bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultraviolet Surface Photometry of Stellar Associations in the Large Magellanic Cloud
Authors: Koornneef Jan
Published: June 2019

Observations of the Large Magellanic Cloud obtained with the 10-channel ultraviolet photometer of the OAO-2 are presented. The aperture was circular and 10 arcmin in diameter whereas the wavelength coverage was from 4250 to 1430Å. A total of 50 fields has been measured. Using photometric criteria, the data fall into three groups which are also spatially separated. The different characteristics are easily understood in terms of population and reddening differences, but only if the previously reported MLMC extinction law” (Koornneef and Code 1981; Nandy et al. 1980) is adopted.
A Radio Continuum Survey of the Magellanic Clouds
Authors: Mills B. Y. , Turtle A. J.
Published: June 2019

Continuum radio emission from the Large Magellanic Cloud was first detected just 30 years ago (Mills & Little, 1953). Subsequently, surveys of the Clouds were made after each new advance in southern hemisphere instrumentation and the principal surveys are listed and briefly described in Table I. They cover a range of frequencies from 19.7 MHz to 8.4 GHz. The early surveys at low frequencies were chiefly concerned with the overall synchrotron emission from the Clouds but, as resolution improved, emphasis has shifted to the individual sources, both emission nebulae and supernova remnants, which can be recognized as Cloud members. Of recent years the unique position of the Clouds for studying radio sources in external galaxies has been undermined by the development of powerful synthesis telescopes in the northern hemisphere; these have provided equivalent sensitivity and better spatial resolution on M31, and several other northern galaxies can also be studied effectively. However, with the commissioning of the Molonglo Observatory Synthesis Telescope (MOST) in 1981 the Clouds have been restored to their rightful place befitting the closest galaxies.
Radio Continuum Emission from Magellanic-Type and Dwarf Irregular Galaxies
Authors: Klein Ulrich , Gräve Roland , Beck Rainer
Published: June 2019

High-frequency radio continuum observations of a small sample of irregular and dwarf galaxies (Klein et al., 1983) have revealed that their integrated radio continuum spectra tend to be significantly flatter than those found for normal spirals (Gioia et al., 1982). This indicates a higher relative amount of thermal (free-free) emission.
Kinematics of the HII Region N11 in the LMC
Authors: Hänel A.
Published: June 2019

Four mechanisms have been proposed to explain the structures and large scale mass motions of HII regions: the Champagne model, the ‘classical’ picture of an expanding Strömgren sphere, interaction of stellar winds and supernova blast waves with the surrounding medium. Especially the last three models have been used to explain the LMC shell structures. Braunsfurth et al. (1983) showed that it is not possible to distinguish the driving mechanisms for individual regions from their global appearance (dimension). Detailed studies of individual shells however, have been limited to the largest and brightest (30 Dor) or the more symmetric ones (e.g. N70, rings around WR stars).
High Resolution Observations of the Photosphere
Authors: Title A. M. , Shine R. A. , Tarbell T. D. , Topka K. P. , Scharmer G. B.
Published: June 2019

High resolution observations, theoretical models, and simulations are discovering many new and exciting phenomena in the solar atmosphere. In recent years, there have been a number of very high quality observations of the solar surface and lower photosphere made on the ground at Sacramento Peak Observatory, Pic du Midi, and at the Swedish Solar Observatory, La Palma. In space the Solar Optical Universal Polarimeter (SOUP) has made diffraction limited (30 cm aperture) time sequences completely free from atmospheric disturbances. The recognition that significant progress is possible in non-linear dynamics has encouraged a number of theoretical groups to attack the problem of convection in the solar atmosphere. Two, two and a half, and three dimensional simulations yield the geometry of the flow below the surface and a prediction of the response of the atmosphere above the surface. Models of magnetic flux tubes are now very sophisticated, and modern high resolution observations should be able to test these theories. The development of the technique of Local Correlation Tracking (LCT) has allowed the direct measurement of horizontal velocities in the atmosphere near disk center. The combination of Doppler and LCT measurements allows a direct measurement of the photospheric vector flow field. Measurements from SOUP, Sacramento Peak, Pic du Midi, and La Palma have shown that mesoscale flows cover the surface and that there exist still larger scale flows associated with emerging pores and active regions. Much of the recent experimental and theoretical progress in processing and understanding high resolution data has resulted from the availability of powerful scientific workstations for user interaction, large amounts of memory for image storage, and supercomputers for the massive fluid dynamics calculations. We are now in the very early stages of learning how to use these new computer tools to identify and follow processes in the solar atmosphere.
Small-Scale Magnetic Features Observed in the Photosphere
Authors: Martin Sara F.
Published: June 2019

Small-scale solar features identifiable on the quiet sun in magnetograms of the line-of-sight component consist of network, intranetwork, ephemeral region magnetic fields, and the elementary bipoles of ephemeral active regions. Network fields are frequently observed to split into smaller fragments and equally often, small fragments are observed to merge or coalesce into larger clumps; this splitting and merging is generally confined to the borders and vertices of the convection cells known as supergranules. Intranetwork magnetic fields originate near the centers of the supergranule convection cells and appear to increase in magnetic flux as they flow in approximate radial patterns towards the boundaries of the cells.
Model Calculations of the Photospheric Layers of Solar Magnetic Fluxtubes
Authors: Steiner Oskar , Stenflo J.O.
Published: June 2019

Multi-dimensional radiative energy transport is coupled self-consistently to magnetohydrostatic solutions for fluxtubes with rotational symmetry. It is shown that the photospheric layers of plage and network fluxtubes are heated by radiation by as much as 300 K at equal geometrical height. The amount of heating depends on the density reduction within the tube. The results are compared with observational data and the most recent semi-empirical model.
Order and Chaos in the Solar Cycle
Authors: Ruzmaikin A.A.
Published: June 2019

Solar activity varying with an 11-year cycle is chaotic at large time scales. The evidence comes from an analysis of observations of the sunspot number and of radioactive carbon. Thereby an estimate of the dimension of the solar attractor can be obtained.

