bluuebunny/crossref_metadata_2025 huggingface dataset:
Lunar Tidal Phenomena and the Lunar Rille System
Authors: Middlehurst Barbara M.
Published: June 2019

Two types of tide-linked lunar phenomena now exist: the so-called lunar transient events, short-lived changes in brightness or colour and obscurations in small areas of the Moon (the reported duration is typically from a few seconds to a few hours and the areas involved are usually a few kilometres or less in diameter); and the A- and B-type seismic signals relayed back from the Apollo 12 passive seismic experiment. The frequency diagrams of both the lunar transien t events and the seismic signals show strong peaks at perigee when the Earth is closest to the Moon, with a smaller peak at apogee (transient events and B-type signal only) and both have been attributed to endogenous causes.

Both sets of data appear to be linked to areas where cracks and rilles exist. The most likely interpretation of the two sets of observations is that they are complementary and that some sort of gas release and excitation (in the cases of the glows and colour changes) is involved.

Association of the lunar events with craters with seamed and cracked floors and with other crack systems is described. Only a few associated areas of the Moon-wide lineament systems have been noted, but the Apollo 12 seismometer signals seem likely to originate from the nearby system of parallel features in the Fra Mauro area. The case for possible gas volcanism is examined.
Summary-Introduction: Velocity Fields and Associated Thermodynamic Variations in the External Layers of Intrinsic Variable Stars
Authors: Ledoux P. , Whitney C. A.
Published: June 2019

By intrinsic variable stars, we mean those stars which present variations in light, spectrum and radial velocity which cannot be accounted for in terms of purely geometrical or orbital factors; so that we have to appeal to some kind of periodic physical modification of the star.
Summary-Introduction
Authors: Schatzman E.
Published: June 2019

There is astrophysical evidence for the existence of shocks propagating in regions of variable density. We have therefore the choice of discussing first the physics of shocks in a variable density atmosphere, or the astrophysical phenomena. Following Kaplan's preliminary report, we shall describe first the astrophysical facts, for the reader to be able to understand the connection of the physics with the astrophysics.
General surveys of the Bulge in the infrared
Authors: Glass I.S.
Published: June 2019

The Bulge has been mapped in the infrared both coarsely from balloon platforms and in detail from the IRAS satellite and the ground. The information revealed by each type of survey is discussed. Particular attention is given to surveys of the Sgr I clear field, its content of long-period variables and their implications for distance determination and the high end of the Bulge luminosity function.
Long-period variables and carbon stars in the Galactic Bulge
Authors: Whitelock Patricia
Published: June 2019

The review covers the properties of red variables in globular clusters and the Galactic Bulge. Details are given of our current understanding of the Mira evolutionary phase. There is evidence that Miras in the LMC and the Bulge occupy different parts of the instability strip but obey the same PLC relation. The Bulge contains at least 2 × 104 Miras of which 100 or so have luminosities in excess of Mbol = — 5 mag. The Mira phase lasts more than 105 yr. These objects originate from stars with a wide range of metallicity, but it is currently unclear if the most metal-rich stars reach the top of the AGB to become Miras. Preliminary data suggest that the distribution of the Miras along the minor axis of the Bulge is different from that of the late-M stars but similar to the 2.4 μm luminosity.

Our knowledge of the Bulge carbon stars is briefly reviewed. It is suggested that, by analogy with the carbon-rich dwarfs, these stars are probably best understood as the products of binary evolution.
Hubble Space Telescope observations of Baade's window
Authors: Baum W. A. , Light R. M. , Holtzman J. , Hunter D. , Kreidl T. , O'Neil E. J. , Groth E. J.
Published: June 2019

This is a status report on a continuing program using the Hubble Space Telescope (HST) Wide–Field Camera (WFC) to probe the stellar population of the Galactic bulge to fainter magnitudes. We seek the mean age of the stars and the initial mass function (IMF). Galactic bulge stars offer the only opportunity to investigate the IMF of a super metal–rich population. They are 100 times closer than the next nearest sample.
Comparing M and K giants in the Bulge
Authors: Morrison Heather , Harding Paul , McGregor Peter
Published: June 2019

Kinematics and abundances of samples of K and M giants in a field at the edge of the bulge are compared. Despite a higher mean abundance, the M giants have the same kinematics as the metal-richer K giants.
Stellar Population Synthesis in the Bulge of our Galaxy
Authors: Robin Annie C.
Published: June 2019

Using a model of population synthesis well tuned to reproduce the disc and halo populations of the Galaxy, we show how to set constrains on the bulge population parameters using magnitude and colour distribution in 4 directions near the bulge. This first analysis result in a bulge with a disc-like LF but a density law much steeper in the center.
Studies of the galactic bulges using the post-theoretical mass method
Authors: Suran Doru Marian , Popescu Nedelia Antonia
Published: June 2019

The results of a new method, Post Theoretical Mass Method (PTM) are presented in order to investigate radial pulsating stars (RPS) in galaxies. Further implications for stellar and galactic astrophysics are also discussed.
Orbital structure of triaxial equilibrium models of various shapes
Authors: Udry S. , Martinet L.
Published: June 2019

To bring out the link between the observed shapes of triaxial dynamical systems (ellipticals, bulges, bars) and their orbital populations inferred from models.
Globally stable equilibria of collisionless self-gravitating matter
Authors: Wiechen Heinz , Ziegler Harald J.
Published: June 2019

