bluuebunny/crossref_metadata_2025 huggingface dataset:
Summary-Introduction
Authors: Pecker J.C. , Thomas R. N.
Published: June 2019

This paper is an introduction to the astronomical material underlying the Varenna Symposium on Aerodynamical Phenomena in Stellar Atmospheres. The term « aerodynamical phenomena » rather than simply « velocity fields » is used in the title of the symposium to imply that primary concern centers as much on the physical phenomena and consequences associated with the presence of velocity fields as it does simply on the velocity fields themselves. To fully appreciate this distinction between aerodynamical phenomena and velocity fields from the astronomer's viewpoint, one must consider it against the background of the classical theory (*) of stellar atmospheres, which assumes that all the properties of the atmosphere are strictly controlled by the radiation field. The thermodynamic state of the classical atmosphere is fixed by the three conditions of radiative equilibrium (no energy transport other than by radiation), hydrostatic equilibrium (no mechanical momentum transport), and local thermodynamic equilibrium at a temperature fixed by the local energy-density of the radiation field (complete coupling between radiation field and atomic degrees of freedom). Analyses of stellar spectra under the framework of this classical atmospheric model take account of the presence of velocity fields (other than thermal) only in their effect upon the atomic absorption coefficient, not in their energetic or momentum coupling to the thermodynamic state of the atmosphere. Thus, if we become interested in aerodynamic phenomena in stellar atmospheres, we must investigate the possible perturbation these velocity fields may have upon the thermodynamic state of the atmosphere. We develop a primary concern with differential motions, velocity gradients, and dissipation mechanisms — all quantities which may produce a local non-relative energy source — rather than directing our attention only at stellar rotation and uniform expansion of an atmosphere. Thus, what we call aerodynamic phenomena embraces not only velocity fields but also their influence upon the thermodynamic state of the atmosphere.
What is the connection between ellipticals and bulges?
Authors: Franx Marijn
Published: June 2019

The structure of bulges and ellipticals, and their relation to galaxy halos are reviewed. Since many ellipticals contain faint disks, the qualitative distinction between bulges and ellipticals is more accurately described as a quantitative variation in Bulge/Disk ratio. The exception may be the brightest ellipticals, which are usually Bright (est) Cluster Members. The available evidence suggests that the spheroid properties are determined by more than halo properties alone. This is clearest for systems with low B/D ratio, where bulge velocity dispersions are similar to disk velocity dispersions. Constraints on the stellar formation scenarios are discussed.
Long-period variable AGB stars in a field towards the Galactic Bulge
Authors: Blommaert J.A.D.L. , Brown A.G.A. , Habing H.J. , van der Veen W.E.C.J. , Ng Y.K.
Published: June 2019

We study two different samples of long-period variable Asymptotic Giant Branch (AGB) stars in a field of low and homogeneous extinction towards the Galactic bulge, the Palomar-Groningen field Nr. 3. The samples were selected to study the evolution of the late phases on the AGB. One sample consists of 486 variables (mostly Miras) optically detected and studied by Plaut (1971) and by Wesselink (1987). The other sample is selected from the IRAS Point Source Catalogue and consists of 239 sources. We made additional infrared measurements between 1.2 and 13 μm for a large fraction of both samples. This information was used to identify the IRAS sources and derive the apparent bolometric magnitudes. The samples of Miras and variable IRAS sources have a similar apparent bolometric magnitude distributions, but are displaced by an amount significantly less than expected from the Mira period-luminosity relation (Feast et al. 1989; 0.3 magnitudes as opposed to 0.6 magnitudes). The surface density distribution along the minor axis of the bulge is the same for both samples. We conclude that both samples have evolved from the same parent population and that they represent different evolutionary stages on the AGB. The IRAS sources with the longer periods (on average 450 days (Whitelock et al. 1991) versus on average 250 days for the optical sample) are the further evolved objects. As the IRAS sources have higher mass loss rates we conclude that mass loss increases during the late stages of the evolution. However, we find indications that in some stars the mass loss process has been interrupted for some time; mass loss could be an intermittent process although its overall rate increases in time.
Cross-correlation of the two micron galactic survey with IRAS
Authors: Mahoney T. , Garzón F. , Calbet X. , Hammersley P. , Selby M. J.
Published: June 2019