The origin of the sunspots can be associated with the interactions of the regular, large-scale, chaotic, and intermittent magnetic fields.
Spectroscopic CCD Search for the Unseen Companions in Three Single-Line WNL Binaries
Authors: Grandchamps Andre , Moffat Anthony F. J.
Published: June 2019

We have obtained some 15 CCD spectra at different orbital phases for each of the SBl systems HD193928 (WN6), HD197406 (WN7), and CQ Cep (WN7), in an attempt to detect the companion, determine its orbit and estimate the masses. The wavelength range of the spectra is λλ3600 - 4200Å, with 1.4Å/pixel and < S/N > ≈ 100/pixel. We compare the best emission-line (N IV λ4058Å) with the best absorption-line (H9 λ3835Å).
The Dynamics of the AG Car Ring Nebula
Authors: Smith Linda J.
Published: June 2019

High spatial (2”) and spectral (7 km s–1) resolution observations of the ring nebula surrounding the LBV star AG Carinae are presented. The data, covering Hα and [NII], were obtained with the UCL echelle spectrograph on the AAT and cover 4 radial slit positions centred on AG Car. The nebular motions are clearly resolved and show that it is a single shell expanding at 70 km s–1 which was ejected from the central star some 7 000 years ago. The data also reveal the presence of a high velocity (~ 83 km s–1) bipolar mass outflow which has distorted the northeastern back edge of the shell.
Spectrophotometry of Ring Nebulæ Around Wolf-Rayet Stars
Authors: Pakull Manfred W.
Published: June 2019

Long-slit spectrophotometric observations of HII regions around Wolf-Rayet stars in the Galaxy, the Magellanic Clouds and in IC1613 are employed to measure the ionizing radiation from these stars. The sample includes all known WR stars in the SMC and 14 regions in the LMC comprising most of the known ring nebulæ. Apart from previously known HeII λ4686 emitting nebulæ around a few WO stars, HeIII regions were discovered within the nebulæ N76 and N79W which are being photoionized by the strong He+ Lyman continuum emission from the MagellanicWNE stars AB7 and Br2, respectively. The Zanstra blackbody temperatures and luminosities (80 000, 95 000 K and log L/LO = 5.4, 6.0, respectively) agree remarkably well with more elaborate WN model calculations and suggest a much higher bolometric correction (BC = −5.8 mag) than generally admitted for WN stars. The nebular λ4686 fluxes are several times higher than the corresponding broad stellar features suggesting that the highly ionized gas would remain detectable at much larger distances (i.e. in HII galaxies) than the underlying hot WNE population.
A Search for High Excitation Nebulae Around Wolf-Rayet Stars in the Magellanic Clouds
Authors: Niemela V. S. , Heathcote S. R. , Weller W.G.
Published: June 2019

We have serendipitously discovered that the WN+O binary Br 40a in the LMC is surrounded by a remarkable high excitation nebula, showing extended, narrow emission lines of HeII. This prompted us to make a systematic search for similar high excitation nebulae around other WR stars in the Magellanic Clouds. This survey revealed a second even more extreme example surrounding the WN+O binary AB-7 in the SMC, and one other marginal detection AB-5 (SMC). The detection of nebular HeII emission implies that these WN stars emit a much harder UV spectrum than is traditionally expected. For each of these nebulae we have taken narrow band CCD images at the CTIO 0.9-m telescope, and have obtained spectrophotometry with the 2D-Frutti photon counting detector on the CTIO 1.0-m telescope.
Stellar Content of the Giant HII Region NGC 604
Authors: Debray Bernard
Published: June 2019

BVRI CCD frames of NGC 604, the brightest Hll region in the nearby spiral galaxy M33, have been obtained at the Nordic Optical Telescope (Canaries Islands). The seeing was 0.6″ which corresponds to ~ 2 pc. at the distance of M33. The magnitude of the stars were obtained using the Capella stellar photometry package (Debray et ai., 1989).
New Observations of LBV Environments
Authors: Walborn Nolan R. , Evans Ian N. , Fitzpatrick Edward L. , Phillips Mark M.
Published: June 2019

Two observational programs which provide new information about particular LBVs through investigations of their immediate surroundings are described. (1) Digital spectral classification of OB supergiants in compact groups apparently associated with Radcliffe 127 and S Doradus has revealed several interesting objects and indicates which of them are likely to be generically related to the LBVs. (2) Velocity-resolved images of the Eta Carinae shell show qualitatively new features, which will contribute substantially to the interpretation of its complex spatial/kinematical structure.
The Influence of WR Like Stellar Wind Mass Loss Rates on the Evolution of Massive Core Helium Burning Stars
Authors: Vanbeveren D.
Published: June 2019

