Showing papers by "European Southern Observatory published in 1992"

The starburst model for active galactic nuclei : the broad-line region as supernova remnants evolving in a high-density medium

Abstract: It is shown that the broad permitted and semipermitted emission lines of the broad-line region (BLR) of radio-quiet active galactic nuclei (AGN) can be generated by strongly radiative (rapidly cooling) supernova remnants expected to occur in the central regions of early-type galaxies undergoing a starburst. Supernova remnants interacting with a circumstellar medium with a density of about n ∼ 10 7 cm -3 become strongly radiative while still expanding at several thousand km/s, and miss the adiabatic Sedov track. Radiative cooling becomes important well before the thermalization of the ejecta is completed, and the shocked matter undergoes a fast condensation behind both the outgoing forward shock and the reverse shock. Two concentric, high-density, and fast-moving thin shells are then formed

A Spectroscopic Study of UM 673 A and B: On the Size of Lyman-Alpha Clouds

Abstract: A study of the Lyα forest spectra (2 A resolution) obtained for the A and B images of the gravitationally lensed high-redshift quasar UM 673 is presented Higher resolution data of the brightest (A) image are also presented In the 2 A resolution spectra, all the absorption lines detected at 5 σ in the spectrum of the fainter B image are present in the A image; however, two anticoincidences are found, ie, two lines in A which do not have a counterpart in B at more than a 3 σ confidence level Given the fact that corresponding Lyα lines in the spectra of A and B have their equivalent widths well correlated, this proves that both light beams actually cross the same clouds

Optical counterpart of the east radio lobe of M87

Abstract: RADIO galaxies are divided into two broad classes1 according to whether they show extended radio lobes ending in hotspots (compact bright sources of radio emission) at some distance from the nucleus (Fanaroff–Riley class II) or instead have bright regions or knots close to the nucleus (FR I). Hotspots and knots emit synchrotron radiation, sometimes even in the optical range2,3. The galaxy M87 (NGC4486) in Virgo is the closest giant elliptical galaxy showing a jet and double-lobe radio structure, and is classified as FR I mainly because of the morphology of the jet-dominated west radio lobe, even though the east radio lobe resembles typical FR II objects and shows a hotspot4. We report here the detection of the optical counterpart of this hotspot. Because the lifetime of optically emitting synchrotron electrons is short, we suggest that this hotspot is fed by an unseen counterjet. This conclusion adds weight to the idea that apparently one-sided jet sources are intrinsically two-sided, with the visibility of the receding jet reduced by beaming effects.

Dust in the neutral globules of the Helix nebula, NGC 7293

Abstract: Images of the ionized knots and spokes within the helical structure of the Helix nebula have been obtained with subarcsecond resolution in the light of the Hα + [N II], [O III] 5007 A and [O II] 3727 A emission lines The [O III] image reveals the knots and their tails in absorption The globules and tails must be dusty and are seen against the diffuse, central [O III]-emitting volume of the Helix nebula The curved heads of these neutral knots are radiatively ionized by the light of the cetnral star to produce spatially resolved, bow-shaped Hα + [N II] and [O II]-emitting structures displaced (measured for the most prominent globule) by 165 arscec (31 × 10 15 cm) towards the central star from the peak of the absorption

Bowshocks and the formation of the narrow-line region of Seyfert galaxies

Abstract: Considerable evidence exists for a relationship between the optical narrow-line region (the NLR) and the nuclear non-thermal radio emission in Seyfert galaxies. In a previous paper we introduced a model for the NLR, which we here develop and describe more fully, in which the linear motions of the radio components drive bowshocks into the ambient nuclear medium. The cooled shocked gas, photoionized by the UV nuclear continuum, produces the optical NLR emission. The radio components may take the form of bubbles of plasma ejected from the nucleus, or may represent the working surfaces of jets

The Galactic globular cluster NGC 5897 and its population of blue stragglers

Abstract: UBVI CCD photometry for 2709 stars in the loose Galactic globular cluster NGC 5897 is presented. The measurements carried out in the very central regions reach below the main-sequence turn-off. An analysis of the colour-magnitude diagrams is made

Is there a relationship between BL Lacertae objects and flat-spectrum radio quasars?

Abstract: The properties of a complete sample of flat-spectrum radio quasars are analysed and compared to those of radio-selected BL Lacs, studied in a previous paper, to look for any relationship between the two classes. It is shown that microlensing of quasars by stars in foreground galaxies can probably be ruled out as an explanation for the BL Lac phenomenon, while an evolutionary connection between flat-spectrum radio quasars and BL Lacs does not seem to be supported by the present data. The intrinsic properties of the two classes, that is extended radio emission and line luminosities, are significantly different

Public software for the astronomer - An overview

Abstract: Sources of public software are described that are available over wide-area research networks in journals and from government sources which may be valuable to the astronomer and astrophysicist. A very large amount of high-quality public software is accessible at all times. Locations with material useful for research are emphasized with practical suggestions regarding access.

