Stability of interplay motions

A family of rectilinear periodic solutions of the three-body problem, in which the central body collides alternately with each of the two other bodies, is investigated numerically for all values of the three masses. It is found that for every mass combination there exists just one solution of this kind. The linear stability of the orbits with respect to arbitrary three-dimensional perturbations is also investigated. Domains of stability and instability are displayed in a triangular mass diagram. Their boundaries form one-parameter families of critical orbits, which are tabulated. Limiting cases where one or two masses vanish are studied in detail. The domains of stability cover nearly one half of the total area in the mass diagram: this reinforces the conclusion that real triple stars might have motions of a kind entirely different from the usual hierarchical arrangement.     

On the Structure of Symplectic Mappings. The Fast Lyapunov Indicator: a Very Sensitive Tool

It is already known (Froeschlé, Lega and Gonczi, 1997) that the Fast Lyapunov Indicator (FLI), that is the computation on a relatively short time of the largest Lyapunov indicator, allows to discriminate between ordered and weak chaotic motion. We have found that, under certain conditions, the FLI also discriminates between resonant and non-resonant orbits, not only for two-dimensional symplectic mappings but also for higher dimensional ones. Using this indicator, we present an example of the Arnold web detection for four and six-dimensional symplectic maps. We show that this method allows to detect the global transition of the system from an exponentially stable Nekhoroshevs like regime to the diffusive Chirikovs one.     

The Universe behind the Milky Way

Summary. Due to the foreground extinction of the Milky Way, galaxies appear increasingly fainter the closer they lie to the Galactic Equator, creating a “zone of avoidance” of about 25% in the distribution of optically visible galaxies. A “whole-sky” map of galaxies is essential, however, for understanding the dynamics in our local Universe, in particular the peculiar velocity of the Local Group with respect to the Cosmic Microwave Background and velocity flow fields such as in the Great Attractor region. Various dynamically important structures behind the Milky Way have only recently been made “visible” through dedicated deep surveys at various wavelengths. The wide range of observational searches (optical, near infrared, far infrared, radio and X-ray) for galaxies in the Zone of Avoidance are reviewed, including a discussion on the limitations and selection effects of these partly complementary approaches. The uncovered and suspected large-scale structures are summarized. Reconstruction methods of the density field in the Zone of Avoidance are described and the resulting predictions compared with observational evidence. The comparison between reconstructed density fields and the observed galaxy distribution allow derivations of the density and biasing parameters and b. Received 4 April 2000 / Published online 18 July 2000     

Massive cosmic strings in Bianchi type II

We study a massive cosmic strings with BII symmetries cosmological models in two contexts. The first of them is the standard one with a barotropic equation of state. In the second one we explore the possibility of taking into account variable “constants” (G and Λ). Both models are studied under the self-similar hypothesis. We put special emphasis in calculating the numerical values for the equations of state. We find that for ω∈(0,1], G, is a growing time function while Λ, behaves as positive decreasing time function. If ω=0, both “constants”, G and Λ, behave as true constants.     

On the Temporal Variability of the “Strahl” and Its Relationship with Solar Wind Characteristics: STEREO SWEA Observations

The “strahl” is a specific population of the solar wind, constituted by strongly field aligned electrons flowing away from the Sun, with energies >60 eV. Using the Solar Wind Electron Analyzer (SWEA) onboard STEREO, we investigate the short time scale fluctuations of this population. It is shown that its phase space density (PSD) at times presents fluctuations larger than 50% at scales of the order of minutes and less. The fluctuations are particularly strong for periods of a few tens of hours in high-speed streams, following the crossing of the corotating interaction region, when the strahl is also the most collimated in pitch angle. The amplitude of the fluctuations tends to decrease in conjunction with a broadening in pitch angle. Generally, the strongly fluctuating strahl is observed when the magnetic field is also highly perturbed. That SWEA is able to perform a very rapid 3D analysis at a given energy is essential since it can be demonstrated that the observed magnetic turbulence can only marginally perturb the PSD measurements.     

