Showing papers by "European Southern Observatory published in 1987"

Active optics. I. A system for optimizing the optical quality and reducing the costs of large telescopes.

Abstract: A system of ‘active optics’ control for the optical imagery of astronomical telescopes has been under development in the European Southern Observatory for about ten years. Its first application will be in the 3·5 m New Technology Telescope (NTT) scheduled for operation in 1988. A model test with a thin 1 m mirror (aspect ratio 56) has given remarkably successful results which will be reported in Part II of this paper. Part I gives a complete presentation of the theoretical principles of this technique of active optics and its scope of application. The subject is treated from the viewpoint of the temporal band-pass of error sources, ‘active optics’ being concerned with the low-frequency band-pass. The high-frequency band-pass (‘adaptive optics’) is principally concerned with atmospheric correction and is only briefly referred to for comparison. ‘Active optics’ correction of the low-band-pass system errors should bring major improvements in image quality together with a large cost reduction. While ...

A new case of gravitational lensing

Abstract: Even before the discovery of the first gravitational lens system1 in 1979 (see ref. 2 for a recent review of the other published candidates), it was recognized that a statistical evaluation of the occurrence of gravitational lensing within a well-defined sample of quasars is important to understand better the quasar luminosity function and possibly the QSO phenomenon itself3, to test cosmological models4,5 and to probe the luminous and dark matter distribution on various scales in the Universe4–6. A year ago, we began a systematic search from the European Southern Observatory (ESO) at La Silla, Chile, for gravitational lens systems in a selected sample of highly luminous quasars; Mv<–29.0. These objects are promising candidates for gravitationally lensed QSO images with arc-second and sub-arc-second separations (J.S. et al., manuscript in preparation and refs 7 and 8). Since December 1986, we have identified four possible cases. Here we give a brief description of our first identified gravitational lens system UM673 = Q0142–100 =PHL37039–11. It consists of two images, A (mR = 16.9) and B (mR = 19.1), separated by 2.2arcs at a redshift zq = 2.719. The lensing galaxy (mR≈ 19, ZL≈0.49) has also been found. It lies very near the line connecting the two QSO images, ˜0.8 arc s from the fainter one. Application of gravitational optometry to this system leads to a value M0≈2.4×1011M⊙ for the mass of the lensing galaxy and to δt≈7 weeks for the most likely travel-time difference between the two light paths to the QSO (assuming H0= 75 km s–1 Mpc–1,q0 = 0).

Shocked bipolar outflow from the evolved star OH231.8 + 4.2

Abstract: Low- and intermediate-mass stars lose significant fractions of their mass during the asymptotic-giant-branch evolution, creating massive circumstellar shells. The deeply enshrouded OH infrared stars are presumably at the end of asymptotic-giant-branch evolution. I report here the discovery of shocked bipolar bubbles expanding supersonically at right angles to a dense dust disk around the red-giant OH infrared star OH231.8+4.2. The relative radial velocity between the northern and southern bubble fronts is over 200 km s-1, and with a total physical extent of 0.42 pc and a known inclination, this gives a dynamical age of about 1,500 yr. Herbig–Haro objects are observed at the front of the bubbles, and are probably formed as instabilities in the bow shocks, where two collimated flows ram into the ambient medium. This new phenomenon represents a brief phase of late stellar evolution, immediately preceding the formation of a planetary nebula.

Detached nuclear-like activity in the radio galaxy PKS 2152–69

Abstract: The effects of nuclear activity in galaxies are known to extend well beyond the immediate nuclear environment. This is particularly true in some quasars and the active ellipticals or radio galaxies where the influence of the central engine can be apparent on scales of up to several megaparsecs. In addition to the large-scale regions of non-thermal radio emission there are sometimes sources of optical, both line and continuum, emission which appear to be associated in some way with the transport of energy from the nucleus to the larger structures. This letter reports the discovery of a bright cloud of gas at a projected distance of 8 kpc (H0 = 50 km s-1 Mpc-1 used throughout) and approximately along the radio axis of the radio galaxy PKS 2152–69. It is shown to radiate a nebular spectrum including lines from the highly ionized species He+, Ca4+, Fe6+ and Fe9+. A compact source of optical continuum radiation within the cloud has an energy distribution which is considerably bluer than the residual light from the underlying galaxy.