The Two Micron Galactic Survey (TMGS) is described by Calbet et al. (this symposium) and Mahoney et al. (1990). It has been cross-correlated with the IRAS point source catalogue (IRPS) using a 15-arcsec search radius, and preliminary results are presented here.
CCD BVRI photometry of stars in the Galactic Bulge
Authors: Ruelas-Mayorga A. , West S. , Peimbert-Torres A.
Published: June 2019

We present preliminary results from BVRI CCD photometry of stars in the galactic bulge. The importance of disc contamination is shown. Several statistical criteria to separate disc from bulge stars are used.
A study of the abundance distributions along the minor axis of the Galactic Bulge
Authors: Tyson Neil D. , Rich R. Michael
Published: June 2019

We derive the heavy element abundances for hundreds of K-giants in seven windows of low extinction, along or near the minor axis of the Galactic bulge. By using the recently-calibrated Washington photometric filter system, the distribution function in [Fe/H] is determined for each field. Within 8° of the Galactic center (∼ 1 kpc) our data are consistent with no gradient in the distribution of [Fe/H], which may hint to a dissipationless collapse, and/or sufficient mixing during the star-forming epoch when Fe was produced in the bulge. The mean abundance over this region is between two and five times solar. The form of these distributions is well-fitted by the simple (closed box) model of chemical evolution where the bulge is self-enriched by processing its original gas content to completion. Beyond 8° from the Galactic center, our data show that the mean of the abundance distributions drops precipitously. This is consistent with the notion that the inner bulge is chemically distinct from the halo.
HB morphology and integrated spectra of globular clusters: a theoretical approach
Authors: Malagnini M.L. , Bono G. , Morossi C. , Pulone L. , Brocato E.
Published: June 2019

The aim of the project is to investigate the role of different horizontal branch morphologies on the integrated light of globular clusters.
Orbital elements of different Galactic population objects
Authors: Ossipkov L.P. , Kutuzov S.A.
Published: June 2019

The study of prevalent orbits in galactic subsystems can help us understand galactic structure and clarify its history. The classical analysis of flat orbits and metallicities of old stars led Eggen et al. (1962) to formulate the rapid collapse of the primordial Galaxy. On the other side Yoshii & Saio (1979) studied three-dimensional orbits that separate in spherical coordinates. They found the Galaxy contracted quasi-stationary after the formation of halo objects. Here we shall briefly discuss the results of numerical orbit calculations (with Merson's method) for selected galactic subsystems. The axially symmetrical two-component model of the Galaxy (Kutuzov, Ossipkov 1989) was adopted. One-component models (Barkhatova et al. 1987, Kutuzov 1988) were used also but no significant difference in orbit elements was found (Kutuzov & Ossipkov 1992). Pericenter and apocenter distances, Rp and Ra, and the maximal height of objects over the galactic plane, zm, were used as orbit elements as well as dimensionless quantities e = (Ra — Rp)/(Ra + Rp) (eccentricity) and c = 2zm/(Ra — Rp) (the flatness of box filled by orbit projection on the meridional plane).
Secular evolutionary trends in ellipticals and bulges due to mergers
Authors: Chatterjee Tapan K.
Published: June 2019

Using basically the impulsive approximation and a modification of the method used by Alladin S.M., (1965, Ap.J.,141, 768), and described in detail in Chatterjee T.K., (1990, IAU Col.,124, 519, 569) we study the evolution of binary interacting galaxies, leading ultimately to mergers. Each collision is characterised by the initial separation between the galaxies and the relative velocity therein. In each case the orbital evolution and largescale structural changes in the galaxies are studied by taking into account the change in relative velocity due to dynamical friction, leading ultimately to mergers. The evolution is considered from a time when the gravitational interaction between the progenitor pairs is physically significant (Chatterjee, 1992, Astroph. Sp.Sc., in press).
Photometric properties of box-shaped galaxies
Authors: dos Anjos S. , de Souza R. E.
Published: June 2019

A bulge/disk decomposition technique using a thin and a thick disk model is applied to a sample of 11 objects. In the thick disk modeling, the profiles are obtained in five differents radial directions. The results show that the model of thick disk is more realist.
Stellar population in the nuclei of early–type galaxies
Authors: Sil'Chenko O. K.
Published: June 2019