We discuss the problem how to determine globally stable equilibrium states of collisionless self-gravitating matter with non-vanishing total angular momentum.
SPH Simulations of the Chemical and Dynamical Evolution of the Galactic Bulge
Authors: Tsujimoto T. , Nomoto K. , Shigeyama T. , Ishimaru Y.
Published: June 2019

We simulate the chemical and dynamical evolution of the galactic bulge with the smoothed particle hydrodynamics (SPH) method. We calculate the early phase of galaxy formation in which the bulge is formed through a burst of star formation. The calculated abundance distribution function of stars in the bulge is consistent with the observations of bulge K giants, if the heavy element yields are three times larger than those expected from Salpeter's IMF.
Velocity dispersions and metallicities of box-shaped galaxies
Authors: de Souza R. E. , dos Anjos S. , Barbuy B.
Published: June 2019

Velocity dispersions and Mg2 indices were measured from CCD spectra of 13 S0 box-shaped galaxies and 5 ellipticals. Evidence for a different relation between velocity dispersion and metallicity, relative to ellipticals, was found for our S0 sample; we attribute this difference to the S0 disk component, affecting both the mean velocity dispersion and mean metallicity.
Extracting kinematics from spectra: a new method and NGC 4406
Authors: Williams T. B. , Saha P.
Published: June 2019

The observed spectrum of a galaxy is the composite spectrum of its stellar population broadened by the stellar velocities. Our topic is the extraction of the broadening functions (i.e., line-of-sight velocity distributions) from spectra. In the past, observers tried to extract only mean velocities and dispersions. But recent work on deconvolution [Franx & Illingworth ApJL327, L55 (1989), Bender A&A229, 441 (1990), Rix & White MNRAS254, 389 (1992), van der Marel & Franx ApJ, to appear] shows that more information can be recovered. The general idea is to compare a galaxy spectrum with one or more stellar ‘template’ spectra; but various methods differ widely in the treatment of noise and the control of sources of systematic error.
A triaxial model for the bulge of NGC 4697
Authors: Zeilinger W.W. , Winsall M.L. , Dejonghe H.
Published: June 2019

There is now mounting evidence that the intrinsic shape of elliptical galaxies and bulges of disk galaxies may generally be triaxial. There are different signatures of non–axisymmetric structures observed: velocity gradients along the minor axis (e.g. Davies & Birkinshaw 1986), twisting of the isophotes (e.g. Williams & Schwarzschild 1979) and misalignment between bulge and disk major axes (e.g. Bertola, Vietri & Zeilinger 1991). Therefore, a framework of reliable algorithms for the dynamical modelling of such triaxial potentials is needed.
Linear and Circular Polarization in the Diffuse Interstellar Medium
Authors: Martin P. G.
Published: June 2019

A brief introduction is provided to the origin of interstellar polarization, grain model calculations, and alignment. The characteristics of the observed linear and circular continuum polarization are described, with an emphasis on various constraints that might be placed on the nature of the polarizing grains and what additional observations would be of value. Complementary information can be obtained from polarization structure at spectral features in the extinction curve. Interstellar polarization has been measured in some other galaxies, indicating an interesting frontier for further research.
Diffuse Interstellar Bands – The Unidentified Part of the Interstellar Absorption Spectrum
Authors: Krełowski Jacek
Published: June 2019

The unidentified (since 1921) diffuse interstellar bands (DIBs) are discussed together with their relations to other interstellar absorptions sucn as: continuous extinction, polarization and atomic or molecular absorption lines. It is shown that DIBs do not form the absorption spectrum of one agent, but probably of several (3 or more). DIBs as well as other interstellar absorptions are usually formed in several clouds along a line-of-sight. Thus, they suffer Doppler splitting; the first high resolution profiles free of the latter effect are described. Since single interstellar clouds may differ not only in radial velocities but also in many physical (optical) parameters, the observed interstellar absorptions are ill-defined averages over all clouds situated along any line-of-sight. It is of basic importance to determine not only the single cloud profiles of diffuse bands, but also their relations to other interstellar absorptions in the same clouds. Intensity ratios of DIBs are shown to be sensitive to the shapes of extinction curves, depletion patterns of elements and molecular abundances in the considered clouds. The sensitivity of the DIBs to the variation in polarization is less documented but probably also present. Thus the diffuse lines are presented as the unidentified part of the absorption spectrum of interstellar matter. Their identification depends on the determination of their relations to other interstellar absorptions which must be determined precisely.
A Critical Assessment of the PAH Hypothesis
Authors: Donn B. D. , Allen J. E. , Khanna R. K.
Published: June 2019

The proposal that polycyclic aromatic hydrocarbons (PAHs) are the source of the unidentified infrared bands has several serious deficiencies that have not been discussed or satisfactorily treated: (1) no collection of neutral PAHs has been found which matches the observed wavelengths; (2) ion or dehydrogenated molecules have been predicted to be the dominant species in some regions, but no infrared spectra of either species have been obtained; (3) the restriction to small grains is based on grain temperatures ∼ 1000-1500 K which in turn followed from the infrared continuum color temperature – there is now a question whether that is thermal emission; if it is, the source cannot be molecules; (4) recent observations of the 12/100 μm flux ratio as a function of stellar temperature do not conform to predictions of the PAH hypotheses; (5) the photon excitation mechanism for infrared emission by neutral molecules should produce strong visible-ultraviolet fluorescence which is not observed – ions may not do this, but their infrared spectra are not known. There does not seem to be a ready explanation for these problems with the PAH hypotheses. Until definitive experimental study and analysis yielding unambiguous results have been carried out for PAHs, hydrogenated amorphous carbon or other forms of carbonaceous material, it is premature to assume any type of grain is the source of the infrared bands.
Chemical, Optical, and Infrared Properties of Quenched Carbonaceous Composites (QCCs)
Authors: Sakata Akira , Wada Setsuko
Published: June 2019