Unveiling the chemistry of hot protostellar cores with ALMA

High angular resolution mm-wave observations of the Orion-KL region, made with the IRAM Plateau de Bure interferometer (PdBI), reveal the presence of several cores of size 10³ AU, which have distinct spectral signatures. Complex molecules such as ethanol, vinyl cyanide and dimethyl ether show different distributions and their relative abundance varies from core to core by orders of magnitude. The molecular column densities derived in the cores also differ widely from the beam-averaged column densities observed with large single-dish telescopes. Obviously, the predictions of hot core chemistry models must be checked against high resolution observations. ALMA, which allies sensitivity and high angular resolution, will be a key instrument for this type of studies. The PdBI observations were part of a search for interstellar glycine, also carried out with the IRAM 30-m telescope and the Green Bank Telescope. We derive a 3σ upper limit on the column density of glycine of 1×10¹⁵ cm⁻² per 2″×3″ beam in the Orion Hot Core and Compact Ridge. Based on observations made with the IRAM PdB Interferometer, the IRAM 30-m telescope and the NRAO Green-Bank telescope. IRAM is supported by CNRS, MPG and IGN.     

Observations of the OH Radical in Comet C/1995 O1 (Hale-Bopp) with the Nançay Radio Telescope

We present OH 18-cm observations of comet Hale-Bopp (C/1995 O1) at the Nançay radio telescope. On nucleus and offset position observations allowed us to obtain both OH production rates and quenching radii. The maximum OH production rate was reached around perihelion, at about10³¹ s^-1.     

Solar radio pulsating structures during September 9, 2001 at meter wavelength

Pulsating structures recorded at 237 MHz that are associated to decimetric continuum enhancement during the September 9, 2001 solar radio burst are described. We analyzed the radiopolarimetric data recorded at the Trieste Solar Radio System (INAF—Trieste Astronomical Observatory—Basovizza Observing Station) with very high time resolution (1 ms) at metric frequencies. Two different types of pulsations that occur in about 4 minutes at the same frequency are described. The possible mechanisms are analyzed and some parameters of the associated magnetic structure are estimated.     

A new model for QPOs in accreting black holes: application to the microquasar GRS 1915+105

In this paper we extend the idea suggested previously by Pétri (Astron. Astrophys. 439:L27, 2005a; 443:777, 2005b) (papers I and II) that the high frequency quasi-periodic oscillations (HF-QPOs) observed in low-mass X-ray binaries (LMXBs) may be explained as a resonant oscillation of the accretion disk with a rotating asymmetric background (gravitational or magnetic) field imposed by the compact object. Here, we apply this general idea to black hole binaries. It is assumed that a test particle experiences a similar parametric resonance mechanism such as the one described in paper I and II but now the resonance is induced by the interaction between a spiral density wave in the accretion disk, excited close to the innermost stable circular orbit, and vertical epicyclic oscillations. We use the Kerr spacetime geometry to deduce the characteristic frequencies of this test particle. The response of the test particle is maximal when the frequency ratio of the two strongest resonances is equal to 3:2 as observed in black hole candidates. Finally, applying our model to the microquasar GRS 1915+105, we reproduce the correct value of several HF-QPOs. Indeed the presence of the 168/113/56/42/28 Hz features in the power spectrum time analysis is predicted. Moreover, based only on the two HF-QPO frequencies, our model is able to constrain the mass M _BH and angular momentum a _BH of the accreting black hole. We show the relation between M _BH and a _BH for several black hole binaries. For instance, assuming a black hole weakly or mildly rotating, i.e. a _BH≤0.5?G? M _BH/c ², we find that for GRS 1915+105 its mass satisfies 13?M _⊙≤M _BH≤20?M _⊙. The same model applied to two other well-known BHCs gives for GRO J1655-40 a mass 5?M _⊙≤M _BH≤7?M _⊙ and for XTE J1550-564 a mass 8?M _⊙≤M _BH≤11?M _⊙. This is consistent with other independent estimations of the black hole mass. Finally for H1743-322, we found the following bounds, 9?M _⊙≤M _BH≤13?M _⊙.