Interstellar calcium towards supernova 1987A in the Large Magellanic Cloud

Abstract: We present high-resolution and high signal-to-noise spectra of interstellar calcium towards SN1987A in the Large Magellanic Cloud (LMC). They supplement previously published observations1,2 for Ca II and provide completely new results for the Ca I spectrum, in which eleven components are detected, not only at velocities corresponding to our Galaxy and the LMC, but also at intermediate velocities. Our spectra, that allow us to estimate the ionization balance in these interstellar clouds, provide some clues about their physical state and location. In particular, the components between 150 and 200 km s−1 show a much lower ionization degree than other components. This may be due to recent compression of the gas by a shock, possibly associated with a former supernova explosion. This interpretation would require the corresponding clouds to be located inside the LMC, indicating that at least some intermediate velocity components are not of halo origin.

Possible large-scale structure at redshifts ≲ 0.5

Abstract: Large-scale structure has been measured using galaxies out to distances corresponding to redshifts of ∼0.051. At much higher redshifts, marginal statistical evidence for the presence of clustering has been obtained using quasars2,3, but the magnitude and evolution of this clustering are not yet well determined. In this paper evidence is given for the possible existence of very large-scale structure at intermediate distances, corresponding to redshifts ≲0.5. This structure peaks in our direction of motion towards the microwave background, and could conceivably be the cause of it.

Optical flash background rates

Abstract: Flash background rates are measured from August 18-September 5, 1985 using the Lowell 0.61-m telescope at CTIO and the 28-cm f/10 Schmidt-Cassegrain telescope at ESO. During 230.1 hours of observing 40 flash events were detected, and it was observed that the background events were dominated by meteors and satellites passing through the field-of-view. The measured flash rate is compared with that of Pedersen et al. (1984), and it is determined that the background flash rate that is applicable to the data of Pedersen et al. for February 8, 1984 is 0.023 events/hr. The morphology of that flash, which appears to be controlled by a gamma-ray burster, is examined.

The Cosmic Distance Scale

Abstract: The zero-point of the extragalactic distance scale, defined by about two dozens of nearby, late-type galaxies, has remained nearly unchanged for the last decade, in spite of the advent of new techniques and great efforts The distances are essentially tied to trigonometric parallax stars and hence independent of the Hyades modulus; they are consistent with RR Lyr stars The mean zero-point is therefore probably secure to better than 10%

An Extended Nebulosity Surrounding the S DOR Variable R 127

Abstract: New high resolution spectrograms of R 127 show the presence of an extended (~4" ≙ 1 pc) expanding (v = ±28 km s-1) gaseous nebula around this high-mass loss S Dor variable.

Quasar spectra and the gravitational lens hypothesis

Abstract: Quasar spectra have been used to identify gravitational lens candi-dates1–6, often in the absence of other evidence, and it has even been suggested7 that they can be used as reliably as fingerprints to uniquely identify individual objects. Here we point out that quasar spectra are sufficiently similar that, in the presence of noise, it can be extremely difficult to distinguish between different objects solely from spectral characteristics.

Recent Developments on the Crab Nebula

Abstract: The spectrum of the Crab Nebula is strongly affected by interstellar absorption, especially in the ultraviolet. For a B-V color excess of 0ṃ4 a smooth spectrum is obtained from the radio to the X-ray frequencies, except for a “bump” in the infrared, presumably caused by dust emission. The composition of the filamentary shell depends on the validity of the photoionization models which still have much uncertainty. Apart from a large and variable helium abundance, no clear anomalies have been firmly established. Because of this and because of the uncertainty in the distance, the nature of SN 1054 remains in doubt. There are important asymmetries in the kinematics and composition of the shell and in the energy input into the Nebula.