Quenched carbonaceous composite (QCC) was synthesized by quenching the plasma of methane gas. Chemical properties as well as optical and infrared spectra of the QCC and “oxidized” QCC were measured. Good agreement of the IR spectra of the QCCs to the unidentified infrared (UIR) emission bands was obtained. Correspondence of their features to molecular structures in the QCC was estimated. It is concluded that a “cross-conjugated ketone” structure (CCK) caused the 6.2, 7.7, and 8.6 μm features and “solo” H atoms on carbon are responsible for the 3.3 and 11.3 μm features.
Far Infrared Emission from Galactic and Extragalactic Dust
Authors: Helou George
Published: June 2019

This review examines the question of how similar far infrared and submillimeter dust properties are among galaxies including the Milky Way, and within the Milky Way. The IRAS data are examined, and no evidence is found for variations in the broad-band dust properties between 10 and 100 μm. Submillimeter data are examined in a search for a power-law index describing the emissivity of dust at λ > 100 μm. The data are consistent with a universal value of the index between 1 and 2. The evidence for large amounts of cold dust (10 to 15 K) in galaxies is reviewed and found quite soft in light of conflicting data. In most spiral galaxies, the dust-to-gas ratios are comparable to their values in the Milky Way, with a few well-documented exceptions such as the Magellanic Clouds and Blue Compact Dwarf galaxies, which have a lower ratio. No compelling evidence is found for gradients in the dust-to-gas ratio within disks of galaxies. Very small grains with fluctuating temperatures appear to be present in all galaxies examined, and to behave just like those observed in the Milky Way.
Relativistic Beaming Models and VLBI Observations of A Complete Sample of Radio Sources
Authors: Readhead A. C. S. , Pearson T. J. , Barthel P. D.
Published: June 2019

We have conducted a systematic study of the milliarcsecond structure of a complete, flux-density limited sample of strong radio sources selected at 5 GHz. We have made 5 GHz maps at two epochs of the 45 compact sources in the sample, and third-epoch observations are in progress. Our intention was to explore the full range of morphologies exhibited by compact radio sources, to search for new superluminal sources, and to determine how widespread such phenomena as parsec-scale jets, alignment of parsec-scale and kiloparsec-scale jets, and superluminal motion are. In addition, we hoped to use this well-defined sample for statistical tests of the beaming theories.
VLBI Structure of NGC 1275 at 3 Millimeters
Authors: Wright Melvyn
Published: June 2019

We have made λ 3mm VLBI observations of 3C84 at 6 epochs between 1981 and 1987, using 6m antennas at the Hat Creek Radio Astronomy Observatory (HCRK), 10m antennas at the Owens Valley Radio Observatory (OVRO), the 12m Kitt Peak telescope (KTPK), and the 14m telescope at the Five College Radio Astronomy Observatory (QBBN). Model fitting suggests a core-halo structure, which we associate with the active nucleus of NGC 1275. The unresolved core, which is smaller than 0.1 pc (0.2 mas; z=0.018; H =50 km s−1 Mpc−1) shows a slow decay corresponding to the decrease in total flux density since the flare in 1980. In 1985 we resolved the core component on a transcontinental baseline (109λ) into a compact nucleus, < 0.1 mas, and a “jet” extended at a position angle 205° and unresolved in width, < 0.1 mas. There is also evidence for a compact emission region located 10 mas from the nucleus at a position angle 170°. This structure corresponds closely with that observed at the same epoch in VLBI maps at 22 GHz.
Relativistic Beaming and the Nuclei of Double-lobed Radio Quasars
Authors: Hough D. H. , Readhead A. C. S.
Published: June 2019

We have defined a complete sample of double-lobed 3CR quasars with minimal orientation bias. Properties of these objects on the >kpc scale are consistent with the simple beaming model for their nuclei. VLBI maps of six nuclei reveal “core-jet” structures. Both nuclei mapped at multiple epochs are modestly superluminal (3C245, 3.1c; 3C263, 1.3c; H0=100 km/s/Mpc, q0=0.5). The distribution of apparent velocities in this sample will provide a stringent test of beaming.
The Radio Structure of Compact Steep Spectrum Radio Sources
Authors: Fanti C. , Fanti R. , Parma P. , Nan R. , Schilizzi R.T. , Spencer R.E. , van Breugel W.J.M. , Venturi T.
Published: June 2019

Compact Steep Spectrum (CSS) radio sources are defined as having straight steep spectra typical of extended double-lobed radio sources and structures unresolved by conventional interferometers (Peacock and Wall 1982; van Breugel et al., 1984; Fanti et al., 1985). Their spectra remain straight up to quite high frequencies, indicating that no dominant core is present. Sometimes the spectrum bends at frequencies lower than 1 GHz. The angular sizes imply projected linear sizes generally < 10 kpc.
The Radio Structures of the Compact Steep Spectrum Sources 3C119, 3C287 and 3C343
Authors: Ren-dong Nan , Schilizzi R. T. , Fanti C. , Fanti R. , van Breugel W.J.M. , Muxlow T. W.
Published: June 2019

