Showing papers in "Astronomy and Astrophysics in 1984"

Identification of the 'unidentified' IR emission features of interstellar dust?

Abstract: We propose a consistent explanation of the so-called unidentified IR emission features of interstellar dust. Following Sellgren (1984) we consider the transient heating of very small grains (∼ 50 atoms) to a peak temperature of ∼ 1000 K by the absorption of a single U.V. photon and estimate the subsequent IR emission. We show that graphite is one of the few solids that would not sublime under such conditions.As pointed out by Duley and Williams (1981), due to the large abundance of hydrogen, H atoms are expected to be bound to the periphery of the graphitic planes of such grains. The stretching modes of the resulting CH radicals give an excellent fit to the observed 3.28 μm emission band. Evaluating the rest of the grain emission spectrum, we show that bulk graphite optical constants cannot be used because of an important size effect: the small number of atoms introduces an essential difference in the electronic transitions of the system for energies less than ∼ 1 eV. Those very small grains of carbon should be considered as large polycyclic aromatic moleculea rather than graphite "spheres". The expected emission of a middle size polycyclic aromatic molecule (coronene = C 24 H 12 ) heated to an average temperature of 600 K is computed. The main spectral features are found at 3.3, 6.2, 7.6, 8.8 and 11.9 μm. Although the interstellar species are thought to be a complex mixture of molecules with a higher molecular weight, the agreement with most of the observed emission bands is already impressive (3.28, 6.2, 7.7, 8.6 and 11.3 pm). We conclude that large pre-graphitic molecules heated by a single UV photon are probably the origin of the IR emission features. A fraction of a few percent of the cosmic abundance of carbon involved in these molecules can account for the observed emission flux.

The Water Molecule in Comets: Fluorescence Mechanisms and Thermodynamics of the Inner Coma

Abstract: On essaie d'evaluer l'emission IR et microonde d'eau dans une atmosphere cometaire. Le principal mecanisme d'excitation est l'excitation IR par le rayonnement solaire

Spectral scan of Orion A and IRC+10216 from 72 to 91 GHz

Abstract: The spectra of the Orion KL molecular cloud and the envelope of the carbon star IRC+10216 are surveyed at 1 MHz resolution over the interval 72.2-91.1 GHz to a sensitivity usually better than 0.1 K. Multitransition analysis of several molecular species is presented which gives information on the physical conditions and chemical abundance in the Orion KL region. The excitation conditions and relative abundances in the IRC+10216 envelope are surveyed. It is stressed that the low detection rate of molecular lines in this object does not necessarily imply a poorer chemistry compared with that in interstellar clouds, but is partly a reflection of low abundances and unsatisfied excitation requirements.

Numerical integration with Lie-series

Abstract: Le but de ce travail est la mise au point d'une methode d'integration rapide des equations differentielles, particulierement les equations du mouvement des corps celestes. On presente un schema d'integration numerique du systeme classique a n corps. On a pu simplifier le calcul des termes de Lie et obtenir finalement une formule de recurrence pour les termes de Lie

Supernova explosions of massive stars - The mass range 8 to 10 solar masses

Abstract: Etude de l'evolution finale d'une etoile avec un cœur initial d'helium de masse 2,2 M solaire, correspondant a une masse de la sequence principale d'environ 9 M solaire, au travers d'un effondrement du cœur et une propagation d'onde de choc

Model Calculations of Magnetic Flux Tubes - Part Two - Stationary Results for Solar Magnetic Elements

Abstract: Modeles des elements du flux magnetique solaire, construits sur la structure de base du champ magnetique photospherique

Type III radio bursts in the interplanetary medium - The role of propagation

Abstract: Interplanetary type III radio burst observations are analyzed in order to ascertain the role played by propagation effects between the true source and the observer. Large source altitudes are noted, together with an increasing angular size of sources with increasing angular distance from the sun's center. These and other observations furnish strong evidence for the theory that propagation effects, group delays, ducting and/or scattering significantly affect the observed heights, sizes, and brightness temperatures of interplanetary type III bursts. This would be true irrespective of whether the bursts are due to plasma radiation at the fundamental or at the harmonic, and the effects would extend to the arrival times of the radiation to a greater or lesser extent, depending on the path from the source to the observer.

Statistical equilibrium of lithium in cool stars of different metallicity

Abstract: Etude des effets non ETL sur les raies de resonance Li II du soleil, d'une supergeante K3 Ib, de naines du halo et d'une geante du halo

Type II bursts, shock waves, and coronal transients - The event of 1980 June 29, 0233 UT

Abstract: The metric Type-II solar burst event of June 29, 1980, is characterized on the basis of spatially resolved radioheliograph observations obtained at Culgoora, Australia, and visible-light observations obtained with the coronograph/polarimeter of the SMM satellite. The data are presented in images, diagrams, and graphs and discussed in detail. The Type-II emission is found to arise in the dense moving material behind the transient loops, which have sky-plane width 0.5 solar radius and line-of-sight depth 0.1-0.4 solar radius. A faint arc observed moving ahead of the transient loops at about 900 km/sec and not associated with the Type-II burst is attributed to a shock front, and the compression ratio and Alfven Mach number of the enhanced-density region are estimated as n2/n1 = 1.3-3 and M(A) = 1.2-3. The ambient material at 3 solar radii is determined to have Alfven speed 250-625 km/sec and magnetic-field strength 50-120 mG. The total mass of the event is calculated as 700 Tg; the total magnetic energy of the loops is (1.5-15) x 10 to the 29th ergs.

