Showing papers in "Astronomy and Astrophysics in 1988"

Secular evolution of the solar system over 10 million years

Abstract: Construction de l'evolution seculaire du systeme solaire en fonction de la masse, de l'excentricite et de l'inclinaison des orbites en tenant compte des effets lunaires et relativistes. Le systeme seculaire est integre sur 30 millions d'annees, puis analyse, par transformee de Fourier, pour obtenir les parametres quasi periodiques des orbites planetaires

A bump in the ultra-high energy cosmic ray spectrum

Abstract: Interpretation physique du spectre du rayonnement provoque par des protons de tres haute energie. L'interaction avec le rayonnement de fond cosmologique a 2,7 K K laisse sa marque dans la signature spectrale sous la forme d'une bosse due a la production d'une paire electron-positron, d'un creux de production de paire, d'une nouvelle bosse due a la production d'un pion par interaction proton-photon et par une coupure du spectre de corps noir

Planetary Theories in rectangular and spherical variables: VSOP87 solution.

Abstract: Les solutions analytiques VSOP (Variation Seculaire des Orbites Planetaires) du mouvement des planetes generalement donnees sous forme elliptique sont ici presentees sous leur forme cartesienne et spherique bien plus maniable. Differentes versions sont accessibles par bande magnetique

The light curves of soft X-ray transients.

Abstract: We compute the light curves of soft X-ray transients in the framework of the mass transfer instability model of Hameury et al. /1/. In this model, the atmosphere of the secondary star is heated by X-rays from the primary and develops an instability when the X-ray flux becomes comparable to the intrinsic stellar flux. We calculate the structure of the illuminated mass losing envelope during the transient event and find that the atmosphere is isothermal down to about 100 Thompson optical depths, as a result of inward radiative heat transport. This strongly reinforces the instability. The accretion rate on to the compact object is then computed using a time-dependent numerical code to describe the accretion disc. We find that the observed light curves are well reproduced if one takes the Shakura-Sunyaev α parameter of the order of unity, the rise times we obtain are of the order of days, and decay times are of the order of weeks.

The CO and N2 abundance in comet P/Halley

Abstract: A preliminary evaluation of the mass 28 amu/e signal observed in the neutral mode of the Giotto neutral gas mass spectrometer (NMS) is presented. At 1000 km from the nucleus we obtain for the CO number density n (CO)/n (H2O)≦0.07. The production rate of CO as a parent molecule directly from the nucleus is thus less than 7% of the H2O production rate. However, CO is also produced from an extended source in the inner coma (R < 20 000 km) and at 20 000 km from the nucleus we obtain for the total equivalent CO production rate 0.05≦Q (CO)/Q (H2O)≦0.15. For N2 an upper limit Q (N2)/Q (H2O)≦0.1 is derived. No parent molecule for the CO could be identified which would be in agreement with the NMS measurements. It is proposed that CO or a very short-lived parent is relased in the coma from cometary dust grains, such as the “CHON” particles.

The dust distribution within the inner coma of comet P/Halley 1982i: encounter by Giotto's impact detectors.

Abstract: Analysis of the data from Giotto’s Dust Impact Detection System experiment (DIDSY) is presented These data represent measurement of the size of dust grains incident on the Giotto dust shield along its trajectory through the coma of comet P/Halley on 1986 March 13/14 First detection occurred at some 287000 km distance from the nucleus on the inbound leg; the majority of the DIDSY subsystems remained operational after closest approach (604 km) yielding the last detection at about 202000 km from the nucleus

The Mark III stellar interferometer

Abstract: The Mark III interferometer is an operational long baseline stellar interferometer on Mt. Wilson with four-possible baseline configurations from 9m NE-SW to 20m N-S. The interferometer was designed to be a highly automated astronomical instrument to measure stellar positions and diameters to a magnitude limit of seven. Initial fringe observations were made in September 1986 with a 12-m N-S baseline. In the following months, semi-automated astrometric and stellar diameter measurements were also made. This paper describes the hardware and software components of the instrument and its operational characteristics. The interferometer has several novel features. One is the use of optimal estimation and control algorithms (e.g. Kalman filters) in the control loops. Another is the ability to operate both as a closed-loop phased interferometer and eventually as an open-loop or absolute coherent interferometer. High thermal stability and mechanical accuracy should permit the instrument to point blind at an astronomical object and maintain optical path equality to within the limits set by the atmosphere. In this absolute interferometric mode of operation, it should be possible to observe faint astronomical objects that are too dim for phase tracking. In theory, measurements of amplitude, group delay, and closure phase will be possible to 14 mag.