The compact steep spectrum radio sources, 3C119, 3C287 and 3C343, are identified with QSO'S of redshift 0.46, 1.055 and 0.988 and visual magnitude 20.0, 17.7 and 20.6 respectively. The overall physical extent of each source is −500 pc (Fanti et al. 1985, 1986). Here we present the results of new global VLBI 18 cm observations which have been combined with MERLIN data to improve the U-V coverage. The data were mapped using the CALTECH and AIPS software packages and also using OLAF software system developed in Jodrell Bank for 3C119 and 3C343.
The Working Surface of a Powerful Radio Source: Global MkIII Observations of the Southern Lobe of 3C205
Authors: Lonsdale Colin J. , Barthel Peter D.
Published: June 2019

The radio source 3C205 is identified with a quasar of redshift 1.53, and is, at first glance, a standard, powerful double radio source with hotspots. Upon closer examination (MERLIN, VLA A-array), the southern lobe is seen to consist of two distinct hotspots plus some more diffuse emission, some 50 kiloparsecs from the quasar (Ho ∼ 75). The arcsecond-scale morphology and polarization structure of this hotspot complex strongly suggests that the larger and weaker off-axis secondary hotspot has been formed by outflow of material from the more compact primary hotspot (Lonsdale and Barthel 1984,6). This interpretation implies a collision between the presumed energy supply beam and something capable of deflecting the flow energy towards the secondary. Because the flow is so energetic, this obstacle would have to be both massive and dense. The main constraint on the density of the obstacle comes from measurements of the internal energy density of the compact primary hotspot coupled with model-dependent estimates for the maximum advance velocity of this feature, thus yielding a balance with the ram-pressure exerted by the obstacle medium.
The Southern Hemisphere VLBI Experiment (SHEVE)
Authors: Preston R. A. , Jauncey D. L. , Meier D. L. , Tzioumis A. K. , Ables J. , Batchelor R. A. , Faulkner J. , Greene B. , Hamilton P. A. , Harvey B. R. , Haynes R. F. , Johnson B. , Lambeck K. , Louie A. P. , McCulloch P. , Moorey G. , Morabito D. D. , Nicolson G. D. , Niell A. E. , Roberts J. A. , Robertson J. G. , Royle G.W.R. , Skjerve L. J. , Slade M. A. , Slee O. B. , Stolz A. , Watkinson A. , Wehrle A. E. , Wright A. E.
Published: June 2019

Six radio telescopes were operated as the first southern hemisphere VLBI array in April and May 1982. Observations were made at 2.3 and 8.4 Ghz. This array produced VLBI images of 28 southern hemisphere radio sources, high accuracy VLBI geodesy between southern hemisphere sites, and subarcsecond radio astrometry of celestial sources south of declination −45 degrees. This paper discusses only the astrophysical aspects of the experiment.
VLA Polarimetry of VLBI Core–Jet Sources
Authors: Wrobel J. M. , Pearson T. J. , Cohen M. H. , Readhead A.C.S.
Published: June 2019

Radio imaging of VLBI core-jet sources can be used to examine the case for continuity of jet-like features between parsec and circumgalactic scales. Futhermore, polarimetry of such sources allows investigation of the dominant magnetic field topologies as a function of linear offset from the central engine. Examination of these continuity and field topology issues is essential for an understanding of how energy is channeled from the nuclear regions to the circumgalactic environment.
Determinations of Distances to Radio Sources with VLBI
Authors: Bartel Norbert
Published: June 2019

The most direct method of determining distances to galactic sources outside the Solar System is the measurement of annual parallaxes. Using as an example astrometric observations of the quasars 3C345 and NRAO512 which are separated by ∼ 0°.5 at the sky, we show that under certain conditions, annual parallaxes of galactic radio sources can be determined with VLBI with an accuracy of 50 μas (standard error). This accuracy is about tenfold higher than any annual parallax accuracy ever obtained with optical astrometry.

The, so far, most direct method of determining distances to extragalactic sources combines VLBI measurements of a supernova's angular expansion velocity with optical spectroscopic measurements of the supernova's radial expansion velocity. We have estimated the distance to SN1979C in M100 in the Virgo cluster to be a lower bound on the distance to SN1980K in NGC6946 to be 5 Mpc, and H0 to be with the uncertainties intended to represent standard errors.
VLBI Observations of the Gravitational Lens System 2016+112
Authors: Heflin M. B. , Gorenstein M. V. , Falco E. E. , Shapiro I. I. , Burke B. F. , Hewitt J. N. , Rogers A. E. E. , Lawrence C. R.
Published: June 2019

On June 1, 1984 we conducted a seven station 18-cm VLBI observation of the 2016+112 gravitational lens system. Preliminary brightness distributions for A and B have been obtained via model fitting. Weak correlated flux density was detected in the C component region.
VLBI Studies of High-Redshift 21cm Absorption Lines
Authors: Briggs F. H.
Published: June 2019