Principal Components Analysis

Abstract: We have seen in Chapter 1 how the n × m data array which is to be analysed may be viewed immediately as a set of n row-vectors, or alternatively as a set of m column-vectors. PCA seeks the best, followed by successively less good, summarizations of this data. Cluster Analysis, as will be seen in Chapter 3, seeks groupings of the objects or attributes. By focussing attention on particular groupings, Cluster Analysis can furnish a more economic presentation of the data. PCA (and other techniques, as will be seen in a later chapter) has this same objective but a very different summarization of the data is aimed at.

Adaptive optics for the European very large telescope

Abstract: Adaptive optics is one of the main features of the Very Large Telescope (VLT) of the European Southern Observatory (ESO) - an array of four 8 meter telescopes. These telescopes can be operated individually, in an incoherent and in a coherent interferometric beam combination mode. Each telescope will be equipped with adaptive optics systems for real-time correction of atmospheric turbulence effects. First results with a prototype system developed for the VLT demonstrated the feasibility and the significant gain of this technology for astronomical imaging. This paper describes the VLT adaptive optics system and its implementation program.

Preliminary survey of collapsed cores in the Magellanic Clouds' globular clusters

Abstract: A preliminary report on a surface photometry survey for collapsed cores in the Magellanic Clouds' globular clusters is presented. Core morphology classifications are given for the 33 globular clusters examined so far. One cluster, NGC 2019, shows definite signs of a collapsed core, and two others, NGC 1774 and NGC 1951, appear as strong candidates. This detection of postcollapse cores outside the Milky Way opens some interesting prospects for future dynamical studies. However, the fraction of collapsed-core clusters appears to be smaller in the Magellanic Clouds than in the Galaxy. This may be due in part to their younger ages, or to the limitations imposed by the seeing effects. It is also possible that the relative scarcity reflects the physical difference in the tidal field environments between the Galaxy and the Clouds, in agreement with a trend found earlier, viz., that the tidal shocks from disk passages accelerate dynamical evolution and enhance the propensity for core collapse. 32 references.

Radio Observations of Young Planetary Nebulae

Abstract: A review is given of recent developments in radio observations of young planetary nebulae. The use of radio continuum spectra and high resolution radio maps of young planetary nebulae to obtain the details of planetary nebula formation is discussed. Recent HI and OH observations support the idea that very young planetary nebulae are surrounded by neutral gas. The use of general radio properties of planetary nebulae to derive their distances is questioned.

Data Coding and Initial Treatment

Abstract: Data Analysis has different connotations for different researchers. It is however definitely part of the chain of reduction and exploitation of data, located somewhere between, let us say, the taking of observations and the eventual, published article.

Changes in the soft and hard X-ray light curves of the AM Her object E1405-451

Abstract: The preliminary results of EXOSAT and contemporaneous optical observations of E1405-451 (V834 Cen) in 1985 and 1986 are presented. In the latter of the two observations the soft X-ray light curve was observed to be quite different to that seen in all previous observations, but similar to the optical light curve and the new soft X-ray light curve of E2003+225. A phase shift of the broad soft X-ray eclipse was also observed. The hard X-ray and optical light curves have also undergone small changes.

The Distribution of X-ray Emitting Quasars in Space

Abstract: This review is mainly concerned with the discussion of the statistical properties of X-ray and optically selected samples of quasars and Sy 1 nuclei and their intercomparison. The inconsistencies which have been found are clearly exemplified by the discrepancy between the observed and the predicted X-ray source counts. It is shown that a satisfactory understanding of these problems has not yet been reached. The last section deals with the long debated contribution of AGNs to the extragalactic X-ray background (XRB), which is still uncertain within a factor of 2, although a lower bound of 30% at the (now convenient) energy of 2 keV can be set.

A Study of the Hubble Flow

Abstract: It is found that the Hubble diagrams for quasars and giant radio galaxies are very similar. Either the evolution of giant branch stars in the galaxies closely matches the evolution of the quasar nonthermal emission or the cosmological deceleration parameter qo is close to 3.