catégorie de Physique


This paper describes the numerical evolution of an N-body system with a slight “temperature inversion”; i.e. the maximum velocity dispersion occurs not at the centre but further out. Fluid models predict that the core of such a system expands on a time-scale of thousands of central relaxation times, and here this behaviour is qualitatively confirmed for an N-body system of over 3000 bodies

publié le 20-11-2019 - 164 vues
Pre-main sequence lithium burning I. Weak T Tauri stars ⋆

We report high-resolution spectroscopic observations for a sample of 38 T Tauri stars (TTS), complemented by UBVRI photometry of 13 TTS, and CCD VRI photometry for 2 visual binaries. Based on these observations and data taken from the literature, we derive lithium abundances in 53 TTS, concentrating on weak-line TTS (WTTS). The sample spans the range in spectral types from K0-M3, aproximately cor- responding to masses between 1.2 and 0.2 M⊙.

publié le 20-11-2019 - 136 vues
The Quantum Propagator for a Nonrelativistic Particle in the Vicinity of a Time Machine

We study the propagator of a non-relativistic, non-interacting particle in any non-relativistic “time-machine” spacetime of the type shown in Fig. 1: an external, flat spacetime in which two spatial regions

publié le 20-11-2019 - 143 vues
Exact superstring motivated cosmological models

We present a number of new, exact scalar field cosmologies where the potential con- sists of two or more exponential terms. Such potentials are motivated by supergravity or superstring models formulated in higher dimensional spacetimes that have been compactified to (3+1)-dimensions.

publié le 20-11-2019 - 119 vues
On the mechanism of trapped surface formation

By a combination of analytical and numerical methods, the density pro le of a momentarily at rest spherical star is varied, and the corresponding response in the area of the spherical shells is monitored. It is shown that the inner apparent horizon (if it exists) must lie within or at most on the star's surface, while no such restriction is found for the outer apparent horizon. However, an apparent horizon lying in the vacuum region will always have non vanishing area, as long as the ADM mass of the system is non zero. Furthermore for density pro les not decreasing inwards, it appears that all spherical trapped surfaces lie on a thick spherical shell. Finally for a uniform density star a simple criterion is found, relating density and proper radius that guarantees the presence or absence of trapped regions

publié le 20-11-2019 - 155 vues
Fate of Inhomogeneity in Schwarzschild-deSitter Space-time

The inflationary scenario is a favorable model to explain the homogeneity and isotropy of the present universe. In this scenario, vacuum energy of the matter field plays a role of the cosmological constant, and the universe enters the phase of deSitter expansion. Initial inhomogeneity of the universe dumps due to the rapid cosmological expansion. To utilize these aspects of inflation,

publié le 20-11-2019 - 164 vues
The Updated MSW Analysis and the Standard Solar Model Uncertainties

We update the analysis of the MSW and general astrophysical solutions to the combined solar neutrino observations by including the GALLEX II result. We also show that our parametrized flux uncertainties are equivalent to the Monte-Carlo results of Bahcall and Ulrich.

publié le 20-11-2019 - 183 vues

The possible solution of dark matter problem with neutralinos of supersymmetric models within the supergravity framework is reviewed. A novel correlation between the neutralino relic abundance Ω χ and the soft supersymmetry breaking patterns is demonstrated. It is explained that, this generic result together with the proton-decay constraint could significantly reduce the allowed parameter space of the minimal SU(5) supergravity model, and therefore makes this model more easily testable. The prospect of obtaining further cosmological constraints from underground experiments for the minimal SU(5) supergravity model is also briefly discussed.

publié le 20-11-2019 - 166 vues
QED corrections to Planck’s radiation law and photon thermodynamics

Leading corrections to Planck’s formula and photon thermodynamics aris- ing from the pair-mediated photon-photon interaction are calculated. This interaction is attractive and causes an increase in occupation number for all modes. Possible consequences, including the role of the cosmic photon gas in structure formation, are considered.

publié le 20-11-2019 - 136 vues

Stars are gravitationally stabilized fusion reactors changing their chemical composition while trans- forming light atomic nuclei into heavy ones. The atomic nuclei are supposed to be in thermal equilibrium with the ambient plasma.

