two existing methods
Long-Ranged Orientational Order in Dipolar Fluids
Recently Groh and Dietrich claimed the thermodynamic state of a dipolar fluid
depends on the shape of the fluid's container. For example, a homogeneous fluid
in a short fat container would phase separate when transferred to a tall skinny
container of identical volume and temperature. Their calculation thus lacks a
thermodynamic limit. We show that removal of demagnetizing fields restores the
true, shape independent, thermodynamic limit. As a consequence, spontaneously
magnetized liquids display inhomogeneous magnetization textures.Comment: 3 pages, LaTex, no figures. Submitted as comment to PRL, May 199
Spin Stiffness in the Hubbard model
The spin stiffness of the repulsive Hubbard model that occurs
in the hydrodynamic theory of antiferromagnetic spin waves is shown to be the
same as the thermodynamically defined stiffness involved in twisting the order
parameter. New expressions for are derived, which enable easier
interpretation, and connections with superconducting weight and gauge
invariance are discussed.Comment: 21 Pages LaTeX2e, to be published in Journal of Physics
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