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At least 613 records · Page 34Linked to original sources
arXiv2008

Situational diagnosis for preservice elementary teachers on some basic astronomical phenomena

The present situation of the teaching-learning relationship in astronomy is rarely investigated in our country. This is so, even when it is widely recognized that astronomy is an integral discipline, at the crossroads of advances in physics, geology, chemistry, etc. In this project we aim at contributing with the situational diagnosis in topics of astronomy of preservice elementary teachers, and to try and develop didactic tools that better collaborate with their formal education. We work with an open written questionnaire, designed to put in evidence some of the most frequently used models on a few basic astronomical notions. We here describe the main questions included in the test, and show our first results and some conclusions after having addressed the questionnaire to a number of preservice elementary teachers.

physics.ed-ph↗
arXiv2008

Dynamical tomography of gravitationally bound systems

We study the inverse problem of deducing the dynamical characteristics (such as the potential field) of large systems from kinematic observations. We show that, for a class of steady-state systems, the solution is unique even with fragmentary data, dark matter, or selection (bias) functions. Using spherically symmetric models for simulations, we investigate solution convergence and the roles of data noise and regularization in the inverse problem. We also present a method, analogous to tomography, for comparing the observed data with a model probability distribution such that the latter can be determined.

astro-ph↗
arXiv2008

Using SPICA Space Telescope to characterize Exoplanets

We present the 3.5m SPICA space telescope, a proposed Japanese-led JAXA-ESA mission scheduled for launch around 2017. The actively cooled (<5 K), single aperture telescope and monolithic mirror will operate from ~3.5 to ~210 um and will provide superb sensitivity in the mid- and far-IR spectral domain (better than JWST at lambda > 18 um). SPICA is one of the few space missions selected to go to the next stage of ESA's Cosmic Vision 2015-2025 selection process. In this White Paper we present the main specifications of the three instruments currently baselined for SPICA: a mid-infrared (MIR) coronagraph (~3.5 to ~27 um) with photometric and spectral capabilities (R~200), a MIR wide-field camera and high resolution spectrometer (R~30,000), and a far-infrared (FIR ~30 to ~210 um) imaging spectrometer - SAFARI - led by a European consortium. We discuss their capabilities in the context of MIR direct observations of exo-planets (EPs) and multiband photometry/high resolution spectroscopy observations of transiting exo-planets. We conclude that SPICA will be able to characterize the atmospheres of transiting exo-planets down to the super-Earth size previously detected by ground- or space-based observatories. It will also directly detect and characterize Jupiter/Neptune-size planets orbiting at larger separation from their parent star (>5-10 AU), by performing quantitative atmospheric spectroscopy and studying proto-planetary and debris disks. In addition, SPICA will be a scientific and technological precursor for future, more ambitious, IR space missions for exo-planet direct detection as it will, for example, quantify the prevalence exo-zodiacal clouds in planetary systems and test coronographic techniques, cryogenic systems and lightweight, high quality telescopes. (abridged)

astro-ph↗
arXiv2009

User Guide for the Discrete Dipole Approximation Code DDSCAT 7.0

DDSCAT 7.0 is an open-source Fortran-90 software package applying the discrete dipole approximation to calculate scattering and absorption of electromagnetic waves by targets with arbitrary geometries and complex refractive index. The targets may be isolated entities (e.g., dust particles), but may also be 1-d or 2-d periodic arrays of "target unit cells", allowing calculation of absorption, scattering, and electric fields around arrays of nanostructures. The theory of the DDA and its implementation in DDSCAT is presented in Draine (1988) and Draine & Flatau (1994), and its extension to periodic structures (and near-field calculations) in Draine & Flatau (2008). DDSCAT 7.0 includes support for MPI, OpenMP, and the Intel Math Kernel Library (MKL). DDSCAT supports calculations for a variety of target geometries. Target materials may be both inhomogeneous and anisotropic. It is straightforward for the user to "import" arbitrary target geometries into the code. DDSCAT automatically calculates total cross sections for absorption and scattering and selected elements of the Mueller scattering intensity matrix. This User Guide explains how to use DDSCAT 7.0 to carry out electromagnetic scattering calculations. DDfield, a Fortran-90 code DDfield to calculate E and B at user-selected locations near the target, is included in the distribution.