publié le 20-11-2019 - 119 vues
Clustering in the 1.2 Jy IRAS Galaxy Redshift Survey I: The Redshift and Real Space Correlation Func

Strictly speaking, the process of 2 minimization is valid only if the errors of the correlation function at a xed separation are Gaussian distributed and if the values at di erent separations are uncorrelated. The power law model is usually t to the data for r <  20 h

publié le 20-11-2019 - 141 vues

Usually inflation ends either by a slow rolling of the inflaton field, which gradually becomes faster and faster, or by a first-order phase transition. We describe a model where inflation ends in a different way, due to a very rapid rolling (‘waterfall’) of a scalar field σ triggered by another scalar field φ. This model looks as a hybrid of chaotic inflation with V ( φ) = m 2 φ 2 2 and the usual theory with spontaneous symmetry breaking with V ( σ) = 14λ ( M 2 − λσ 2 ) 2 . The last stages of inflation in this model are supported not by the inflaton potential V ( φ) but by the ‘non-inflationary’ potential V ( σ).

publié le 20-11-2019 - 163 vues

We present a simple, closed form expression for the potential of an axisymmetric disk of stars interacting through gravitational potentials of the form V ( r) = −β/r + γr/2, the potential associated with fundamental sources in the conformal invariant fourth order theory of gravity which has recently been advanced by Mannheim and Kazanas as a candidate alternative to the standard second order Einstein theory. Using the model we obtain a reasonable fit to some representative galactic rotation curve data without the need for any non-luminous or dark matter. Our study suggests that the observed flatness of rotation curves might only be an intermediate phenomenon rather than an asymptotic one.

publié le 20-11-2019 - 121 vues

In the first paper in this series we presented a typical set of galactic rotation curves associated with the conformal invariant fourth order theory of gravity which has recently been advanced by Mannheim and Kazanas as a candidate alternative to the standard second order Einstein theory.

publié le 20-11-2019 - 131 vues
The Environment of Lyman α Absorbers in the Sightline towards 3C273

We present new ground-based data following up on the HST discovery of low-redshift Lyman α absorption in the sight-line to the quasar 3C273. Our goal is to investigate the relationship between the low-column-density absorbers and higher column-density objects such as galaxies or H II regions. Narrow-band filter observations with a corono- graph show that there are no H II regions or other strong Hα line-emitting gas within a 12 kpc radius of the line-of-sight to the quasar, at the velocities of three of the absorbers. Broad-band imaging in Gunn r shows that there are no dwarf galaxies at Virgo distances with absolute magnitude above M B ≈-13.5 and within a radius of 40 kpc from the line- of-sight to the quasar

publié le 20-11-2019 - 203 vues
The Local Galaxy Density and the Arm Class of Spiral Galaxies

We have examined the effect of the environmental density on the arm classi- fication of an extensive sample of spiral galaxies included in the Nearby Galaxy Catalog (Tully, 1988a). We have also explored the dependence of the arm class of a galaxy on other factors, such as its blue absolute magnitude and its disk- to-total mass ratio, inferred in the literature either from the gradient of a good galaxy rotation curve or from a photometric mass decomposition method.

publié le 20-11-2019 - 232 vues

CBS is a new program for binary system light curve analysis, it generates synthetic light curves for a binary system, accounting for eclipses, tidal distortion, limb darkening, gravity darkening and reflection; it is also possible to compute the light contribution and eclipses of an accretion disk. The bolometric light curve is generated, as well as curves for the U,B,V,R,I colour bands. In the following we give a brief description of the first version of the program and show some preliminary results.

publié le 20-11-2019 - 123 vues

We show how to use 21-cm emission and absorption studies to estimate the heat inputs to the neutral gas in low pressure environments, such as in outer disks of spiral galaxies, in galactic halos or in intergalactic space

publié le 20-11-2019 - 202 vues

Observations indicate that some extended outer disks have a sharp cut off in the surface density of neutral hydrogen when this approaches the value of ∼ 2 × 1019 cm−2 . In this paper we model these HI edges as places where the ratio of neutral to ionized hydrogen drops rapidly due to ionizing radiation.