Slow scintillation in the interstellar medium.

Abstract: On montre que le lent affaiblissement des pulsars sur des echelles de temps des jours aux mois, est du aux scintillations interstellaires (SIS) Les fluctuations de la densite electronique qui provoquent l'affaiblissement font partie du meme spectre en loi de puissance qui produit les SIS normales (des minutes aux heures) On suppose que d'autres variations lentes de l'intensite des sources peuvent egalement etre dues au milieu interstellaire

The CH radical in diffuse interstellar clouds

Abstract: On presente des observations de la raie interstellaire CH A-XR 2 (1) a 4300,3 A pour 29 lignes de visee s'etendant de 140 a 2800 pc dans le plan galactique. La comparaison des densites de colonne de CH et de H 2 montre que N(CH)αN(H 2 ) comme prevu pour une production de CH par un reseau de reactions en phase gazeuse. Le reseau reproduit egalement le rapport observe pour N(CH)/N(H 2 ) pour ces nuages interstellaires diffus

A Catalog Of Selected Compact Radio-Sources For The Construction Of An Extragalactic Radio Optical Reference Frame

Abstract: A catalog of 234 strong compact extragalactic radio sources that display optical counterparts is presented. This catalog identifies proposed sources for establishing an almost inertial reference frame against which the motions of the earth, solar system, galactic objects, and spacecraft may be measured. This catalog also defines those sources for which precise optical positions should be determined in order to relate this reference frame with the optical FK 5 fundamental system. The accuracy of the radio source positions of these sources is not greater than 0.1 arcsec with the majority not greater than 0.01 arcsec. Further refinement to not greater than 0.005 arcsec by future observations is expected soon.

Properties of solar magnetic fluxtubes as revealed by Fe I lines

Abstract: On explore l'information contenue dans le spectre solaire pour modeler les tubes de flux magnetique par une analyse des proprietes statistiques des profils de raie de Stokes I et V de 400 raies Fe I non jointes

Type III bursts in interplanetary space - Fundamental or harmonic?

Abstract: ISEE-3 spacecraft observation of 120 relatively simple, isolated bursts in the 30-1980 kHz range are the basis of the present study of Type III bursts in the solar wind. Several characteristics are identified for many of these bursts which imply that the mode of emission changes from predominantly fundamental plasma radiation during the rise phase to predominantly second harmonic during decay. The fundamental emission begins in time coincidence with the start of Langmuir waves, confirming the conventional belief in these waves' causation of Type III bursts. Attention is given to the characteristics of fundamental components, by comparison to harmonics, at km-wavelengths.

Anti-gravity and galaxy rotation curves

Abstract: Une modification de l'attraction gravitationnelle Newtonienne qui apparait dans le contexte des tentatives modernes visant a unifier la gravite avec les autres forces dans la nature peut produire des courbes de rotation pour les galaxies spirales qui sont presque plates de 10 a 100 kpc, peut lier les amas de galaxies et peut fermer l'univers avec la densite de matiere baryonique compatible avec la nucleosynthese originelle

The 3D dynamics of barred galaxies

Abstract: Etude numerique de l'allure et de la stabilite des orbites possibles dans des modeles realistes de galaxies barrees 3 D. On decrit toutes les familles principales d'orbites periodiques; l'influence des familles principales sur les orbites non periodiques est evaluee a l'aide des surfaces de section. On analyse les changements dans la stabilite dus a la variation de la masse et aux rapports axiaux de la barre

Magnetic susceptibility of neutron matter and nuclear effective interactions

Abstract: On utilise les interactions effectives neutron-neutron de type Skyrme pour etudier la susceptibilite magnetique d'un gaz uniforme de neutrons a des densites de l'ordre de celles des etoiles a neutrons stables gravitationnellement. Une transition ferromagnetique entraine une densite d'environ 1,5 fois superieure a la densite d'equilibre de la matiere nucleaire symetrique

Magneto-acoustic-gravity waves on the Sun. I - Exact solution for an oblique magnetic field

Abstract: Propagation d'ondes magneto-acoustiques de gravite dans une atmosphere isotherme conductrice compressible en presence d'un champ magnetique oblique homogene

Star disturbances in gravitational lens galaxies.

Abstract: Etude du dedoublement des images et des variations de flux provoques par une etoile isolee dans une galaxie-lentille gravitationnelle etendue. On trouve que le nombre des sous-images peut etre nul dans certains cas de sorte que l'une des images disparait completement a cause de l'heterogeneite representee par une etoile (ou un amas globulaire). La troisieme image manquante du QSO double 0957+561 A,B pourrait etre attribuee a cet effet

The nature of magnetic activity in lower main sequence stars

Abstract: L'emission chromospherique et coronale d'etoiles de la basse sequence principale simples et binaires est correlee avec un parametre d'activite de la forme R=g(B−V)P −1 , ou P est une periode de rotation et g(B−V) est une fonction de la couleur

The typical interstellar extinction in the Small Magellanic Cloud.

Abstract: On donne une courbe d'extinction UV moyenne pour le PNM qui correspond au cas le plus commun et par consequent a la loi d'extinction la plus probable La courbe est presque lineaire en λ ―1 Elle est compatible avec des grains interstellaires de silicate presque pur et des petits grains de graphite

Model calculations of magnetic flux tubes. I: Equations and method

Abstract: Description des equations, conditions aux limites, geometrie, approximations et methodes numeriques utilisees dans une etude numerique de la physique des tubes de flux magnetiques dans les zones de convection et les atmospheres stellaires