Evolutionary computations of massive close binaries (MCB) including the effects of stellar wind (SW) and convective core overshooting predict that all massive primaries with ZAMS mass larger than 10 M⊙ start their core helium burning phase (CHeB) as bare helium cores; the hydrogen rich layers are removed on a timescale of the order of 104 yrs as a consequence of Roche lobe overflow (RLOF). The CHeB remnant after RLOF resembles closely a zero age CHeB star and its further evolution is entirely independent from its binary nature. Similarly as has been done previously by Vanbeveren and Packet (1979, A.&A.80, 242), I have performed a phenomenological study on the evolution of massive hydrogen less CHeB stars including the effect of SW mass loss using updated M determinations of van der Hucht et al. (1986, A.&A.168, 111). The SW mass loss rate formalism used in the computations is based on the following requirements:
Evolution of the Progenitor of SN 1987A in the HR Diagram
Authors: Yamaoka H. , Saio H. , Nomoto K. , Kato M.
Published: June 2019

We calculate the blue-red-blue evolution of the progenitor of SN 1987A in the HR diagram by adopting the Schwarzschild criterion for convection and by choosing appropriate parameters for mass loss and mixing [Fig. 1 (left) where metallicity is Z = 0.005]. During helium burning, the star moves from the blue to the red due to mass loss from 23 M⊙ to 16 M⊙. Figure 2 (right) shows the lifetime in effective temperature bins normalized to unity (solid) compared with the number of supergiants with −8 < Mboi < � 9 in the LMC, which is also normalized to unity (dashed). The model is qualitatively consistent with the observed histogram.
Wolf-Rayet Stars in Local Group Giant HII Regions
Authors: Drissen Laurent
Published: June 2019

We present the results of our search for WR stars in three of M33's most massive giant HII regions, namely NGC 604, NGC 595 and NGC 592. A few of the putative “superluminous” WR stars are shown to be crowded multiple systems. We also compare the WR populations of the most massive GHRs in the Local Group. WN stars are 5 times more numerous on the average than WC stars in these large complexes. The variations among GHR WR populations are discussed in the context of metallicity and age differences.
Massive Stars in M31 and M33 OB Associations
Authors: Cananzi K. , Azzopardi M.
Published: June 2019

We observed several OB associations of M31 (OB 33, 42, 59 and 69) and of M33 (OB 4 and 137) in UBVI and through interference filters centered on lines of HeII and CIII, as well as a narrow continuum comparison filter at 4749 å (Armandroff & Massey, 1985). The observations were made at the CFH 3.6m telescope with a RCA-CCD. The stellar photometry was obtained using DAOPHOT.
CCD Controllers
Authors: Leach Robert W.
Published: June 2019

The requirements of current and next generation CCD controllers in the areas of CCD device and system architectures, readout noise, number and speed of readouts are reviewed together with such operational requirements as system flexibility, power consumption, cost and weight. The basic components of a CCD controller are described, including the timing sequencer, clock drivers, video processor and computer interface. The capabilities and implementation of the CCD controller developed at San Diego State are reviewed. An upgraded controller is described to overcome limitations in the area of readout speed and efficient support of multiple readout capability.
Multi-Fiber Spectroscopy by Wide-Field Telescopes
Authors: Watson F. G.
Published: June 2019

Fiber-fed multi-object systems are now the preferred instruments for gathering spectroscopic data on survey scales. The technique lends itself particularly well to telescopes with fields of 30 arcminutes or more. This paper gives a broad overview of the instrumental considerations involved in its implementation.
Effects of Shutter Timing on CCD Photometry
Authors: Galadí-Enríquez D. , Jordi C. , Trullols E.
Published: June 2019

CCD-shutters take some time to open and close. This results in a difference between the recorded and real exposure times. The pixel-to-pixel pattern δ(x, y) in which this difference varies depends on the characteristics of the device. These timing errors affect photometric measurements in two ways: directly, since the recorded exposure time is not the real one, and indirectly, via the flatfielding process, because the shutter effect is mixed up with the flatfield pattern. The direct effect may be negligible when exposure times are long. The indirect effect is present irrespective of the exposure times of the frames, if flatfield exposure times are short.
Development of a 7000 × 4000 Pixel Mosaic CCD Camera
Authors: Kashikawa Nobunari , Yagi Masafumi , Yasuda Naoki , Okamura Sadanori , Shimasaku Kazuhiro , Doi Mamoru , Sekiguchi Maki
Published: June 2019

The CCD we use is TC215 manufactured by TI, Japan. The pixel size is 12 microns square. It is a virtual phase CCD which has a peak QE of 60% at 700 nm and 15% QE at 350 nm. It is commercially available in a package, which is too big to meet our requirement for CCD spacing. We therefore put the CCD in a specially made compact package. Each chip is mounted on a machined ceramic spacer whose thermal coefficient is matched with that of the CCD package. We glue each CCD chip on the spacer under a microscope to measure YHE x − y position and height. Then we screw the CCD chip + spacer on a copper motherboard with the help of the gauge which has a planned grid with a good accuracy. The alignment accuracy we can get with such set-up procedures is an order of 5 microns (< 0.5 pixel) in the horizontal direction and 10 microns in the vertical direction.
JHK Photometry of Extragalactic Sources Using an Infrared Array Camera
Authors: McLean I. S. , Liu T. , Teplitz H.
Published: June 2019