The results of nuclear stellar population investigation are presented for 100 “normal” galaxies. The significant fraction of intermediate–age stars (T being about 1 billion years) is found in the nuclei of 50% of early–type disk galaxies.
Triaxiality in the bulges of spirals: dynamical implications
Authors: Varela A.M. , Simonneau E. , Muñoz–Tuñón C.
Published: June 2019

The presence of triaxial bulges has been suggested as a mechanism for sweeping out the interstellar gas into the nuclear zones of normal spiral galaxies, giving rise to star formation processes (Zaritsky and Lo, 1986). In an initial phase of the present study, we have analysed the bulge component of a sample of spiral galaxies, obtained using the 4.2m WHT at La Palma Observatory, selected from among those which exhibit moderate activity. The results obtained are: (a) There is evidence, from the misalignment between the semi–major axes of the isophotes of the bulge and disc components, to support the presence of triaxial bulges in these galaxies; (b) The high spatial resolution of the images allows us to sample the bulges, showing that the surface brightness distribution is not always represented by a family of homologous coaxial ellipses, but rather by a set of ellipses which precess around a common centre, and whose ellipticity varies (see Beckman et al., 1991; Varela, 1992). It is possible to reproduce this structure with a family of concentric triaxial ellipsoidal shells. The presence of a disc offers the advantage of enabling us to interpret the geometry of the bulge correctly, but poses the disadvantage of requiring to interpret the disc emission in order to reconstruct that associated only with the bulge, i.e. to decontaminate the bulge from the disc light. To this end we carried out a photometric analysis, which enabled us to effect a valid bulge/disc decomposition. The preliminary results yielded by the analysis of the bulge component (after decontaminating the disc) show the presence of “transparent” discs in the galaxies analysed.
IR imaging of spiral galaxy bulges
Authors: Zaritsky Dennis , Rieke Marcia , Rix Hans-Walter
Published: June 2019

Imaging in the infrared (2.2μ) minimizes the impact of dust obscuration and allows reliable mapping of the mass-tracing stellar population in spiral galaxies. We find dramatic differences compared to photometry at shorter wavelengths (e.g. 0.8μ). As an example, the observations of the mini-bar and inner spiral arms of M 51 are discussed.
Visible/UV Scattering by Interstellar Dust
Authors: Witt A. N.
Published: June 2019

Studies of interstellar dust based on observations of scattered and dust-emitted radiation from grains are reviewed.
Aromatic Infrared Emission in Cirrus Clouds
Authors: Puget J. L.
Published: June 2019

We discuss the fraction of the infrared cirrus emission radiated out of thermal equilibrium using the IRAS data. Recent spectrophotometric data (Giard et al., 1988a, 1988b) are presented which confirm that aromatic infrared bands account for at least a fraction of their near and mid-infrared cirrus emission as was suspected from indirect arguments. Large variations of the energy distribution among the IRAS bands are shown to be present in molecular clouds. Abundances of polycyclic aromatic hydrocarbons (PAH's) required to account for the emission are discussed.
The Heating of Interstellar Gas by Dust
Authors: Hollenbach David J.
Published: June 2019

Interstellar grains serve as an intermediary in transferring stellar radiant energy or gas chemical energy into the heating of interstellar gas. Grain photoelectric heating, a process in which ultraviolet photons eject energetic electrons from grains into the gas, dominates the gas heating of the intercloud medium, diffuse clouds, and most photodissociation regions (PDRs); it is also significant in HII regions. Grain photoelectric heating in PDRs can explain the observed correlations of CII(158 μm) with CO J=1-0 intensities and CII(158 μm) + OI(63 μm) with infrared continuum (grain) luminosities. Gas-grain collisions and the infrared emission from grains can dominate the gas heating in molecular clouds with embedded stars. The ejection of newly-formed, vibrationally-excited H2 molecules from grains behind dense, dissociative shocks leads to postshock gas heating which produces strong far-infrared and millimeter line emission as well as H2O maser emission.
Dust and the Gas Phase Composition of Dense Clouds
Authors: Walmsley C. M.
Published: June 2019