Evidence from zinc abundances for dust fractionation in chemically peculiar stars

Abstract: A SMALL number of intrinsically luminous, low-mass stars have recently been shown to have an extremely peculiar elemental abundance pattern. Their photospheric carbon, nitrogen, oxygen and sulphur abundances are within an order of magnitude of solar values, but all other normally abundant metals are present in only trace amounts; in two stars, iron is deficient by nearly five orders of magnitude. Two possible explanations are that the low iron content is primordial, implying a very great age, whereas the CNO and S abundances have been acquired during evolution, or that the CNO and S abundances reflect the initial stellar composition and the low iron content is the result of chemical separation by dust formation. The latter hypothesis arises mainly because the abundance pattern of these stars is similar to that of interstellar gas1, in which fractionation to dust plays an important part, but it is not easily understood how a process that must occur in the circumstellar envelope can so strikingly affect the photospheric abundances. Here we report the detection of appreciable amounts of zinc in the star HD52961, and argue that, because zinc will condense into dust only at rather low temperatures, its detection in near normal amounts is convincing evidence for the fractionation hypothesis.

Optical line-emitting gas and radio emission: evidence for correlation in low-luminosity radio galaxies

Abstract: Narrow-band Hα + [N II] images of a sample of low-luminosity B2 radio sources are used to investigate the correlations between radio and optical emission-line properties. It is shown that the correlation between radio and emission-line luminosity, previously established for high-power radio galaxies, extends over five decades in radio luminosity and includes the low radio power sources. It is also shown that the compact radio sources have emission-line excesses relative to the other sources in the sample of the same radio power, as well as far-infrared emission characteristics of emission from cool dust.

Optical Performance of Large Ground-based Telescopes

Abstract: Images taken with ground-based telescopes are dominated by atmospheric seeing. Analytical expressions of long-exposure optical functions, namely the modulation transfer function, point spread function and encircled energy are established, under the assumption that dome and telescope seeing are brought to negligible values, and that the diameter of the telescope is larger than the atmospheric coherence length. The influence of guiding errors and axisymmetrical telescope aberrations is also assessed, and a definition of optical quality is proposed. The results are generalized and the optical performance of a ground-based telescope is expressed in terms of effective diameter and signal-to-noise ratio.

Condensation of dust around the Wolf–Rayet star WR 125

Abstract: Infrared photometry of the WC7-type Wolf-Rayet star WR 125 (MR 93) in 1981-91 shows a 10-fold rise in the 3-4 μm flux during 1990-91 This, together with observations at longer wavelengths, is interpreted in terms of the condensation of dust in its wind, similar to the events observed from episodic dust-making WR stars like WR 48a in 1979-80 and WR 140 in 1977 and 1985 Dust condensation started in 19906 and has continued for at least a year at a rate of 23 × 10 -7 M ⊕ /yr, about 04 per cent of the mass-loss rate

Anisotropic Ionizing Radiation in Seyfert Galaxies. I. The Extended Narrow-Line Region in Markarian 573

Abstract: The morphology, kinematics, and ionization state of the nuclear extended narrow-line region (ENLR) of the Seyfert 2 galaxy Mrk 573 are studied using narrow-band images of a grid of long-slit spectra. The entire ENLR is mapped spectroscopically, and velocity structure is studied. The velocity field map shows a typical galactic rotation picture with some important deviations. A simple geometric model, in accordance with the 'unified schemes', is employed to study the effects of various parameters of the observed picture. The best match is achieved when a biconical radiation field illuminates the ISM of the host galaxy that takes part in a normal galaxy rotation but also has radial motions close to the nucleus. The emission-line images reveal an ENLR elongated along the radio axis in the northwest-southeast direction, but a map of the flux ratio forbidden O III 5007/(H-alpha + forbidden N II) shows a different structure, with the highest excitation peak offset by about 4 arcsec along the radio axis to the southeast.

Rotational Parameters of PSR 0540-69 as Measured at Optical Wavelengths

Abstract: The rotation frequency of the 50 ms pulsar PSR 0540 - 69 utilizing data collected at the ESO 3.6 m telescope at La Silla is measured. The observations were acquired in the interval 1989 January-1991 April. The average pulse shape displays a broad enhancement with a superposed double-peaked structure. It was possible to phase-relate the data between 1990 March and 1991 April indicating the pulsar did not display any large spin discontinuities during that time span. Three solutions, incorporating a pulse number ambiguity of +/- 1, are presented. The preferred solution yields a braking index of 2.04 +/- 0.02, comparable with GINGA measurements in 1987. Discrepancies between these results and extrapolation of the GINGA results are discussed.