The UCLA two-channel IR camera has been used to perform crowded-field photometry in two distinct classes of extragalactic objects. DAOPHOT was used to reduce images of M 33 and provide JHK color-magnitude diagrams of over 1,700 stars. Galaxy photometry in crowded clusters, e.g. Abell 370, was obtained in JHK and analyzed using FOCAS. In both cases, extensive reductions were performed using synthetic aperture photometry for comparison. Methods and results are summarized.
EEV and Electron Corp. Virtual Phase CCDs in the Near IR Region, He I λ 10830 Å
Authors: Shcherbakov A. G. , Shcherbakova Z. A. , Ilyin I. , Tuominen I.
Published: June 2019

He I, λ 10830 å observations of late-type stars provide valuable information about the chromospheric and low chromospheric-coronal transition regions. High resolution measurements of the helium λ 10830 å line profile offer a convenient way to survey the coronal emission of the Sun as a point source, as well as a variety of stars (O'Brien and Lambert, 1986, Shcherbakov and Shcherbakova, 1991).
Opening of the Magnetic Field Lines in a Fast Rotating Magnetosphere, with an Application to Jupiter
Authors: Aly J. J.
Published: June 2019

In this Communication, we consider a simple model of magnetosphere around a fast rotating Jupiter–like object possessing a spin–aligned dipolar moment μ. In this model, low–energy plasma released by inner sources located beyond the corotation radius diffuses outward through closed lines, forming a thin equatorial disk in which there is a quasistatic balance between the centrifugal force and the magnetic tension. At some critical radius r0, however, the magnetic field is no longer strong enough to hold the matter, and blows open, the matter escaping freely (Fig. 1) (this model has been introduced by Hill and Carbary, 1978).
“Sweeping Pinch” Mechanism and the Acceleration of Jets in Astrophysics
Authors: Uchida Yutaka , Shibata Kazunari
Published: June 2019

A magnetodynamic mechanism of jet formation, in which a packet of the toroidal component of the magnetic field Bϕ plays a role, is proposed. Such a packet of toroidal field, produced by the rotational motion in the β=Pg/Pm≫1 region, relaxes itself in the β≪1 region when brought up into such a region, for example, by the process of flux emergence due to magnetic buoyancy. In the β≪1 region, a progressive pinch is caused by this relaxation and the mass is swept out by the pinch near the axis and also by the j x B force in the twisted field region surrounding the axis.
Self-Confined Cosmic Rays
Authors: Wentzel Donat G.
Published: June 2019

Cosmic rays do not stream freely through the galaxy, contrary to earlier expectations. Streaming cosmic rays are slowed down by the emission of resonant Alfven waves that scatter the cosmic rays. The theory of self-confinement explains the isotropy of the bulk of the cosmic rays but not of cosmic rays above 103 Gev; it has been a stimulus to the theory for cosmic-ray acceleration at supernova shocks; and, on inclusion of diffusion in a galactic wind, it may explain the uniform cosmic-ray density out to 18 kpc in our galaxy. Rapidly streaming electrons in clusters of galaxies, in supernova remnants, and near solar flares are accomodated by the theory when it is expanded to include the effects of hot plasmas and other wave modes. A “resonance gap” may prevent the turning backwards of streaming particles and thus allow streaming near the particle speed.
Cosmic Rays and the Parker Instability
Authors: Nelson A.H.
Published: June 2019

Parker (1966, 1969, 1979) has shown that the magnetic buoyancy of a uniform horizontal magnetic field will destabilize the Galactic gas layer. Perturbations of the form shown in Fig. 1 will grow in time with the magnetic loops ballooning up into the Galactic halo, and the interstellar gas draining down the field lines to collect in the mid-plane. Parker also showed that if the dynamical effect of the cosmic ray component of the interstellar medium is included, using an isotropic cosmic ray pressure, then the instability is enhanced.
Parallaxes with Large Reflectors
Authors: Harrington R. S.
Published: June 2019

Reflecting telescopes designed to do astrometry can obtain parallaxes essentially free of systematic errors. Improvements in detection and mensuration now make it possible to produce parallaxes of unprecedented accuracy without undue cost or effort.
The Milliarcsecond Core of 3C147 at 6 cm
Authors: Preuss E. , Alef W. , Whyborn N. , Wilkinson P.N. , Kellermann K.I.
Published: June 2019

3C147 is a compact (≲1″), steep spectrum radio source identified with a quasar at z = 0.545 (0″.001 = 7.4 pc; c/Ho = 6000 Mpc and qo = 0.5). The radio structure shown by VLBI observations at 18 cm (Readhead & Wilkinson, 1980; Simon et al., this volume), at 50 cm (Wilkinson et al., 1977), and at 90 cm (Simon et al., 1980 and 1983) shows a bright ‘core’ (60 pc at one end of a ‘jet’ ~0″.2 (1.5 kpc) in length oriented in p.a. ~ −130°. In this sense 3C147 is typical of the one-sided ‘core-jet’ structures commonly found in the centres of other extragalactic radio sources. However, MERLIN observations at 6 cm (Wilkinson, this vol.) and VLA observations at 2 cm (Crane & Kellermann, unpubl.; Readhead et al., 1980) show a larger elongated feature extending ~0″.5 (3.7 kpc) to the North East of the bright core in p.a. ~25° or on the opposite side to the 0″.2 jet.
VLBI Observations of 3C298
Authors: Graham D. A. , Matveyenko L. I.
Published: June 2019

The quasar 3C298 (1416+06) has a complex radio-frequency spectrum, with a spectral bend below 100 MHz and a steep slope of −1.2 in the range 150 MHz – 3 GHz. At higher frequencies the spectrum flattens with some evidence of variability.
Sub-milliarcsecond Radio Structure of AO 0235+164
Authors: Jones Dayton L. , Davis Mike M. , Unwin Steve C.
Published: June 2019