A review is given of our current knowledge of the gas phase abundance of interstellar molecules. A comparison is made between cold dust clouds, where presumably gas phase processes dominate the chemistry, and regions of high mass star formation such as Orion where both shocks and evaporation of dust grain mantles may play a role. In dust clouds, carbon rich molecules often show enhanced abundances, whereas in hot turbulent star formation regions, saturated species such as methanol and ammonia are often favored. In both types of source, there is evidence for abundance gradients and inhomogeneities. A probable cause is that the chemistry is far from a steady state situation. A useful diagnostic in such cases is the relative abundance of deuterated species and our current knowledge concerning deuterium fractionation is discussed. Recent results show that the observed degree of deuterium enhancement in species such as water, methanol, and ammonia in the Orion-KL region is much too great to be explained on the basis of normal gas phase chemistry. It is argued that a plausible explanation of the observations is that one is observing material which has recently been evaporated from dust grain mantles. One important consequence of this hypothesis is that molecular line observations may yield important information on the composition of the solid phase.
The Composition of Dust in Stellar Ejecta
Authors: Whittet D. C. B.
Published: June 2019

I review the observational evidence relevant to the composition of dust ejected from circumstellar shells. Emphasis is placed on those types of stars believed to be the principal sources of refractory stardust in the interstellar medium. In stars with normal abundances (O > C), the predominant species appears to be an amorphous silicate, characterised by smooth spectral features at 9.7 and 19μm, a far infrared emissivity close to unity, and a featureless ultraviolet extinction curve. Similarly, the characteristics of carbon stars (C > O) are consistent with the presence of amorphous carbon rather than graphite. Silicon carbide is a minor constituent of carbon-rich stardust and is therefore unlikely to be an important ingredient of interstellar grains. A major goal for future studies will be to establish the evolutionary link between amorphous carbon stardust and the carriers of the unidentified infrared emission features observed in planetary nebulae.
Recent Observations and Superluminal Monitoring of the Quasar 3C 395
Authors: Simon R. S. , Johnston K. J. , Spencer J. H.
Published: June 2019

Until recently, the compact structure in 3C 395 could be described as follows (Simon et al. 1987):

(1)

A bright, unresolved, flat spectrum core to the northwest. The core brightened by about 0.4 Jy between 1983.3 and 1985.4, from 0.6 Jy to 1.0 Jy.

(2)

A moderately resolved component 15.8+/−0.2 mas to the southwest along P.A.I 18o. The position of this component relative to the core has changed by less than 0.2 mas in 6 years and the flux density of this component has remained roughly constant.

(3)

A component which was moving rapidly away from the core towards component (2) with a proper motion of 0.64+/−0.1 mas yr−1. For 3C 395 (z=0.635), this implies (v/c)∼20c for Ho =100 km/s/Mpc and qo =0.5.
Expanding Quasars and the Expansion of the Universe
Authors: Cohen M. H. , Barthel P. D. , Pearson T. J. , Zensus J. A.
Published: June 2019

The μ–z diagram (Figure 1) plots the observed internal proper motion μ versus redshift z for 32 extragalactic radio sources associated with active galactic nuclei. The observed points fall below an upper bound which decreases with redshift; there is a statistically significant anticorrelation between redshift and internal proper motion.
The Large Scale Structure of Superluminal Radio Sources
Authors: Simon R. S. , Johnston K. J.
Published: June 2019

The standard model for apparent superluminal motion is that of relativistic motion in a jet pointed nearly along the line of sight. Variants of this model give a natural explanation for the extremely asymmetric morphology observed in the radio cores of these sources. Despite this, examination of the large scale structure in superluminal radio sources strongly suggests that large scale symmetrical structure with two large radio lobes placed on either side of the compact emission is a common phenomenon among these objects. When a comparison is done between the large scale structure typically seen in double quasars and that seen in superluminal sources, it is found that the de-projected physical sizes estimated for superluminal quasars are similar to the size scales observed in the double quasars. Large scale radio structure is probably as common among apparent superluminal sources as it is in radio quasars.
Superluminal Motions and Large Scale Structure in 3C 120
Authors: Walker R. C. , Benson J. M. , Unwin S. C.
Published: June 2019

