Showing papers by "European Southern Observatory published in 1985"

Nitrogen and oxygen evolution in dwarf irregular galaxies

Abstract: On a calcule des modeles numeriques de l'evolution chimique d'un echantillon de 40 galaxies irregulieres naines en supposant l'existence de sursauts de formation stellaire et en tenant compte des vents galactiques. On a suivi l'evolution de l'azote et de l'oxygene a l'aide des resultats les plus recents concernant l'evolution stellaire

How common are ‘dust-lanes’ in early-type galaxies?

Abstract: La fraction des galaxies contenant de la poussiere a ete evaluee a ∼40% pour les galaxies elliptiques proches et un peu plus pour les galaxies SO. On a observe des enveloppes autour de ∼25% des galaxies elliptiques de l'echantillon etudie, mais il n'y a pas de correlation forte entre la presence d'enveloppe et la presence de poussiere

Simple three-dimensional periodic orbits in a galactic-type potential

Abstract: We study the families of simple periodic orbits in a three-dimensional system that represents the inner parts of a perturbed triaxial galaxy. The perturbations depend on two control parameters. We find the regions where each family is stable, simply unstable, doubly unstable, or complex unstable. the stable and simply unstable families produce other families by bifurcation. Several families reach a maximum (or minimum) perturbation and then are continued by other families. The bifurcations are direct or inverse. The transition from one type of bifurcation to the other is theoretically explained. Another important phenomenon is the splitting of one family into two, or the joining of two families into one. We do not have any complex instability in the limiting cases of two-dimensional motions (when one control parameter is zero).

JHKL properties of emission-line galaxies

Abstract: On donne les couleurs JHKL dans le nouveau systeme photometrique SAAO pour des echantillons de galaxies de Seyfert 1, de Seyfert 2 et de galaxies a regions H II. On compare aux couleurs de galaxies «ordinaires»

The infall velocity toward Virgo, the Hubble constant, and a search for motion toward the microwave background

Abstract: The perturbation of the local velocity field due to the overdensity of the Virgo complex (VC) is investigated theoretically, using an idealized spherically symmetric Virgocentric flow model and testing for a correlation between cos theta (where theta is the angle from the VC direction to eight groups and clusters with velocity 800-2500 km/s) and the relevant velocity residuals. The data and results are presented in extensive tables, graphs, and a map in extragalactic coordinates. The infall of the Local Group (LG) toward the VC is estimated as 200 + or - 50 km/s, corresponding to a mean mass/light ratio M/L(B) = about 70 for a sphere centered at the VC core and extending to the LG. The rate of free expansion within the VC is estimated as 52 + or 2 km/s Mpc for a VC distance of 21.6 Mpc. It is pointed out that the LG infall toward the VC cannot account for observations of an apparent 630-km/s motion of the LG toward the microwave background.

Narrow and variable lines in the ultraviolet spectrum of the Seyfert galaxy NGC4151

Abstract: The Seyfert galaxy NGC4151 observed when the nucleus is at a minimum has two emission lines of full width at half maximum <7 and 16 A respectively and varying in intensity by a factor of three in 10 days. These lines are too narrow to be emitted by the whole broad-line region and must arise instead from two localized regions which have a special excitation mechanism, possibly a two-sided jet.

Two remarkable bright supernova remnants

Abstract: Statistical studies of supernova remnants (SNRs) indicate that the objects of high surface brightness are young1. This is confirmed by measurements of angular expansion for a number of such remnants and by the secular decrease in their radio flux densities2. Studies of these young SNRs are of particular interest as they can impose constraints on models of supernova outbursts and can reveal details of the initial interaction between the rapidly-expanding stellar material and the ambient interstellar medium3. Also, a high percentage of bright remnants belong to the comparatively-rare class of filled-centre SNRs. However, because of their small angular sizes, many have not been observed with sufficient resolution to determine detailed structures or, in some cases, even to verify an SNR classification. Between July 1981 and August 1984, we used the Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO) in its B, C and D configurations to clarify the morphology of several high-brightness sources from the Milne SNR catalogue4. Of these, the two sources G349.7+0.2 and G357.7−0.1 were found to be most unusual.

Detailed observations of NGC 4151 with IUE – IV. Absorption line spectrum and variability

Abstract: Analyse detaillee du spectre d'absorption UV (λλ 1150-3200 A) du noyau de NGC 4151. On a identifie plus de 30 raies d'absorption UV

Radio observations of the young planetary nebula NGC 6302

Abstract: On presente des observations VLA de la nebuleuse bipolaire NGC 6302 a 2,6 et 21 cm, en emission continue, a 2 cm dans la raie 476α et a 21 cm dans la raie HI. La plus grande partie de l'emission continue provient d'un toroide ionise de diametre angulaire exterieur +10 secarc. Les caracteristiques de NGC 6302 et de son etoile centrale sont celles d'une nebuleuse planetaire jeune

Studies of violent star formation in extragalactic systems – I. Population synthesis model for the ionizing clusters of giant H II regions and H II galaxies

Abstract: On presente des modeles de synthese de la population pour des amas ionisants dans des regions HII geantes extragalactiques et de galaxies HII. On suppose que ces amas sont des sursauts uniques de formation stellaire caracterises par une distribution de la masse en loi de puissance. On presente une tabulation detaillee des parametres globaux observables des amas

Optical spectrophotometric study of supernova remnants in the Large Magellanic Cloud.