VLBI experiments can contribute to our understanding of absorbing gas observed in the spectra of high-redshift quasars by measuring the spatial extent of the absorbers. An optical survey conducted by Wolfe, Turnshek, Smith, and Cohen (1986) has turned up a class of absorbers characterized by large HI column density and a rich spectrum of metal absorption lines. In these respects the absorbers resemble spiral galaxies, but their occurence is too frequent for interception probabilities based on galaxy cross sections at the present epoch. Recent radio observations to determine the nature of such an absorber at z=2.04 in the spectrum of PKS0458-020 include both continuum observations at 606 and 1590 MHz to define the structure of the background quasar as well as spectral line VLBI at 467 MHz. The observations reveal structure in the continuum source on a wide range of angular scales, making the source nearly ideal for spectral line VLBI on a range of baselines. Early spectroscopic results show that the absorber must have a spatial extent of galaxian size, consistent with the hypothesis that these absorbers are galaxies. The further implication is that galaxies must have been larger and richer in gas at the epoch around z=2 in order to provide the likelihood of interception that is observed optically. (A complete description of the result is in preparation by Briggs, Wolfe, Liszt, Davis and Turner.)
Very High Resolution Observations of the Luminous Water Masers in NGC 4258
Authors: Claussen M. J. , Reid M. J. , Schneps M. H. , Lo K.-Y. , Moran J. M. , Gusten R.
Published: June 2019

We summarize the results and interpretation of a four station transcontinental VLBI experiment of the luminous water masers in the nearby galaxy NGC 4258. At a distance of 5 Mpc, the longest baseline of the experiment provides spatial resolution of less than 1016 cm. The strongest maser emission was detected on all baselines, and was found to consist of at least two features separated by about 0.1 mas (1016 cm). Weaker features are possibly spread over a region up to 1 mas in size. These results provide evidence that supports the scenario described by Claussen and Lo (1986) which suggests that the very luminous water masers reside in molecular gas that immediately surrounds the central, active nucleus.
VLBI Mapping of 43 GHz SiO Emission Associated with IRc2 in Orion-KL
Authors: Greenhill L. J. , Moran J. M. , Reid M. J. , Predmore C. R. , McIntosh G. C. , Rogers A. E. E.
Published: June 2019

Information on the structure of the molecular flow within 1″ of IRC-2, in Orion-KL, is sparse. Measurements of the continuum at 7.8μ and 12.5μ show a disk of size and suggest that the center of the disk may be dust free (Lester et al. 1985). Aperture synthesis mapping of water maser shell features (Sylber 1986) has provided information on the scale. Smaller scales can be studied by mapping SiO maser emission. We observed the 43 GHz, v=1, J = 1 → 0, transition of SiO using a 2 station interferometer with a 74 km baseline between Haystack Observatory, Westford, MA and Five College Radio Observatory, New Salem, MA. The fringe spacing was 20 milliarcseconds (mas) and the velocity resolution was 0.25 km-s−1. Our results provide the highest resolution view to date of what is likely to be the inner of IRC-2.
The Influence of Propagation through the Irregular Interstellar Plasma on VLBI Observations
Authors: Rickett B.J. , Coles W.A.
Published: June 2019

It has been known since pulsars were discovered that the interstellar medium (ISM) can cause angular broadening of radio sources. However, the amount of scattering along typical lines of sight to extra-galactic sources is not large enough to cause any loss of visibility, even on intercontinental baselines, unless the frequency is below about 100 MHz. Recent studies have shown that interstellar scattering (ISS) can have considerably stronger effects; this paper addresses their relevance to VLBI.
Limits on Refractive Interstellar Scattering of H2O Masers
Authors: Gwinn C. R. , Moran J. M. , Reid M. J. , Schneps M. H.
Published: June 2019

We limit the spectral index of density fluctuations in the interstellar plasma by comparing maser sizes, due to small-scale fluctuations, with wander of masers from constant velocity, possibly due to refraction by large-scale fluctuations. The wander of the masers about constantvelocity motion limits power-law spectra of the fluctuations to indices less than 4.0.
The Low Frequency Variability of Extragalactic Radio Sources: A Relativistic Effect or Galactic Scintillation?
Authors: Padrielli L. , Fanti R. , Ficarra A. , Gregorini L. , Mantovani F. , Spangler S.
Published: June 2019

Results obtained on the Low Frequency Variability (LFV) phenomenon, by means of combined multifrequency observations of 50 sources, on a period of more than ten years on a frequency grid of 0.4, 2.3, 4.8, 8.0, and 14.4 GHz and two epoch VLBI observations at 18 cm can be summarized as follows:

1.

15–20% of variables appear to have variations consisting either of quasi-simultaneous outbursts at all frequencies or of bursts which drift to lower frequencies with time and decreasing amplitude. In our sample, we find five good cases: 3C 120, 0605-085, 1510-089, 3C 345, BL Lac. Three of these are famous superluminals; the other two show significant structural changes between our 18 cm VLBI measurements. The corresponding expansion rate for these five sources is in agreement with the γ's derived from LFV with the usual causality arguments. For the sources of this class, the observations are therefore in agreement with models that explain the phenomenon of the variability as synchrotron emission of relativistic electrons beamed in a direction close to the line of sight.

2.

35% of variables show only low frequency (<1 GHz) variability and little or no intermediate high frequency variations. In DA 406, prototype of the category, no superluminal motions have been observed, even if the resolution of our VLBI observations should allow the detection of the structural change expected on the basis of intrinsic LFV. In this case we do not find direct evidence of relativistic motions associated with the LFV and the process is most easily explained if the variations are extrinsic (propagation effects through the interstellar medium as the slow refractive scintillation).

3.