The jet in the superluminal source 3C 120 has been observed on all scales from under a parsec to about 100 kpc. The jet brightness decays rather smoothly with core distance, indicating that the basic characteristics of the jet are established on parsec or smaller scales and evolve simply with distance.
High Dynamic Range VLBI Observations of the 3C120 Radio Jet
Authors: Benson J. M. , Walker R. C. , Muxlow T.W.B. , Wilkinson P. N. , Booth R. , Pilbratt G. , Unwin S. C.
Published: June 2019

3C 120 is a nearby radio galaxy (z = 0.033) that shows superluminal motions in its radio jet within one parsec of the nucleus. The apparent velocities of the moving features are approximately 1.5 mas/yr. This object is of particular interest because it is clearly identified with an N-type galaxy, unlike all other superluminal radio sources which are only seen in quasars and BL Lac objects. Because of the close proximity of 3C 120, we have been able to make a series of VLBI and VLA images at stepped resolutions, and thereby map the radio jet continuously from the radio nucleus (0.5 h−1 parsec) into the diffuse 400 h−1 kpc structure.
Sub-luminal Motion in M87
Authors: Biretta J. A. , Reid M. J. , Junor W. , Spencer R. , Muxlow T.
Published: June 2019

The galaxy M87 (3C274, NGC4486) contains one of the nearest jet sources in the northern hemisphere, and one of our best opportunities to study a jet in detail. We have made MkII VLBI observations of the nuclear jet at 1.66 GHz using 18 stations of the “World Array.” The array included Arecibo which greatly improved the north-south resolution. The data were global fringe fitted, and self-calibrated to remove both antenna and baseline amplitude and phase errors. The resulting map (Fig. 1) has a dynamic range of 2300:1 and reveals a very complex structure. At distances ≥50 mas from the core, the jet is resolved across its width and appears limb-brightened. The brightness centroid oscillates from side to side by an amount roughly equal to the jet's width; this may be caused by brightness variations at the edges of a limb-brightened jet. Within 50 mas of the core the opening angle (at the FWHM points) is 6.5°±0.3°; this is larger than that of the 20″ jet, and suggests there is recollimation between the pc and kpc scales. The jet is aligned within 1° of the 20″ jet. There is no evidence for a counter-jet; the jet/counter-jet brightness ratio is R≥200.
Uniform Apparent Superluminal Motion in 4C39.25
Authors: Schalinski C.J. , Alberdi A. , Elósegui P. , Marcaide J.M.
Published: June 2019

The radiosource 4C39.25 (0923+392) is an 18 mag quasar with a redshift of 0.699. Throughout the seventies its compact structure at centimetric wavelengths consisted of 2 components separated by an angle of about 2 milliarcseconds (mas). In the eighties the structure has been found to consist of 3 components (Marcaide et al., 1985; Shaffer et al., 1987). The angular separation between the “a” and “c” components (see Fig. 1, which shows our most recent map of this source made at a wavelength of 2.8 cm) has remained constant at about 2 mas, while component “b” is separating from component “c” at a rate which we estimate from 2.8 cm observations as 0.18± 0.01 mas/yr, and which translates into an apparent linear separation velocity of 4.0 ± 0.2 c (HO=100 km/s/Mpc, qO=0.5, c is the speed of light).
Bulk Relativistic Motion in a Complete Sample of Flat Spectrum Radio Sources
Authors: Schalinski C. J. , Witzel A. , Krichbaum Th.P. , Hummel C. A. , Biermann P. L. , Johnston K. J. , Simon R. S.
Published: June 2019

As part of a multi-epoch and multi-wavelength study of the physics of core dominated radio sources we have investigated the occurrence of apparent superluminal motion and other indications for bulk relativistic motion (b.r.m) in a statistically complete, flux density limited (S5GHz ≥ 1Jy) sample of 13 objects with flat radio spectra These sources come from the S5-survey (Kühr et al., 1981) and are optically identified as 7 quasars and 6 BL-Lac objects. They have been observed over a wide range of frequencies, from radio through X-rays (s. Eckart et al., 1986 and references therein). Radiomaps have been obtained at frequencies from 327 MHz to 22.2 GHz with resolutions from arcseconds to 0.2 mas, using the VLA, MERLIN and telescopes of the US- and European-VLBI networks. A recent status report on the VLBI-observations is given by Witzel (1987). In this paper we summarize the results on the direct evidence for b.r.m. in this sample as derived from repeated VLBI-observations at 5 GHz, as well as supporting evidence from SSC-calculations and flux density variability of the 5GHz VLBI core components (Table 1).
VLBI Cores in a Sample of Radio Galaxies
Authors: Comoretto G. , Feretti L. , Giovannini G.
Published: June 2019