Abstract: On presente des spectrogrammes des filaments de neuf restes de supernovae brillants et d'une region HII du GNM. On trouve une distribution uniforme de l'abondance de l'helium atomique: Z(He)=N(He)/N(H)=0,085 dans toutes les sources mesurees

A self-consistent box-shaped galaxy

Abstract: On presente un modele a N corps d'une galaxie en forme de boite qui est auto-compatible et en equilibre. Le schema de rotation est cylindrique

Ultraviolet continuum variability of the quasar 3C273

Abstract: Several quasars show an excess of radiation in the ultraviolet over the power law defined by their infrared/optical continuum. Several authors1–4 have fitted the infrared to ultraviolet spectrum of these quasars with a combination of a power law component, line emission, blends of Fe II lines, Balmer continuum and a black-body component which dominates the ultraviolet spectrum. We report here the first observations of variability in the ultraviolet spectrum of the quasar 3C273 (redshift, z = 0.158) as observed by the International Ultraviolet Explorer (IUE)5 in the period 1978–84. The flux at λobserved = 1,675 A increased by 1.25 between April and June 1982, then decreased by a factor 2 between June 1982 and April 1983. The amplitude of these variations and the constancy of the intensity of the Lyα emission line during the same period are indications that the ultraviolet variations are caused by variations of the black body and/or non-thermal components and not by variations of the continuum or blend of lines originating from the broad-line region. The flux variations are accompanied by variations of the spectral shape. We present an interpretation of the observed variability in terms of a discontinuous and variable distribution of the temperature on the photosphere emitting the ultraviolet continuum.

The close QSO pair Q1548+114A, B

Abstract: Observations spectroscopiques IPCS et image CCD des QSO Q1548+114A et B, qui sont separes de 4,8 sec arc et ont des deplacements vers le rouge de 0,44 et 1,90. L'absorption H et K galactique est presente dans les deux spectres. Il s'agit de la paire la plus rapprochee de quasars distincts

Evolution of families of double-and triple-periodic orbits

Abstract: We study the evolution of the families of double-and triple-periodic orbits in a dynamical system that has closed zero velocity curves for arbitrarily large energies. We find three interesting features: (i) the characteristic x=x(h) of the family of double periodic orbits divides the (x,h)-plane into two unconnected parts; (i i) there is a sequence of sixteen closed characteristics, bifurcating from another one, each of them inside the previous one; (iii) inside the innermost characteristic of that sequence there is a sequence of eight pairs of close characteristics which are not connected with any of the previous characteristics.

Panel Discussion on Data Analysis Trends in X-Ray and γ-Ray Astronomy 30/5/84, 11°°–12°°

Abstract: [The text of the panel has been edited by Dr. ozel (with indispensable help from Gabi Breuer, secretary of MPIfR) from a tape recording. The words not completely understandable are noted by (?), while various inclusions for the continuity of the text are indicated by [ ]. The slides and viewgraphs presented in the panel are added as Figures and Tables.]

The gravitational lens effect and the surface density of quasars near foreground galaxies

Abstract: On presente une methode simple pour calculer l'exces de quasars du a l'effet de lentille gravitationnelle, au moyen de comptages nombre-magnitude realistes. Cet exces est, en tres bonne approximation, proportionnel a la masse de la galaxie et pour un echantillon de quasars de magnitude limite egale a 20, il vaut 1,3×10 −13 M H D L −1 quasars par galaxie, ou M H est la masse de la galaxie (halo) en M ○. et D L sa distance en Mpc. On discute du probleme de la conservation de l'energie

Coordinated Optical-Exosat-Tenma Observations of a Burst from 2S1636-536

Abstract: During a coordinated observations of 2S1636-536 with EXOSAT, Tenma and the 1.4 metre Danish telescope at La Silla, a single burst was detected at all three observatories. The burst was bright with a peak flux of 6 x 10-8 ergs/cm2/s and rapid expansion of the blackbody radius. EXOSAT and Tenma agree closely in the values of blackbody radius, temperature and flux which are very similar to the three bursts recently reported by Tenma which appear to reach the Eddington flux limit at their peak. Preliminary analysis of the X-ray and optical timing data reveal that this burst is unusual in having a sharp leading edge in both X-ray and optical data and a very short X-ray-optical delay compared with those previously observed. If confirmed, this result puts significant restraints on the location of the optical reprocessing site.

Dust in elliptical galaxies — how often, how much?