Dynamical development of a ring-like structure in the supernova 1987A nebula

Abstract: CONTINUING observations of the nebula around SN1987A, which is about 3 arcseconds across, show it to be surprisingly regular. The brightest part of it is a highly axisymmetric structure, perhaps a simple ring1–4. This structure, which may be related to similar structures in planetary nebulae and around Wolf–Rayet stars, poses a challenge for current nebular formation theories. Here we discuss the formation of an axisymmetric nebula in terms of the interaction between the slow wind ejected by the progenitor of SN1987A while it was a red supergiant (RSG) and the subsequent fast wind produced by the same star after it had evolved into a blue supergiant (BSG). In our model, an initial small asymmetry in the RSG wind, due to rotation of the red giant, is amplified by the action of the fast BSG wind. Structures that are significantly axisymmetric, and perhaps even ring-like, can be produced. We suggest that such phenomena should be common in the formation of nebulae, as evidenced by the fact that only a small fraction of the observed circumstellar nebulae appear to be spherically symmetric.

Probing photoionization models in two well-studied extended emission-line regions: Centaurus A and 3C 227

Abstract: The observed emission-line ratios of the extended emission-line regions of Cen A and 3C 227 are compared with the theoretical results from photoionization models, assuming a power-law spectrum for the nuclear ionizing radiation. It is shown that a variation of the optical depth of the emitting gas can mimic a variation of the ionization parameter. Most of the observational data are explained if each subregion in the extended emission-line gas is characterized by a different value of the optical depth and shows a small density contrast between a «cloud» and a more dilute gas. However, the observed [O III] line ratio cannot be reproduced by photoionization, and an additional heating mechanism is required

Preliminary analysis of an ultraviolet Hubble Space Telescope faint object camera image of the center of M31

Abstract: A 5161 s exposure was taken with the FOC on the central 44 arcsec of M31, through a filter centered at 1750 A. Much of the light is redleak from visible wavelengths, but nearly half of it is genuine UV. The image shows the same central peak found earlier by Stratoscope, with a somewhat steeper dropoff outside that peak. More than 100 individual objects are seen, some pointlike and some slightly extended. We identify them as post-asymptotic giant branch stars, some of them surrounded by a contribution from their accompanying planetary nebulae. These objects contribute almost a fifth of the total UV light, but fall far short of accounting for all of it. We suggest that the remainder may result from the corresponding evolutionary tracks in a population more metal-rich than solar.

Infrared imaging of IRAS sources near the Galactic Centre.

Abstract: A number of IRAS sources in the direction of the Galactic Centre and suspected of being regions of recent or current star formation have been investigated by means of direct near-infrared imaging and limited spectroscopy in a further attempt to clarify their nature. Near-infrared counterparts to the IRAS sources are selected from the various objects in the images on the basis of their near-infrared colours, coincidence with the 3.5-μm sources of Glass, and general appearance. Photometry is derived from the images, and combined with the spectroscopy and with previously published results to determine whether indeed these sources are close to the Galactic Centre and whether they are young

Whole earth telescope observations of v471 tauri - the nature of the white-dwarf variations

Abstract: Time-series photometric observations of the binary star V471 Tauri were conducted using the Whole Earth Telescope observing network. The purpose was to determine the mechanism responsible for causing the 555 and 277 s periodic luminosity variations exhibited by the white dwarf in this binary. Previous observers have proposed that either g-mode pulsations or rotation of an accreting magnetic white dwarf could cause the variations, but were unable to decide which was the correct model. The present observations have answered this question. Learning the cause of the white dwarf variations has been possible because of the discovery of a periodic signal at 562 s in the Johnson U-band flux of the binary. By identifying this signal as reprocessed radiation and using its phase to infer the phase of the shorter wavelength radiation which produces it, made it possible to compare the phase of the 555 s U-band variations to the phase of the X-ray variations. It was found that U-band maximum coincides with X-ray minimum. From this result it was concluded that the magnetic rotator model accurately describes the variations observed, but that models involving g-mode pulsations do not.

A search for the optical/IR counterpart of the probable Einstein ring source 1830–211

Abstract: The results of deep optical and IR imaging, astrometry, and spectroscopy of the recently identified, probable Einstein ring source 1830-211, are reported. While no definitive counterpart has been identified, several faint objects have been found in the proximity of the radio positions. The brightest object (∼ 20 mag in the red) is nearly coincident with the north-eastern (NE) component of the radio source; however, it has been identified spectroscopically as a foreground M star. Its presence will make the optical or IR identification of the north-eastern radio component very difficult

Overcoming perspective elongation effects in laser-guide-star-aided adaptive optics.