AO 0235+164 is a very compact, flat-spectrum radio source. It is identified with a BL Lac object, and has optical absorption-line systems at z = 0.524 and z = 0.852. A complex set of HI absorption lines is seen at z = 0.524 (932 MHz), and several of these lines change significantly in depth over periods of less than a year. This is the only known case of variable extragalactic absorption lines. A faint nebulosity 2 arcsec south of AO 0235 + 164 has an emission-line redshift of z = 0.52 and may be an intervening galaxy. The radio spectrum of this source (between major outbursts) is remarkably flat, with the total flux density staying between about 1 and 3 Jy over a range of at least 1000 in frequency. Such a flat spectrum would lead one to expect a complex, wavelength-dependent structure consisting of several components with different self-absorption frequencies. However, the observed radio structure of 0235+164 is about as simple as one could imagine – it is a nearly unresolved point source in VLBI experiments from 900 MHz to 22 GHz. Recent VLBI experiments at 6 and 13 cm have shown evidence for some elongation of the source in a generally NE-SW direction, but only at low contour levels (< 15% of the peak). The major portion of the flux density appears to come from a core which is unresolved in VLBI experiments over a range of ~ 25 in frequency.
A Comparison of Compact and Extended Radio Sources in Three Wavebands
Authors: Miller Lance
Published: June 2019

I shall compare the luminosities in the X-ray, optical, and radio wavebands of two types of radio source - compact flat-spectrum sources and extended double sources - and attempt to place limits on models which unify these types.
Paschen Alpha as a Probe of Relativistic Beaming
Authors: Saunders Richard
Published: June 2019

Is the theme of core-brightening by relativistic beaming, so useful for compact sources, helpful in explaining the nuclear properties of classical doubles? Certainly one can construct an attractive model - involving the usual beaming together with an anisotropic dust distribution at r ≲ 1 pc - to explain the differences in nuclear radio and optical properties of radiogalaxies as due differences in orientation with respect to us. There are clear predictions about the strength of broad Pα. Preliminary results indicate that this kind of model is not correct.
Gravitational Lenses and VLBI Structures
Authors: Subramanian K. , Narasimha D. , Chitre S. M.
Published: June 2019

The double quasar 00957 + 561 A,B along with bright radio arches and VLBI structures is modelled using a gravitational lens consisting of an elliptical galaxy and a cluster. The effective time-delay between components A and B comes out to be about a year and this enables one to distinguish between intensity fluctuations resulting from intrinsic quasar variations and minilensing by low mass stars.
Interstellar Scattering
Authors: Cordes James M.
Published: June 2019

Fine scale electron density fluctuations in the interstellar medium (ISM) are manifest as scintillations and temporal broadening of pulsar signals and as angular broadening of galactic and extragalactic sources. Although scattering off the fluctuations is often a nuisance for conventional studies of radio sources, analysis or searches for interstellar scintillations (ISS) can lead to information about the ISM or radio sources that otherwise would not be obtainable. For example, the length scale probed by ISS in the ISM is typically 1011 cm and the characteristic angular size for quenching ISS is less than 1 μarc sec.
Structure of H2O Maser Outburst in Orion KL Nebula
Authors: Matveyenko L. I.
Published: June 2019

The H2O outburst in Orion KL had been observed by the VLBI technique in 1979–1983. The line profile is asymmetrical. The flux density and line width are variable and correspond to a partially saturated maser. The outburst region had a compact and a less compact one whose sizes are equal to 0.3 and 2.5 a.u. respectively. The fine structure of the profile corresponds to knots whose size is smaller 0.2 a.u. The brightness temperature of the region, Tb ≃ 1017K, implies a high emission directivity
VLBI Geodesy: Techniques and Recent Results
Authors: Shaffer David B.
Published: June 2019

From its inception, it was realized that VLBI had the potential for very precise determinations of baselines between antennas. Since the first early, crude results (Cohen and Shaffer 1971), the data collection systems and analysis techniques have been continuously refined, and VLBI is now producing baseline measurements to accuracies of a few centimeters over trans- and inter-continental distances. Since the Mark III system and the basic techniques of delay and delay rate baseline determination have been previously described (Rogers et al. 1983; Shapiro 1976), and there have been several recent meetings that dealt exclusively with VLBI geodesy techniques (NASA 1980; NOAA 1982), I will concentrate in this review on recent developments in equipment and procedures, followed by presentation of new results. This review is based primarily on the efforts and results of the so-called “East Coast VLBI Group” comprised of staff and support contractors at NASA Goddard Space Flight Center (NASA/GSFC), the NEROC Haystack Observatory, MIT, and the Harvard-Smithsonian Center for Astrophysics. Additional input comes mostly from staff of the National Geodetic Survey (NGS), Onsala Space Observatory, and the Jet Propulsion Laboratory.
How Precise are Spectroscopic Abundance Determinations Today?
Authors: de Strobel Giusa Cayrel
Published: June 2019