The remaining 40–45% of variables show uncorrelated high (<5 GHz) and low frequency variability with a minimum of activity at the intermediate frequencies. The explanation of the phenomenon is less clear. It could be attributed to intrinsic (superluminal) variations at high frequencies, coexisting with unrelated processes at low frequencies.
Measurements of Interstellar Scattering in the Cygnus Region
Authors: Spangler S. , Fey A. , Mutel R.
Published: June 2019

We report progress on a study of interstellar scattering in the direction of the constellation Cygnus. A high quality, 1663 MHz VLBI image has been obtained of the radio source 2013+370, whose structure at this frequency is dominated by scattering. The visibility function indicates that the spectral index of the interstellar density irregularities is between 3.7 and 4.0, with the higher value being somewhat more likely. Another project consists of multifrequency observations of eight sources in this region. Pronounced changes in the scattering are observed for sources separated by only a few degrees.
Solar System VLBI Astrometry: The Venus Balloon Experiment
Authors: Preston R. A. , Ellis J. , Finley S. G. , Hildebrand C. E. , Purcell G. H. , Stelzried C. T. , Sagdeev R. Z. , Linkin V. M. , Kerzhanovich V. V. , Kogan L. R. , Kostenko V. I. , Matveenko L. I. , Nazirov R. R. , Pogrebenko S. V. , Strukov I. A. , Blamont J. E. , Armand N. , Alexandrov Yu. N. , Boloh L. , Laurans G. , Petit G. , Bogomolov A. F. , Moiseev I. G. , Biraud F. , Boischot A. , Ortega-Molina A. , Rosolen C. , Kaufmann P.
Published: June 2019

A global array of 20 radio observatories was used to measure the three-dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the Venus atmosphere by the VEGA spacecraft.
The Crustal Dynamics Project
Authors: Coates Robert J.
Published: June 2019

The Crustal Dynamics Project has been developing, deploying, and operating very-long-baseline interferometry (VLBI) systems and satellite laser ranging (SLR) systems for highly accurate geodetic measurements of global plate motion, plate stability, regional crustal deformation, and earth rotation/polar motion. Over the past 10 years, the measurement accuracies of these systems have been improved by a factor of 10 to the cm level. Plans are to continue these developments to reach mm level accuracies. The present deployment of the VLBI systems is primarily in the Northern Hemisphere. This network has produced measurements of the relative plate motion between the North American, Eurasian, and Pacific plates; the stability of the same plates; and the regional deformation at the North American/Pacific plate boundary in California and Alaska.
Joint Uses of VLBI with Astronomical Optical Instruments for Studying Vertical Changes
Authors: Zhi-sen Li , Guo-dong Zhang , Yan-ben Han
Published: June 2019

The description of the gravitational field at the surface of the Earth requires two quantities: the absolute value of the gravitational acceleration and the gravitational direction (deviation from vertical direction). At present, the various gravimeters measure the former quantity, and there is no effective instrument for monitoring the latter. This shortcoming seriously affects the comprehension and further knowledge of the gravitational field.
Conventional Terrestrial System by VLBI: A Kinematic Approach
Authors: Papo H. B. , Saleh J.
Published: June 2019

The paper examines the potentials of VLBI time delay observables for the establishment and maintenance of a Conventional Terrestrial System (CTS). The CTS is defined in 2-D by the standard epoch positions and velocities of a network of control points located on a spherical reference surface.
Phase Connection for Geodesy: Results from a 245-km Baseline
Authors: Ray J. R.
Published: June 2019

In an effort to greatly improve the measurement precision of VLBI observables used to determine geodetic baselines, the potential of utilizing fringe phase has been investigated. The 245-km baseline between Mojave and Owens Valley (in CA) has been most thoroughly studied for this purpose. In trying several different experimental designs it appears that source scheduling is not the critical factor in determining when phase connection can be established. Normally, instrumental instabilities are also not limiting (although nominal functioning of the instrumentation is essential). The most serious factor in limiting opportunities for phase connection seems to be atmospheric variability. However, when the observation SNR is made large (> 50) to give very precise group delays (and hence enhance the chance of resolving phase ambiguities), the improvement in baseline uncertainty obtained using phase delays is only marginal compared with the corresponding group delay solution. For this case, the baseline error budget is dominated by stochastic variations in clock and atmospheric delay contributions rather than by observation noise.
On the Computation of Group Delay Corrections Caused by Radio Source Structure
Authors: Campbell J. , Schuh H. , Zeppenfeld G.
Published: June 2019

We developed a structure algorithm for the computation of phase- and group delay corrections to geodetic VLBI observations. For some of the sources, i.e. 3C454.3, 3C273B and 3C345 the corrections are in the range of several hundred picosecs of group delay.
Monitoring of Extragalactic Radio Sources via Geodetic VLBI Observations
Authors: Tang Guoqiang , Rönnäng Bernt
Published: June 2019

It was pointed out already in the early stage of the development of the Mark-III VLBI that the structure of the quasars could affect VLBI results for geodesy or astrometry (Cotton, 1980; Robertson, 1981). As errors from other sources are gradually reduced, this effect can not be ignored any longer. We should monitor the structure variations of the sources and study their internal kinematics closely. 8GHz maps made from geodetic VLBI observations may give us important information.
The RADIOASTRON Project
Authors: Kardashev N. S. , Slysh V. I.
Published: June 2019