Unusual characteristics of electromagnetic waves excited by cometary newborn ions with large perpendicular energies

Abstract: Solution of the linear kinetic dispersion equation shows that cometary new-born ions with large perpendicular energies (large initial angles between the solar wind velocity and the IMF) can excite a wave mode with rest frame frequencies of the order of the heavy ion cyclotron frequency which exhibits maximum growth slightly away (6°–15°) from parallel propagation, together with high mass density compression ratios and almost linear polarization. The association of these properties is relevant to space observations of MHD-like waves at comets and in planetary foreshocks and has not been reported in the vast literature on possible instability mechanisms. Dilute drifting oxygen ion and photoelectron rings with finite thermal spreads model the free energy source in the solar wind frame. For parallel propagation, the mode of interest is left-hand circularly polarized, moves in the direction of the newborn particles, and is predominantly fed by the perpendicular energy of the heavy ions; as obliquity increases, its polarization and compression change rapidly. Modification of the free energy source parameters (ring density, drift velocity, perpendicular speed and thermal spread) determines their relative influence, defines the domain of existence of the mode, and shows that it might evolve into a nonoscillatory, purely growing structure distinct from the mirror wave. The results are also applicable to non-cometary environments.

Dust in Comet p/ Halley from VEGA Observations

Abstract: Direct measurements of the dust particle spatial and mass distributions in comet Halley have been carried out on Vega spacecraft over the mass range ~10−16−10−6g with the SP-2 dust particle detector consisting of a set of acoustic and impact plasma sensors. These measurements have revealed a large-scale, time-variable structure of the dust coma, the position and characteristics of its boundaries, and a strongly pronounced angular directivity of dust emission from the cometary nucleus. An unexpectedly high concentration of tiny dust particles with masses below 10 −4 g and strong systematic variations of the shape of the particle mass distribution as a function of distance to the cometary nucleus have been observed. The mean dust production rate by the nucleus has been estimated at (10–13)106 g s−1 (Vega 1) and (5–7) 106 g s −1 (Vega 2).

Radiation formation of a non-volatile comet crust

Abstract: Ion irradiation of the outer meters of a cometary surface can produce new molecular species in the solid state. Because of the interfaces with the interplanetary vacuum these species segregate in an irreversible way into a nonvolatile residue and new very volatile species. The latter are ejected directly or lost when the comet enters the inner solar system. Therefore, a comet exposed to background particle radiations in the Oort cloud obtains an outer web of non-volatile material (~102 g cm-2) which will lead to the formation of a substantial “crust”. When a new comet enters the inner solar system there will be early activity, initial fizzures in the crust and the break-off of unstable pieces of the crust, due to warming of subsurface species. If this comet enters a periodic orbit in the inner solar system the remaining mantle should be continuously hardened due, primarily, to thermal processing. There will also be permanently active regions on such a comet which were initially shaded from the cosmic ray radiation when the comet was in the Oort cloud or which subsequently lost their crust.

Radio-holographic reflector measurement of the 30-m millimeter radio telescope at 22 GHz with a cosmic signal source

Abstract: Le profil de l'antenne reflectrice du radiotelescope de 30 m de l'IRAM a ete mesure par une methode radio-holographique dont la precision est de 25 μm rms. La methode d'observation est decrite et des cartes en couleurs de la deviation du reflecteur sont presentees

Cometary MHD and chemistry

Abstract: Our magneto-hydrodynamical (MHD) and chemical comet-coma model has been applied to describe and analyze the plasma flow, the magnetic field and the ion abundances in comet P/Halley in a consistent manner. We assume the volatile composition to consist of 80% water and 20% carbon-, nitrogen-, oxygen-, and sulfur-compounds. The radius of the nucleus is 3.36 km. With hemispherical illumination, this is equivalent to the active area on the nucleus during the Giotto encounter. The physics and chemistry of the coma are modeled in great detail, including photoprocesses, gas-phase chemical kinetics, energy balance with a separate electron temperature, multifluid hydrodynamics with a transition to free molecular flow, fast-streaming atomic and molecular hydrogen, counter and cross-streaming of the ionized species relative to the neutral species in the coma-solar wind interaction region with momentum exchange by elastic collisions, mass-loading through ion pick-up, and Lorentz-forces of the advected magnetic field. A comparison of the results is made with the data from the Giotto mission, especially from the HIS ion mass spectrometer. We resolve the contact surface (magnetopause); its position is in agreement with observations. We also find the enhancement of the ion density just outside of the contact surface and agreement for the three groups in the ion mass spectra, particularly for the first group up to 21 amu.