Abstract: Tracking the instantaneous position of the laser guide star removes its perspective elongation thus allowing an increased separation of the laser transmitter from its telescope.

Two remarkable Herbig–Haro objects in the NGC 2264 region

Abstract: Two groups of Herbig-Haro objects, designated HH 124 and HH 125, have been discovered in the NGC 2264 complex. Direct plates, narrow-band CCD images, low-dispersion spectra and high-resolution line profiles of the sources have been obtained. HH 124, which lies to the north of NGC 2 264, has a typical linear structure with HH knots on both sides of an IRAS source which is situated close to a central emission knot. The knots of HH 124 show spectra typical of low-velocity shocks and the knots on opposite sides of the central one show blue- or redshifted emission, characteristic of a bipolar outflow tilted to the line of sight

The correlation between water production rates and visual magnitudes in comets

Abstract: From the visual magnitudes of the International Comet Quarterly data base and the OH radio lines measured at the Nancay radio telescope, the law log Q(H2O) = 30.74 (+/-0.02) - 0.240 (+/-.003) m(sub h) is derived from a sample of 13 comets.

The origin of the optical emission lines associated with extragalactic radio sources

Abstract: The observed extended emission line regions (EELRs) associated with radio sources are investigated with the objective of determining the characteristics of the ionization mechanisms and the possible effect of star formation. The sources included in the sample are 3C 227, 3C 277.3, 3C 305, Cen A (NGC 5128), NGC 7385, PKS 0349-27, PKS 2152-69, and Minkowski's Object. It is shown that the emission-line ratios of the EELRs considered can be explained by models which account for the coupled effect of photoionization and shock acting at different degrees. It is also shown that the EELR ionization is not due to young stars. The main energy sources of the EELR are identified.

Vaca Muerta mesosiderite strewnfield

Abstract: A field investigation is presented of the strewnfield of the mesosiderite Vaca Muerta, originally found in 1861. The area, 11.5 km long, 2.1 km wide, is located about 60 km southeast of Taltal, Chile, in the Atacama Desert. It has yielded 80 meteorites with a total mass exceeding 3782 kg. Most fragments were found in an undisturbed state, but some had been broken by prospectors. The present studies, in connection with historical records, indicate that the original mass of Vaca Muerta exceeded 6 metric tons. One impact feature, somewhat modified by man, consists of a 10.5-m diameter, 1.7-m deep hole, without an uplifted rim. Small masses were scattered up to 85 m from the hole.

Limb observations of the 12.32 micron solar emission line during the 1991 July total eclipse

Abstract: The limb profile of the Mg I 12.32-micron emission line is determined by occultation in the July 11, 1991 total solar eclipse over Mauna Kea. It is shown that the emission peaks are very close to the 12-micron continuum limb, as predicted by recent theory for this line as a non-LTE photospheric emission. The increase in optical depth for this extreme limb-viewing situation indicates that most of the observed emission arises from above the chromospheric temperature minimum, and it is found that this emission is extended to heights well in excess of the model predictions. The line emission can be observed as high as 2000 km above the 12-micron continuum limb, whereas theory predicts it to remain observable no higher than about 500 km above the continuum limb. The substantial limb extension observed in this line is quantitatively consistent with limb extensions seen in the far-IR continuum, and it is concluded that it is indicative of departures from gravitational hydrostatic equilibrium, or spatial inhomogeneities, in the upper solar atmosphere.

The Luminosity Functions of Galaxies

Abstract: It is pointed out that the different Hubble types of galaxies have different luminosity functions. Because the mix of Hubble types changes with the density of the environment, the overall luminosity function of all types must change with the environmental density, i.e. there is no unique (Schechter) luminosity function of galaxies. For four different type mixes the Schechter parameters M* and α are given.

HST observations of the inner coma of Comet Levy 1990c

Abstract: Data reduction procedures which will make it possible to model the nucleus of future targets of cometary space missions were tested on the basis of observations of Comet Levy 1990c by the HST on September 27, 1990. Faint hemispherical shells expanding in the coma, which is structured as a sunward fan deflected by solar radiation pressure, were observed. The Eddington fountain model makes it possible to infer the ejection times of the expanding shells and the initial velocity of the dust. Three shells ejected at time intervals of 11.5 and 12.5 h with an initial velocity of 0.35 km/s were observed for dust grains of 3-micron size. The lag angle of the fan is between 10 and 15 deg, thus suggesting a time delay of the most active sublimation of 1 h after midday. The observed time periodicities suggest a nucleus rotating in 24 h with a surface characterized by two active areas placed almost exactly in opposite hemispheres, even if a longer rotation period and a more complex nucleus topography can be excluded.