Abstract: We report results showing that dust-lanes are a common feature of nearby elliptical galaxies. About 40% of ellipticals have dust-lanes with diameters of a few kpc and dust masses of 104–106 M⊙. This result has some possible consequences for galaxy surface photometry, since dust-lanes too small to be seen on photographic plates may produce both “twisting” of inner isophotes and off-centre nuclei.

A compilation of UBVRI photometry for galaxies in the ESO/Uppsala catalogue

Abstract: We have compiled a catalogue of all presently available UBVRI aperture photometry for galaxies in the ESO/Uppsala catalogue. It comprises a total of about 1000 galaxies south of -17° declination.

On The Calibration of the uvby Photometric System for Late-Type Stars

Abstract: An attempt has been made to calibrate the indices of the uvby photometric system in terms of MK classes equal to and later than that of the Sun. Results are presented for stars of luminosity classes V and III; b-y and c1data are given. For stars on the main sequence, the relation between m1and b-y is discussed for stars of solar type.

Occultation detection of a Neptune ring segment

Abstract: A stellar occulation event of Neptune revealed a ring segment that was previously undetected. It appears that the object is not a complete ring, but rather a localized swarm of particles which follows a ring orbit over a limited range of longitudes. To avoid confusion with the standard use of the word ring it is suggested that the feature be called an arc. The distance of the arc zone is not precisely known because so far there has been no confirmed occultation by both an arc and the planet.

Formation, Equilibrium and Stability of Jets

Abstract: Consideration of the many observed types of jets on scales ranging from parsecs to megaparsecs seen in radio, optical, infrared and X-ray wavebands with a variety of morphologies both in galactic and extragalactic systems leads to some constraints on their fundamental nature. Jet formation is introduced with the concept of the Laval nozzle and related points include the problem of maintaining the nozzle, Mach disk effects due to under and over-expansion and the potential importance of magnetic confinement and focussing. Current ideas on jet formation at the black hole and accretion disk are given with emphasis on the plasma physics associated with black-hole electrodynamics, thermal and magnetically driven winds and thick disks. Stability of jet propagation is reviewed with emphasis on magnetised and unmagnetised Kelvin-Helmholtz instabilities and the various dominant modes. The particle acceleration physics of shocks, wave-particle interactions and turbulence is summarised while noting some outstanding plasma physics problems. Jet equilibrium associated with the non-linear saturation of instabilities, the formation of cocoons, shock stabilisation and magnetic fields is discussed. Detailed plasma physics studies that could significantly clarify jet physics are indicated.

Detecting supernova remnants in external galaxies

Abstract: Reliable identifications of supernova remnants in external galaxies are slowl becoming available through the combined use of radio, optical, and x-ray surveys. The host galaxies represent a variety of Hubble types. Although the number of SNF detected in each galaxy is currently small, new telescopes and detectors in all three wavelength regimes should allow us to obtain larger samples in the near future.

The stellar content of the A496 cD galaxy

Abstract: J, H, and K radial color distributions obtained for the galaxy A496 cD indicate that while the halo becomes bluer at increasing radii in the B-V, it becomes redder in the visual-IR colors. The combined visual-IR data lead to a unique model for both the main sequence and giant stars in which the main sequence is characterized by a radial increase of an upper mass cutoff, and the giant population is dominated by M4-5III giants. Interpretation of these data in terms of metallicity gradients implies a young population in the halo, while the radial mass cutoff increase, together with the increasing fraction of giant stars, contradicts an aging effect as the reason for the mass cutoff. Several critical predictions of the Valentijn (1983) model, which invokes ongoing star formation out of accreted X-ray gas, are fulfilled.

Evidence for underlying galaxies in a complete sample of variable quasars

Abstract: On note que les quasars selectionnes deviennent plus rouges aux faibles magnitudes probablement par suite de la presence d'une galaxie rouge sous-jacente qui contamine progressivement la couleur integree lorsque la composante nucleaire bleue s'affaiblit

Photographic photometry of 16000 galaxies on ESO blue and red survey plates

Abstract: An ongoing photometric survey of 16,000 galaxies on ESO red and blue plates is described. The galaxies of the ESO/Uppsala Catalog (Lauberts, 1982) are being PDS scanned, and the results are combined with existing photometry and supplementary observational data obtained at the 1-m ESO telescope to form a catalog for paper, magnetic-tape, and possibly video-disk publication. Details of the plate digitization, photometric calibration, and automatic parameter extraction are discussed, and a flow chart of the extraction operation and graphs of sample data are provided.

Nonlinear Problems in Stellar Dynamics

Abstract: We deal with orbits of stars in galaxies (1) on the plane of symmetry of a spiral galaxy (2) on the meridian plane of an axisymmetric galaxy and (3) three-dimensional orbits in non-axisymmetric galaxies. The theory of the “third integral” explains the ordered motions- However, there are also stochastic motions that play an important role near resonances. The transition to stochasticity is studied. Finally some new types of phenomena that appear in 3-dimensional systems are discussed.