The RADIOASTRON mission (Andreyanov et al. 1986) is designed to achieve angular resolutions as fine as 6 microarcsec and will be used to study radio sources with very high brightness temperature. It will form an orbiting radio interferometer between a satellite radio telescope 10 m in diameter and several large ground-based radio telescopes. The orbit of the satellite makes it possible to have baselines from several thousands to 80,000 km. The satellite will be equipped with four dual polarization receivers at 327, 1665, 4830, and 22235 MHz, with local oscillators phase-locked to a ground-based hydrogen maser frequency standard via an S-band microwave link. The IF signals from the receivers will be transmitted to the ground by an X-band high data rate link and recorded on magnetic tape in the VLBA format. Participation of large radio telescopes from many countries is envisaged and encouraged.
The Very Long Baseline Array
Authors: Romney Jonathan D.
Published: June 2019

The Very Long Baseline Array (VLBA) will be the world's first large-scale dedicated VLBI facility. It is planned as a multipurpose instrument, supporting continuum, spectroscopic, astrometric/geodetic, multi-frequency, bandwidth synthesis, polarization, and pulsar measurements – with a variety of bandwidths, sampling rates, quantization schemes, and multi-band modes. The Array will consist of ten new 25-m precision antennas, in an optimized geographical configuration. Extremely low-noise receivers will support observations with dual polarization in nine frequency bands from 330 MHz to 43 GHz, including almost all conventional VLBI bands. A wideband, high-density recording system will allow unattended operation at a sustained data rate of 128 Mbit/s for 24 hours, and peak rates up to 512 Mbit/s. The VLBA correlator, planned as a 20-station system to support processing of all observations involving the Array, will be by far the largest ever built for VLBI. The Array will be operated remotely, from a new operations and data-reduction center to be built in Socorro, NM.
Design of Chinese Second 25-m VLBI Antenna
Authors: Pei-yuan Xu
Published: June 2019

The first Chinese 25-m VLBI antenna has just been completed at the She-Shan Station of the Shanghai Observatory. In accordance with the plan of the Chinese VLBI Network, the second 25-m VLBI antenna will be established at the Urumqi VLBI Station. It is a shaped symmetric dual reflector Cassegrain antenna and will be equipped with nine feeds from 0.3 GHz to 23 GHz. The surface accuracy of the main reflector is 0.65 mm, and the 14-m central part has a better surface accuracy (0.42 mm) to provide the possibility of making VLBI observations at 44 GHz. This antenna has a semihomology design, and its subreflector has a mechanism to focus it in three dimensions. Crossed dipole feeds for the 0.32 and 0.61 GHz bands are located in the middle of the subreflector, and the focussing mechanism will retract it to bring the dipoles to the prime focus. For other frequencies, a corrugated horn will be mounted at the Cassegrain focus. To obtain dual frequency operation at X and S bands, a flat dichroic reflector and an elliptical reflector will be mounted over the on-axis S band horn and off-axis X band horn, respectively. The antenna has a wheel and track mounting, and the pointing will be controlled by an IBM PC/XT microcomputer. The pointing and tracking accuracy is 16 arcsecond. The estimated aperture efficiencies are in the range of 45–65%. The estimated antenna temperatures are in the range of 35–100 K.
Hydrogen Maser Clocks and Deep Space Systems Involving Clocks: Ultra-Long Baseline Interferometry Definition of an Inertial Frame, Searches for Pulsed Gravitational Radiation
Authors: Vessot R.F.C.
Published: June 2019

The present status of research and development in atomic hydrogen maser clocks gives stability of 3×10−16 in Δf/f for intervals of several hours. Long-term drift rates are in the order of 1×10−15/day and uninterrupted operating lifetimes of more than 4 years are fairly common with SAO's VLG-series masers. Experiments with improved techniques for separating atoms in the desired state confirm that stability can be achieved with existing technology at levels below 1×10−16. A four-fold improvement beyond this level appears feasible with more smoothly applied storage volume coatings of fluorinated materials. A recent breakthrough in technology has been made by SAO/Harvard, M.I.T. and, University of British Columbia, by operating hydrogen masers at 0.4K using storage coatings of superfluid liquid helium. This promises stability better than 1 × 10−17 in Δf/f for intervals longer than 30 seconds. Features of a hydrogen maser operating at room temperature, developed at SAO for long-term (∼7 years) unattended operation in space, are described. Possible applications of such masers with stability ∼1×10−16 for intervals 104–105 seconds in a deep space network of 4 (or more) radio astronomy antennas interconnected by time correlated one-way and two-way Doppler systems are discussed. By comparing the phase of signals propagated in both senses around the four (or more) closed triangular paths, changes in the magnitude and direction of the baselines can be determined with reference to inertial space defined by the constancy and isotropy of the velocity of light. The system is also sensitive to pulsed gravitational radiation at wavelengths less than the inter-terminal distances. Sensitivity is limited by the stability of the clocks and the fluctuations of ionization in the paths between stations. Multiple station time correlation of Doppler fluctuation in the signals between the six (or more) inter-terminal paths can be used to determine the direction and polarization of the pulsed gravitational radiation.
Mapping with a Single-Spacing Interferometer
Authors: Doubinsky B. A. , Kardashev N. S.
Published: June 2019