astro-ph↗
arXiv2008

Age determinations of the open star clusters King 14 and NGC 146

We based on 2MASS J and Ks photometry for the open star clusters King 14 and NGC 146, and using color magnitude diagrams with isochrones fit we have found an age of log(age) = 7.8 (63 $\pm$8 Myr) for King 14 and log(age) = 7.5 (32 $\pm$8 Myr) for NGC 146. Our age determination is bigger than given in Lynga [5] (log(age) = 7.2 for King 14 and log(age) = 7.1 for NGC 146) and less than Dias [6] (log(age) = 7.9 for King 14 and log(age) = 7.8 for NGC 146)

astro-ph↗
arXiv2008

Hydrodynamics of a perturbation in the jet of 3C111

We present results on the modelling of the ejection of a superluminal component in the jet of 3C111. We propose that the component is generated by an injection of dense material followed by a decrease in the injection rate of bulk particles in the jet. Our model is supported by 1D relativistic hydrodynamics and emission simulations, and is capable of reproducing the brightness evolution of two features, as revealed by 15 GHz VLBA observations. We show that other scenarios, such as an increase of the Lorentz factor in the material of the perturbation, fails to reproduce the observed evolution of this flare.

astro-ph↗
arXiv2008

A Search for Z=-1 Dark Matter Annihilation Products in Cosmic Rays with AMS-01

The majority of mass in the universe has not been observed optically and is termed dark matter. The supersymmetric neutralino provides an interesting dark matter candidate, which may self-annihilate in our galaxy, producing particles visible in the cosmic ray spectrum. During a ten day space shuttle flight, the AMS-01 detector recorded over 100 million cosmic ray events. This analysis searches for the products of neutralino annihilation in the AMS-01 Z=-1 spectrum, and uses the results to place limits on which supersymmetric and dark matter halo distribution models are compatible.

astro-ph↗
arXiv2009

Galactic Halos Derived from LambdaCDM Cosmology Simulation and their Red-Shift Evolution

Galaxies can form in a sufficiently deep gravitational potential so that efficient gas cooling occurs. We estimate that such potential is provided by a halo of mass $M \gtsim M_{c} \approx 7.0 \times 10^{12} ~ (Δ_{c}(z) (1+z)^{3})^{-1/2} \Msun$, where $Δ_{c}(z)$ is the mean overdensity of spherically virialized objects formed at redshift $z$, and $M_{c} \approx 4.0 \times 10^{11} \Msun$ at $z = 0$. Based on this criterion, our galaxy samples are constructed from cosmology simulation data by using HiFOF to select subhalos in those FOF halos that are more massive than $M_{c}$. There are far more dark subhalos than galaxy-hosting subhalos. Several tests against observations have been performed to examine our galaxy samples, including the differential galaxy mass functions, the galaxy space density, the projected two point correlation functions (CF), the HODs, and the kinematic pair fractions. These tests show good agreements. Based on the consistency with observations, our galaxy sample is believed to correctly represent galaxies in real universe, and can be used to study other unexplored galaxy properties.

astro-ph↗
arXiv2009

Special relativity may account for the spacecraft flyby anomalies

The empirical formula proposed by J. D. Anderson et al. [1] to reproduce the data on the Earth flyby anomalies is derived from special relativity (SR). The transverse Doppler shift together with the addition of velocities account for the Doppler data. Time dilation together with the addition of velocities account for the ranging data.

gr-qc↗
arXiv2008

Determining the WIMP mass from a single direct detection experiment

The energy spectrum of nuclear recoils in Weakly Interacting Massive Particle (WIMP) direct detection experiments depends on the underlying WIMP mass (strongly for light WIMPs, weakly for heavy WIMPs). We discuss how the accuracy with which the WIMP mass could be determined by a single direct detection experiment depends on the detector configuration and the WIMP properties. In particular we examine the effects of varying the underlying WIMP mass, the detector target nucleus, exposure, energy threshold and maximum energy, the local velocity distribution and the background event rate and spectrum.

astro-ph↗
arXiv2011

Mass Distribution, Rotation Curves and Gravity Theories

{Comparison of mass density profiles of galaxies of varying sizes based on some gravity theories from observed galaxy rotation curves and assessing the need for dark matter.} We present an analysis of the rotation curves of five galaxies of varying galactic radii: NGC6822 (4.8 kpc), Large Magellanic Cloud (9 kpc), The Milky Way (17 kpc), NGC3198 (30 kpc) and UGC9133 (102.5 kpc). The mass and mass density profiles of these galaxies have been computed using the scientific computing s/w package MATLAB taking the already available velocity profiles of the galaxies as the input, and without considering any Dark Matter contribution. We have plotted these profiles after computing them according to three different theories of gravity (and dynamics): Newtonian (black line), Modified Newtonian Dynamics (MoND) (green line) and Vacuum Modified Gravity (red line). We also consider how the profile due to the Newtonian theory would modify if we take into account a small negative value of the Cosmological Constant (5 x 10^-56 cm^-2 from theory) (blue line). Comparing these mass and mass density profiles, we try to form an idea regarding what could be a realistic theory of gravity and whether we need Dark Matter to explain the results. Keywords : disk galaxy rotation curves, galaxy mass, mass density profile, dark matter, Newtonian theory, MoND, Vacuum Modified Gravity, negative cosmological constant

astro-ph↗
arXiv2008

Convective hydrocodes for radial stellar pulsation. Physical and numerical formulation