publié le 20-11-2019 - 115 vues
Extremely High Velocity Outflows

Extremely high velocity (EHV) wings, with full widths of 72 to 140 km s−1 , are seen on the CO J = 3→2 lines toward W3 IRS 5, GL 490, NGC 2071, W28 A2 (G05.89–0.39), GL 2591, S140, and Cepheus A. Observations of 12CO and 13CO J = 3→2 and J = 2→1 lines indicate that optical depth generally decreases with increasing velocity separation from the ambient cloud velocity. Maps of the extremely high velocity (|V −V0| >∼ 20 km s−1 ) and the high-velocity (5 <∼ |V − V0| <∼ 20 km s−1 ) CO emission components show that the morphology of the two components is similar in W3 IRS 5 and W28 A2 but may be different in GL 2591, S140, and Cepheus A.

publié le 20-11-2019 - 120 vues

Physics beyond the simplest version of the standard electroweak model is re- quired to reconcile the results of the chlorine and the Kamiokande solar neutrino experiments.

publié le 20-11-2019 - 132 vues

A brief introduction to the gauge invariant classical and quantum theory of cosmological perturbations is given. The formalism is applied to inflationary Universe models and yields a consistent and unified description of the generation and evolution of fluctuations. A general formula for the amplitude of cosmological perturbations in inflationary cosmology is derived.

publié le 20-11-2019 - 125 vues

Early photoionization of the intergalactic medium is discussed in a nearly model-independent way, in order to investigate whether early structures corresponding to rare Gaussian peaks in a CDM model can photoionize the intergalactic medium sufficiently early to apprecia- bly smooth out the microwave background fluctuations.

publié le 20-11-2019 - 122 vues

One still cannot conclusively assert that the universe underwent a neutral phase, despite the new COBE FIRAS limit y < 2.5 × 10−5 on Compton y-distortions of the cosmic microwave background. Although scenarios where the very early (z ∼ 1000) ionization is thermal (caused by IGM temperatures exceeding 104K) are clearly ruled out, there is a significant loophole for cosmologies with typical CDM parameters if the dominant ionization mechanism is photoionization. If the ion- izing radiation has a typical quasar spectrum, then the y-constraint implies roughly h 4/3ΩigmΩ −0.28 0 < 0.06 for fully ionized models. This means that BDM models with Ω0 ≈ 0.15 and reionization at z ≈ 1000 are strongly constrained even in this very conservative case, and can survive the y test only if most of the baryons form BDM around the reionization epoch.

publié le 20-11-2019 - 139 vues

This paper is in two unequal halves. After dealing with the possibility of a genuine continuum background at λ >∼ 1 cm, and showing that it is unlikely to arise in interesting circumstances, the remainder of the discussion is devoted to discrete radio sources, and their consequences for cosmology.

publié le 20-11-2019 - 155 vues
The Inflationary Energy Scale

The energy scale of inflation is of much interest, as it suggests the scale of grand unified physics and also governs whether cosmological events such as topological defect forma- tion can occur after inflation. The COBE results are used to limit the energy scale of inflation at around 60 e-foldings from the end of inflation. An exact dynamical treat- ment based on the Hamilton-Jacobi equations is then used to translate this into limits on the energy scale at the end of inflation. General constraints are given, and then tighter constraints based on physically motivated assumptions regarding the allowed forms of density perturbation and gravitational wave spectra. These are also compared with the values of familiar models.

publié le 20-11-2019 - 134 vues
The Infrared Tully-Fisher Relation in the Ursa Major Cluster

We present new magnitudes derived from 1.65 µm images for 23 galax- ies in the Ursa Major cluster. Magnitudes now exist for all but one spiral meeting our criteria for cluster membership and having H i velocity width greater than 187 km s−1 and inclination greater than 45◦ .

publié le 20-11-2019 - 131 vues
An imaging K-band survey — I: The catalogue, star and galaxy counts

We present results from a large area (552 ′ ) imaging K-band survey of faint objects. The survey is a high galactic latitude blank-field sample to a 5 σ limit of K ≃ 17 .3. The methods for constructing the infrared survey are described, including flatfielding, astrometry, mosaicing and photometry

publié le 20-11-2019 - 130 vues