An energy formula for nonlinear force-free magnetic fields

Abstract: On decrit la relation entre l'energie et les integrales d'helicite pour les champs magnetiques de force nulle non lineaire des atmospheres solaires et stellaires

The dynamics of coorbital satellite systems

Abstract: The dynamical behavior of N coorbital satellites moving with the same average mean motion around a primary has been studied both analytically and by numerical integrations for N from 2 to 9 satellites Simplified dynamical equations are used to determine the different stationary configurations and their local stability against infinitesimal perturbations The ring of equally spaced identical satellites is found to be locally unstable for N of six or less, while for N from two to eight there exists another stable compact stationary configuration with separations of 60 deg or less between adjacent satellites For N of seven or more the equally spaced configuration becomes locally stable, and for N of nine or more it is the only stationary configuration The motion becomes chaotic for large-amplitude perturbations The chaotic motion fills a restricted region of phase space whose outer boundary is determined by the maximum velocity curve

A streaming instability interaction between the solar wind and cometary dust

Abstract: Etude des conditions de debut d'une instabilite de courant entre luecoulement de plasma du vent solaire et la poussiere cometaire

The IRAS two-colour diagram as a tool for studying late stages of stellar evolution

Abstract: A partir de donnees photometriques du satellite IRAS reportees sur des diagrammes 2 couleurs, on etudie l'evolution possible des etoiles a enveloppe de la famille des geantes. Cette methode permet de classer ces etoiles en fonction de leur perte de masse et de distinguer si leur enveloppe est riche en oxygene ou en carbone

The dust coma of comet P/Halley: measurements on the Vega-1 and Vega-2 spacecraft

Abstract: The spatial, temporal and mass distributions of coma dust particles for masses > 10−13 g, which we measured with the DUCMA instruments and reported earlier (Simpson et al., 1986a), have been confirmed and extended to reveal additional properties of the dust coma. We have analyzed the inverse square dependence (∝ R −2) of flux with distance R from the nucleus and show that there are well-defined envelope boundaries for the measured range of masses. Regions of enhanced fluxes above these R −2 ‘baselines’ inside these boundaries are identified as dust jets. Of special interest is the giant flux enhancement near closest approach for Vega-1, for which we show that the mass spectrum is similar to the pre-encounter spectra and that there is a smooth rate of rise of intensity over more than four seconds leading into the peak. This is a flux gradient similar to that found for several other much smaller flux enhancements inbound and outbound. The mass dependence of terminal velocities for some jets is consistent with recent predictions, and recurring jets observed by both spacecraft—when traced back to the nucleus—correlate with the active regions predicted by Sekanina and Larson (1986). We have discovered particles with a wide range of masses arriving in clusters or ‘packets’ (i.e., a non-Poisson distribution of dust particles) throughout the regions beyond the envelope boundaries — namely, the fringe regions of the dust coma. These observations suggest the emission from the nucleus of large conglomerates of small particles which gently disintegrate as they travel outward to account for the 10−13 g particles observed beyond the envelope boundaries. Also, dust fluences over the missions have been determined. Finally, we report that both the DUCMA electronics and dust sensors maintained their calibrations and performance throughout the encounters and during the post-encounter interplanetary missions.

Lithium abundances in metal-deficient dwarfs.

Abstract: Les mesures sur 37 naines du champ, ajoutees a celles de la litterature, montrent que les naines extremement pauvres en metal forment une population plus homogene, par rapport a l'abondance en lithium, que les etoiles plus riches en metal

Dust formation in stellar winds. IV. Heteromolecular carbon grain formation and growth.