Recent VLBI development provides opportunities for aperture synthesis, or super synthesis, with terrestrial interferometer antennas, and also in the near future with orbiting antennas. In this connection it is useful to consider the potential mapping capability of a single interferometer. Usually one would assume the use of image restoration procedures to be indispensable for observations with a single interferometer, because the uv-plane is covered only on an elliptical track, and is very much unfilled. High and distant sidelobes of the synthesized beam, caused by gaps in the coverage of the uv-plane, are usually considered to be influenced only to a small degree by any weighting function by which the visibility function is multiplied in the Fourier image formation algorithm. It has been shown that the mean-squared value of the point-source response function is invariant with respect to the weighting function (Doubinsky 1983). However, this does not prohibit the modulation of the point-source response by the visibility weighting function. This study shows that one can suppress the sidelobes in two symmetrical sectors of the point-source response with the help of such modulation. Thus for a line-shaped source model (that is, linear within the angular resolution of the synthesized beam) one could map without excessive distortion by Fourier transformation alone (Doubinsky 1979). The exclusion of restoration procedures, which are largely empirical, increases the reliability of the image when the source structure is extended and the signal-to-noise ratio is low. It also saves much computer time.
A Novel Method for Suppression of Aliasing Ghosts
Authors: Chikada Yoshihiro
Published: June 2019

This paper proposes a “half-sample delay switching” to suppress aliasing ghosts caused by finite frequency sampling without the clumsiness of a sharp anti-aliasing filter.
FFT Processor as a Digital Lens in Grid Array VLBI
Authors: Daishido T. , Asuma K. , Nishibori K. , Inoue S.
Published: June 2019

Correlator type processors have been currently used in VLBI, in which the number of correlators increases as N(N-1)/2 where N is the number of antennas. In a large elements VLBI array in future, this will introduce difficulty in constructing the processor. If we took the grid array configuration of antenna, FFT processor could be used in VLBI processing instead. FFT processor is used as a digital lens for a directional finding facility of the arrival waves. In FFT, the number of multiplications increases only as N log N. Although it gives maximum redundancy of Fourier components, the next step of the research on crustal motion will require measuring the velocities as a vector fields. Higher S/N ratio is also obtained in the VLBI maps.

The present processor, which was originally constructed for a pilot system of the radio patrol camera at Waseda University, is formed by a FFT Processor and eight Complex Amplitude Equalizers(1,2). When it is used as an VLBI processor, phase and amplitude fluctuations due to local oscillators and/or propagation effects are absorbed by the quick control of the Complex Amplitude Equalizers.

Here, we show examples of the computer controlled phase adjustment using the Equalizers(3). Our telescope is an 8 elements Very Short Baseline Interferometer (only 2m) at 10 GHz. Fig.1 is the block diagram of the present system. In VLBI system the recording facilities should exist between A/Ds and the Equalizers. The FFT processor is an error free ideal lens, and from the sampling theorem it makes eight independent beams in our system. Fig.2 is the dirty beams before removing phase errors, and Fig.3 the beams after the error removed. Both are 180 deg beam switched, while non switched in Fig.4.
Methods of Correction for the “Wet” Atmosphere in Estimating Baseline Lengths from VLBI
Authors: Elgered G. , Davis J. L. , Herring T. A. , Shapiro I. I.
Published: June 2019

The error in VLBI estimates of baseline length caused by unmodelled variations in the propagation path through the atmosphere is greater for longer baselines. We present and discuss series of estimates of baseline lengths obtained using different methods to correct for the propagation delay caused by atmospheric water vapor. The main methods are use of data from a water-vapor radiometer (WVR) and Kalman-filtering of the VLBI data themselves to estimate the propagation delay. Since the longest timespan of WVR data associated with geodetic VLBI experiments was obtained at the Onsala Space Observatory in Sweden, we present results for the following three baselines: (1) Onsala–Wettzell, FRG (920 km), (2) Onsala–Haystack/Westford, MA (5600 km), and (3) Onsala–Owens Valley (7914 km).
Seyfert 1 Galaxies in Calan–Tololo Survey
Authors: Maza José , Ruiz María Teresa
Published: June 2019

Four years ago we started at Cerro Calán an extension of the objective prism survey carried out at Cerro Tololo Inter-American Observatory (CTIO) by Malcolm Smith a decade ago (Smith 1975, Smith, Aguirre and Zemelman 1976). Thanks to the kindness of Dr. P.S. Osmer, then Director of CTIO, we got on loan Smith's original plate collection. Searching his plate material we learned to recognize the most prominent emission line objects. We decided then to extend Tololo's original survey to an area of the southern sky large enough to be of statistical interest but small enough so it could be carried through with our very limited facilities at Cerro Calán.
Emission-Line Variability and the Nature of the Broad-Line Region
Authors: Peterson B. M.
Published: June 2019

Variability of the emission-line spectra of broad-line AGNs is now a well-established phenomenon (see Peterson 1988 for a review). The rapid variability of the emission lines and the strong correlation between continuum and line fluxes suggests broad-line region (BLR) dimensions too small to be consistent with theoretical estimates based on photoionization equilibrium considerations.
Spectropolarimetry and Variability of Seyfert 1.8 and 1.9 Galaxies
Authors: Goodrich Robert W.
Published: June 2019

Twelve AGN classified as Seyfert 1.8s or 1.9s were observed with a CCD spectropolarimeter on the 3-m Shane telescope on Mount Hamilton. The instrument and data reduction procedures are described in Miller, Robinson, and Goodrich (1988). Three of the objects (NGC 2622, NGC 7603, and Mrk 1018) have undergone extreme variability, at times being classified as Seyfert 1.8s or 1.9s and at other times having spectra classifiable as Seyfert 1s. A set of IDS data on these three objects was generously provided by D. E. Osterbrock and collaborators, and allows comparison of the spectra in the “low state” (i.e. Seyfert 1.8 or 1.9) and the “high state” (Seyfert 1 spectrum).