In this paper we describe our convective hydrocodes for radial stellar pulsation. We adopt the Kuhfuss (1986) model of convection, reformulated for the use in stellar pulsation hydrocodes. Physical as well as numerical assumptions of the code are described in detail. Described tests show, that our models are numerically robust and reproduce basic observational constraints. We discuss the effects of different treatment of some quantities in other pulsation hydrocodes. Our most important finding concerns the treatment of the turbulent source function in convectively stable regions. In our code we allow for negative values of source function in convectively stable zones, which reflects negative buoyancy. However, some authors restrict the source term to non-negative values. We show that this assumption leads to very high turbulent energies in convectively stable regions. The effect looks like overshooting, but it is not, because turbulence is generated by pulsations. Also, turbulent elements do not carry kinetic nor thermal energy, into convectively stable layers. The range of this artificial overshooting (as we shall call it) is as large as 6 local pressure scale heights, leading to unphysical internal damping through the eddy-viscous forces, in deep, convectively stable parts of the star.

astro-ph↗
arXiv2008

Double-mode classical Cepheid models - revisited

For many years modeling of double-mode pulsation of classical pulsators was a challenging problem. Inclusion of turbulent convection into pulsation hydrocodes finally led to stable double-mode models. However, it was never analysed, which factor of turbulent convection is crucial. We show that the double-mode behaviour displayed in the computed models results from incorrect assumptions adopted in some of the pulsation hydrocodes, namely from the neglect of buoyant forces in convectively stable layers. This leads to significant turbulent energies and consequently to strong eddy-viscous damping in deep, convectively stable layers of the model. Resulting differential reduction of fundamental and first overtone amplitudes favours the occurrence of double-mode pulsation. Once buoyant forces in convectively stable regions are taken into account (as they should), no stable double-mode behaviour is found. The problem of modeling double-mode behaviour of classical pulsators remains open.

astro-ph↗
arXiv2008

Energy-Momentum of the Gravitational Field: Crucial Point for Gravitation Physics and Cosmology

A history of the problem of mathematical and physical definition for the energy-momentum of the gravity field is reviewed. As it was noted 90 years ago by Hilbert (1917), Einstein (1918), Schrodinger (1918) and Bauer (1918) within Geometrical Gravity approach (General Relativity) there is no tensor characteristics of the energy-momentum for the gravity field. Landau & Lifshitz (1971) called this quantity pseudo-tensor of energy-momentum and noted that Einstein's equations does not express the energy conservation for matter plus gravity field. This has crucial consequences for gravity physics and cosmology, such as negative energy density for static gravity field and violation of energy conservation in expanding space. However there is alternative Field Gravity approach for description of gravitation as a symmetric tensor field in Minkowski space, which is similar to description of all other physical interactions and based on well-defined positive, localizable energy-momentum of the gravity field. This relativistic quantum Field Gravity approach was partially developed by Firz & Pauli (1939), Birkhoff (1944), Thirring (1961), Kalman (1961), Feynman (1963) and others. Here it is shown that existence of well-defined positive energy-momentum of the gravity field leads to radical changes in gravity physics and cosmology, including such new possibilities as two-component nature of gravity - attraction (spin 2) and repulsion (spin 0), absence of black holes and singularities, scalar gravitational radiation caused by spherically symmetric gravitational collapse.

gr-qc↗
arXiv2008

New Possibilities for Observational Distinction Between Geometrical and Field Gravity Theories

Crucial observational tests of gravity physics are reviewed. Such tests are able to clarify the key question on the nature of gravitational interaction: is gravity the curvature of space? or is gravity a matter field in Minkowski flat space as other physical forces? Up to now all actually performed experiments do not allow to distinguish between these two alternatives in gravity physics. The existence of well-defined positive energy-momentum of the gravity field in Poincare-Feynman approach leads to radical changes in gravity physics and cosmology which may be tested by laboratory experiments and astrophysical observations. New possibilities for observational distinction between geometrical general relativity and field gravity theories are discussed. Among them: the contribution of the scalar repulsive force into Newtonian gravitational interaction, post-Newtonian translational motion of rotating bodies, gravitational deflection of light by small mass bodies, scalar gravitational radiation from spherically pulsating stars, existence of limiting radius, surface, magnetic field for massive bodies and absence of singularities and horizons for relativistic compact objects.

gr-qc↗