Abstract: La theorie de la nucleation classique est appliquee a la formation des grains de carbone dans les enveloppes circumstellaires des etoiles carbonees a partir de reactions avec l'acetylene et l'hydrogene

Complex refractive index of grain material deduced from the visible polarimetry of comet P/Halley

Abstract: Visible polarimetry of comet Halley over the phase angle α (sun-comet-earth angle) from 1°.6 to 64°.4 has shown negative values of the polarization p in α ≤20° and linearly increase of p with α > 20°. In addition, a slight increase of p with wavelength λ is likely to exist. By applying the Mie calculations to explain both α -and λ-dependences of p, we have found that it’s likely for the cometary dust to consist of the material with the average complex refractive index (m = n − i• k), i.e. (n, k) = (1.392, 0.024) at λ = 0.365 μm, (1.387, 0.031) at λ= 0.484 μm, (1.385, 0.035) at λ= 0.62 μm and (1.383, 0.038) at λ = 0.73 μm, for grains assumed to have the size spectrum reported by the Vega missions.

The radial distribution of galactic gamma rays. IV - The whole galaxy

Abstract: Etude des correlations entre le rayonnement gamma galactique diffus et le gaz interstellaire a partir des donnees COS-B en HI et CO

Grain growth in turbulent protoplanetary accretion disks

Abstract: Un modele d'accretion de grains de poussiere dans un disque protoplanetaire uniforme et turbulent est propose. Des particules de petite taille sont ajoutees continuellement au systeme en rotation et modifient l'evolution du disque d'accretion. Les simulations numeriques effectuees permettent d'etudier les diverses phases de l'accretion, la croissance des particules, l'opacite du systeme et les effets de la convection

Infrared investigation of water in comet P/Halley

Abstract: Gaseous, neutral water was detected in the coma of comet P/Halley during infrared (IR) observations from the NASA Kuiper Airborne Observatory (KAO). High resolution spectra revealed intense, solar-pumped IR fluorescent emission in the (001–000) band of H2O near λ ~ 2.65 µm. Three lines of the (011–010) “hot” band were also observed. In this paper, we summarize the results of our KAO H2O investigation. Besides detecting H2O and measuring its absolute production rate, we measured its spatial distribution, temporal variability, outflow velocity, and have determined the H2O ortho/para ratio. The measured line intensities imply that water in the coma is rotationally relaxed, as predicted by recent nonthermal equilibrium models. However, significant differences in excitation are detected between a fluorescence equilibrium model and the observed spectra, and these differences may yield important information on the coma kinetic temperature. We find that the aperture-averaged excitation temperatures for the low-lying rotational transitions are ~ 40 K, indicating that the coma kinetic temperature is extremely cold. Excitation differences are also detected between the pre- and post-perihelion data, and between nucleus-centered and offset exposures. Swings effects may explain some of the differences. We have detected significant short-term temporal variations in the H2O line intensities, and find a strong pre- to post-perihelion asymmetry (a factor of ~ 7) in the water production rate at a heliocentric distance of ~ 1 AU. Finally, we comment on possible future directions for high resolution IR spectroscopy of comets.

Ion temperature and flow profiles in comet P/Halley's close environment

Abstract: The HIS instrument of the ion mass spectrometer on board the Giotto spacecraft identified the contact surface at 4800 km distance from the comet nucleus. This boundary is clearly seen by a drastic drop in the temperatures of different ion species from about 2000 K outside to values as low as 300 K inside. Also, inside the contact surface an outflow speed of > 1 km s −1 was measured, in contrast to a value around zero right outside. We discuss how these numbers might be affected by a potential charge-up of the spacecraft. Outside the contact surface, the ion temperature rises gradually with increasing distance. Between 9000 and 10 000 km distance the ion density increases by a factor of 4. This “ion pile-up” is not yet explained uniquely. Between 25 000 and 28 000 km distance there is again a rather abrupt jump to significantly higher temperatures and higher outflow speeds.

The Multi-Resolution CLEAN and its application to the short-spacing problem in interferometry

Abstract: Modification de l'algorithme CLEAN utilise en interferometrie pour l'adapter a l'etude des sources etendues. On tient compte de parametres optimises pris a differentes resolutions pour mettre au point cette technique de deconvolution. On teste l'algorithme en l'appliquant a differentes sources theoriques modelisees