It is shown that a great breakthrough has occurred in the accuracy of spectroscopic abundance analyses with the introduction of solid state light detectors, such as Reticons and CCDs. Because of uncontrolled systematic errors in photographic photometry, abundances derived from high dispersion photographic spectra can hardly be known with an accuracy better than 0.3 dex. This is well exemplified by the recent finding that the observational scatter is large even in the equivalent widths of the Utrecht Solar Atlas. A fortiori these uncertainties are present in the [Fe/H] stellar abundance Catalogue, chiefly based in its present form on photographic material. For the future the calibration of the [Fe/H] Catalogue with spectra taken with Reticon detectors is recommended. A signal/noise ratio of 300 to 500 is more important than an improvement in spectral resolution with a low signal/noise ratio. Then, the remaining uncertainties in the abundances will mostly reflect inaccuracies in atmospheric parameter determinations of the models and in the assumptions underlying model computations.
Approaches to Photometric Calibrations
Authors: Rufener F.
Published: June 2019

In this review an examination will be made of the experimental conditions which must be satisfied by multicolor photometric systems if the observational parameters are to be correlated unequivocally with physical quantities of the star. An inventory of error sources disturbing such calibrations contains: instability of the natural system, procedures of observation and reduction to outside the atmosphere and correlation with a standard system. The requirements desirable for the preparation of lists of standard stars, for the definition of the pass bands, for the “orthogonality” of the calibrated parameter, for the definition of the domain of validity of the calibration are listed. Some remarks are made on the concepts of “open” or “closed” calibrations.
Standard Values and Information in Data Banks
Authors: Jaschek C.
Published: June 2019

This paper is an attempt to provide the background information necessary to find data outside of one's specialized field. The most important computer readable catalogs pertaining to the different stellar parameters are reviewed.
Fifteen Years of Statistical Parallaxes by the Principle of Maximum Likelihood
Authors: Heck A.
Published: June 2019

This paper presents a brief review of the various applications carried out with the statistical-parallax algorithm based on the principle of maximum likelihood.
A New Analysis of the Abundance Standards ι Herculis and τ Scorpii
Authors: Peters Geraldine J. , Polidan Ronald S.
Published: June 2019

Recent continuum and line data in the far ultraviolet for ι Her (B3IV) and τ Sco (BOV) have been combined with published observations in the near uv and visible spectral regions and interpreted with the aid of the Kurucz (1979) line-blanketed model atmospheres. For ι Her and τ Sco, respectively, interpolated models of (Teff/log g) 17500±500K/3.75±0.15, and 31500±1500K/4.3±0.2 fit the observed continua from 900–6500Å and the profiles of Hγ and Hδ. Adopting these models, solar abundances are suggested for both stars.
The Status of the Absolute Calibration of Stellar Fluxes Between 912 and 1200 Å
Authors: Polidan R. S. , Holberg J. B.
Published: June 2019

Recent results have shed new light on the status of the calibration of absolute stellar fluxes between 912 and 1200 Å. Observations of hot white dwarfs, subdwarfs and planetary nebula nuclei with the Voyager ultraviolet spectrometers provide evidence that the current calibration agrees very well with extrapolations of IUE energy distributions shortwards of 1200 Å. Voyager observations of main sequence B-stars used as flux calibration sources have revealed that many are variable in brightness in the 912–1200 Å region. We conclude there is no current observational motivation for any revision of the 912 to 1200 Å calibration described by Holberg et al. (1982).
A Preliminary Luminosity Calibration of the Uvby, β Systems for Supergiant F-Type Stars
Authors: Antonello E.
Published: June 2019

Calibrations of the uvby, β photometric systems have been made by Crawford (1975, 1978, 1979) for F, B and late A-type stars of luminosity class V-III; the calibration for early A-type stars has been made by Hilditch et al. (1983). One of the remaining areas of the HR diagram to be calibrated is that of the bright giant to supergiant region (luminosity class II-I). We have attempted to obtain a calibration in terms of luminosity for supergiant F-type stars. To this end we have used the statistical method of multiple stepwise regression analysis. Before doing the calibration for supergiants, we show the usefulness of this method by a comparison of the results obtained for dwarf F-type stars with the previous calibration given by Crawford (1975).
The Stability and Accuracy of the Uvby System
Authors: Manfroid J.
Published: June 2019

The recent analysis by Olsen (1983) of uvby data demonstrates the difficulty of comparing observations obtained with different equipment. He detected systematic errors in the standard stars of Crawford and Barnes (1970), who merged several sources. The introduction of new photometers with quasi-rectangular passbands (Nielsen 1983), which give theoretically nontransformable output, (Young 1974), could make the problem more acute.
An Improvement in the Visual Surface Brightness Scale for B5-F5 Main Sequence Stars
Authors: Pastori L. , Malaspina G.
Published: June 2019

Angular diameters of 593 B5-F5 main sequence stars listed in the “Catalogue of apparent diameters and absolute radii of stars” (CADARS; Fracassini et al. 1981) have been analysed in order to improve the precision of the visual surface brightness Sv. The new relations between this quantity and the color index (B-V)o turn out to be in good agreement with those found with the interferometric method (Barnes et al. 1978). Moreover, the results suggest that surface gravity effects may bias the Sv-(B-V)o relations.
A Reddening and Metallicity-Free Temperature Estimator for Late M Giants
Authors: Mennessier M. O. , Grenon M.
Published: June 2019

Lockwood's measurements in five-color narrow-band photometry of spectroscopic standard M stars is used to define an estimator of temperature. The TiO and VO indices deduced from this photometry depend in the same way on the metallicity but vary in a different way with the temperature: their ratio is only influenced by the temperature. A relation between T/V and the temperature is calibrated for M giants between 2000 and 3000° K. Both the T and V indices are indicators of metallicity and gravity which allow us to check that the relation (T/V)/temperature is independent of the metallicity. This relation leads to a precise determination of the temperature and can be applied to a wide variety of M giants, independently of their reddening and chemical composition.
The Chemical Evolution of the Galactic Disc, Investigated by Abundance Analysis of F Stars: A Progress Report
Authors: Edvardsson B. , Nissen P. E. , Gustafsson B. , Andersen J.
Published: June 2019

An extensive project for the investigation of abundances, especially of light elements, in F-type stars is described and some preliminary results are given.
Recent Trigonometric Parallax Measurements of MK Late-Type Spectral Standard Giant Stars
Authors: Ianna Philip A. , Culver Roger B.
Published: June 2019

Trigonometric parallaxes are reported for ten bright, southern late-type MK giant stars. The plate material was obtained with the 66 cm Yale-Columbia refractor at the Mt. Stromlo Observatory and measured with the PDS 1010A at Mt. Stromlo. The ten stars include HR 794 (KO III), HR 1247 (M2 IIIab), HR 2245 (M2.5 III), HR 2773 (K3 Ib), HR 3518 (K3 III), HR 3803 (K5 III), HR 5287 (K2 III-IIIb), HR 5603 (M3 IIIa), HR 6832 (M3.5 III), and HR 6913 (K1 IIIb). The modern parallaxes are compared with earlier results and the luminosity calibration for these stars is discussed.
High-velocity Clouds
Authors: Wakker B.P.
Published: June 2019

This contribution describes high-velocity clouds (HVCs), neutral hydrogen moving with velocities inexplicable by differential galactic rotation. They have been invoked as evidence for infall of gas to the Galaxy, as manifestations of a galactic fountain, as energy source for the formation of supershells, etc. It is becoming clear that a single model will not suffice to explain all HVCs. A better understanding is mainly hampered by the fact that the distance remains unknown. Many aspects to the study of HVCs will be discussed here.
Star Formation at Large Galactic z?
Authors: Tobin William
Published: June 2019

It seems very probable that young, luminous B stars occur in the galactic halo. An origin in the disc followed by ejection may account for many of these stars; certain proposed formation-ejection mechanisms involve the infall of halo material. The kinematics and supposed locations of some halo B stars seem incompatible with an origin in the disc, suggesting that these stars may have formed in the halo.
Optical Radiation Field in the Disk and Halo
Authors: Chi X. , Wolfendale A.W.
Published: June 2019

The imminent launch of NASA's Gamma Ray Observatory (GRO) has focussed on the need to provide up to date estimates of the energy density of interstellar radiation away from the Galactic Plane, this parameter being a prerequisite for calculation of the flux of gamma rays coming from the inverse Compton scatterings of cosmic ray electrons. In this work we use recent information on the stellar distributions and on the extinction properties of dust in the interstellar medium in the calculation. An important feature of the radiation field is that the field energy density is still substantial in the halo, as was found in our previous work. The differences between groups are due to the total energy input and the dust distribution. Our ensuing energy densities are probably accurate to about 30%, the uncertainty being due to lack of precise knowledge of the input parameters rather than approximations used in the calculation.
Radio Studies of Cosmic Rays in Nearby Galaxies
Authors: Hummel E.
Published: June 2019

The constraints on cosmic ray electron sources and propagation as derived from radio continuum observations of galaxies are reviewed. Special attention is paid to the inferences which can be obtained from the radio continuum properties of spiral galaxies seen edge-on.
The Influence of Extended Source Distributions on Cosmic ray Spectral Index Variations in the Galactic Wind Model
Authors: Pohl M.
Published: June 2019

The solution of the steady-state transport equation describing the propagation of relativistic electrons perpendicular to the galactic plane in a galactic wind is discussed for extended source distributions. The wind velocity is assumed to be zero in the plane and to increase with galactic height. The electrons undergo simultaneously diffusion, convection, adiabatic deceleration, radiative losses and injection. We contrast the resulting spectra with those derived for an infinitely thin source distribution and discuss the dependence of the variation of the spectral index inside the source distribution on the form of this distribution in any transport model. Apart from a new region at very small energies the asymptotic spectra agree with those of the line-source model (LS). Inside the source distribution the spectral index break is doubled from δγlin =0.5 to δγext=1.0.
Very-High-Eccentricity Librations at Some Higher Order Resonances
Authors: Klafke J.C. , Ferraz-Mello S. , Michtchenko T.
Published: June 2019

Motions near the 3:1, 4:1 and 5:2 resonances with Jupiter are studied by means of numerical integrations of a semi-analytically averaged Sun-Jupiter-asteroid planar problem. In order to have a model including the very-high-eccentricity regions of the phase space, we adopted a set of local expansions of the disturbing potential, adequate to perform the numerical exploration of regions in the phase space with eccentricities higher than 0.9 (Ferraz-Mello and Klafke, 1991). Individual solutions and qualitative results thus obtained are completely reproduced by numerical integration of the complete equations by filtering off the short-period components of these solutions.
Corotation Solutions in the Elliptic Asteroidal Problem with Stokes Drag
Authors: Beaugé C. , Ferraz-Mello S.
Published: June 2019

We have searched for stable stationary solutions of the resonant elliptic restricted problem of three bodies with Stokes drag. Results for the 1/2 external Jovian resonance are shown as example, including a comparison with numerical results of an N-body simulation.
Symplectic Integrators for Hamiltonian Systems: Basic Theory
Authors: Yoshida Haruo
Published: June 2019

Symplectic integrators are numerical integration methods for Hamiltonian systems, which conserves the symplectic 2-form exactly. With use of symplectic integrators there is no secular increase in the error of the energy because of the existence of a conserved quantity closed to the original Hamiltonian. Higher order symplectic integrators are obtained by a composition of 2nd order ones.
The Structure of the Binary Star Nebula NGC 2346
Authors: Walsh J. R.
Published: June 2019

NGC 2346 is a high excitation nebula which has a bipolar appearance and whose obvious central star is of too late a spectral type (A5) to account for the photo-ionization of the nebula. Spectral observations at high and low resolution in the visible, and low dispersion IUE observations in the UV are combined to explain the structure of the object. The radial velocity structure indicates a cylindrical geometry, or possibly two cavities, with the ionized material in the form of thin sheets. The variation of extinction has been investigated at many positions over the nebula; the obscuration is irregular although a ring of low extinction around the central star is apparent. The central star has previously been found to be a single-lined spectroscopic binary, and its zero velocity lies midway between the positive and negative radial velocity components of the ionized material. Visible spectrophotometry of the central star is combined with long and short wavelength IUE spectra in order to determine the parameters of the cool central star and the hot binary companion. The evolutionary status of this peculiar nebula is briefly discussed.
10 μm Spectral Observations of Moderately Extended Planetary Nebulae
Authors: Roche P.F. , Aitken D.K. , Whitmore B.R.
Published: June 2019

We have obtained 8-13 μm spectra of a sample of eight moderately extended planetary nebulae at a resolution of 0.22 μm using a 20″ circular aperture.
Type I Planetary Nebulae
Authors: Peimbert Manuel , Torres-Peimbert Silvia
Published: June 2019

The general properties of PN of Type I are reviewed. A list of 29 PN of Type I is presented, most of them are bipolar. Their bipolar nature might be a direct consequence of the large masses and angular momenta of their progenitor stars. PN of Type I are He and N rich, their observed chemical abundances are compared with theoretical predictions. A group of Type I PN candidates is presented.
Elemental Abundances in Planetary Nebulae
Authors: Kaler James B.
Published: June 2019

To date, we have been able to gather information on the abundances of 16 elements, ranging from helium to iron, relative of course to hydrogen, the seventeenth. Of the lightest 26 elements only the lithium, beryllium, boron trio, aluminum, and the quintet of metals from scandium to magnanese have not been treated. The results of decades of labor on galactic planetaries are presented as succinctly as possible in Table 1, where the elements are shown in order of atomic number. I will take as a general approach that He/H and O/H have readily recognizable gradients and variations, and that the other elements either generally vary in concert with oxygen, or are best studied with respect to that atom. Column (2) classifies the element according to its most prominent behavior. The well-studied ratios that are generally constant, for which a true mean can be derived, are designated “C.” Those that are probably constant, but which are not well studied, are noted as “c.” The four elements with abundances significantly under solar, which are probably depleted from the nebular gas by grain formation, are called “D.” The letters “G” and “E” denote those for which vertical galactic gradients and/or enrichment by the parent star have been clearly established.
Effects of Dust Formation on Chemical Abundances
Authors: Shields G. A.
Published: June 2019

Gas-phase abundances of C, Mg, Si, Ca, and Fe have been measured for a number of planetary nebulae on the basis of optical, ultraviolet, and infrared emission-line intensities. The abundances of Si, Ca, and Fe show characteristic depletions of one to two orders-of-magnitude as a result of grain formation. Magnesium shows a similar depletion in the outer parts of several planetary nebulae, but it is undepleted in their inner parts. Carbon is not detectably depleted by grain formation. Efficient condensation of refractory elements can easily occur during the early stages of formation of a planetary nebula; but the observed, residual gas-phase abundances are not understood. Observations of molecules in the envelopes of late-type stars may provide useful clues.
Fast Winds in Planetary Nebulae
Authors: Kahn F. D.
Published: June 2019

A planetary nebula consists mainly of gas ejected slowly by a red giant. Its dynamics is dominated by the hot central star which is left behind later. In particular a fast wind from this star forms a bubble of hot gas which fills the inner part of the nebula and pushes the envelope into a shell. This shell remains only partly ionized for a considerable time. Its non-ionized part is subject to a Rayleigh-Taylor instability, and is expected to break up into fragments which remain behind in the HII part of the nebula.
Distances of the Central Stars and their Position in the HR Diagram
Authors: Pottasch S.R.
Published: June 2019

Determination of the distances to individual planetary nebulae are discussed. Especially those methods which are independent of assumed nebular properties (mass, absolute flux, etc.) are assembled and discussed. In this way, reasonable approximations to the distance can be obtained for about 50 planetary nebulae. The accuracy of the distances is tested by comparing nebular properties derived from these distances with the properties of nebulae at the galactic center or in the Magellanic clouds. A comparison is also made with the statistical distance determinations; the conclusion is that the assumption of constant mass often leads to an overestimate of the distance, while the assumption of constant Hβ flux leads to distances having individual uncertainties of up to a factor of 3.

The central star temperature determination is summarized. Individual central stars are placed on the HR diagram and compared with theoretical predictions. Deductions concerning the evolution which can be made from the observations are discussed.
Kinematic distances of Planetary Nebulae
Authors: Maciel W. J. , Pottasch S. R.
Published: June 2019

Most distances of planetary nebulae are known roughly within a factor of 2 or larger, except for some special objects for which the uncertainty can be as low as 50%. In the present work both IUE interstellar Lyman alpha profiles and 21 cm HI line surveys are used to infer the distances of four planetary nebulae.