We present the first results of a statistical study of the milliarcsec structure in a complete sample of radio galaxies. We have selected from the B2 and 3CR samples of galaxies the sources which present, at the VLA or WSRT angular resolution, an unresolved core with a flux density at 5 GHz Sc ≥ 100 mJy. The total sample consists of 30 radio galaxies, 17 from the B2 and 13 from the 3CR catalog. This complete sample covers a range of total radio power at 408 MHz log P = 23.5 – 26.5 W/Hz (low-intermediate luminosity). The radio structure of these sources on the arcsec-arcmin scale is well known, thanks to good dynamic range VLA and/or WSRT maps; a large variety of structures is present in the sample, from classical doubles to head-tail sources; flat, inverted and steep spectrum cores are also present.
Shock Models of Time Variability and Superluminal Motion in Compact Extragalactic Radio Sources
Authors: Hughes Philip A. , Aller Hugh D. , Aller Margo F.
Published: June 2019

Following the success of a simple shock model for outbursts in BL Lacertae and 3C 279 (see Aller, Aller & Hughes, this meeting) we have constructed computer codes to study in detail the radiation from shocked, relativistic jets. These codes compute the transfer of synchrotron radiation, accounting for polarized emission and absorption, rotation, and mode conversion for a turbulent collimated flow with one or more shocks propagating parallel to the jet axis. We present results for a flow that evolves adiabatically, with the turbulence represented by a random component to the magnetic field within each computational cell, and with the shocks prescribed analytically following Königl (Phys. Fluids, 23, 1083, 1980). From the evolution of the total and polarized fluxes as a function of frequency, and from the corresponding projection of the source structure on the plane of the sky, we see that this type of model

a.

is capable o f explaining the variability of compact radio sources - see Aller, Aller & Hughes, this meeting,

b.

highlights the care needed when interpreting VLBI maps, in that

i)

the component separations are frequency dependent (see Fig. 1)

ii)

the ‘core’ is not always the brightest component (see Fig. 1)

iii)

the Doppler boosting factor of the shocked flow is not directly related to the Lorentz factor derived from the apparent superluminal motion of a component

iv)

a multiplicity of components can give rise to both apparent contractions and accelerations,

c.

clearly shows the link between time variability of compact sources and evolving VLBI structure, and suggests that both may be understood in terms of weak shocks that tap a small fraction of a jet's flow energy,

d.

enables us to probe the physical conditions of the flow and the ambient material.
Active Extragalactic Radio Sources Which Show Signatures of Shocks in Compact Jet Structures
Authors: Aller M. F. , Aller H. D. , Hughes P. A.
Published: June 2019

We have analyzed the cm-wavelength flux and polarization observations of four active objects (3C 279, OT 081, BL Lacertae, and 3C 446) for which the data exhibit signatures of shocks. Common characteristics of these sources are: high degrees of polarization, large changes in the polarized flux during outbursts; and stable position angles of the electric vector over long periods.
Dual Relativistic Effects in Compact Radio Sources
Authors: Qian S. J.
Published: June 2019

Dual relativistic effects in compact radio sources are discussed, and as an illustrative example we try to show that the observed features of the knot C4 in 3C345 (superluminal motion, superluminal expansion, apparent diameter and flux variation etc.) can be interpreted simultaneously.
3C236 - The Giant Radio Galaxy with a Compact Steep Spectrum Nucleus and a Compact Two-Sided Jet
Authors: Schilizzi R. T. , Skillman E. D. , Miley G. K. , Barthel P. D. , Benson J. M. , Muxlow T.W.B.
Published: June 2019

3C236 is the largest radio source known. It has a 39 arcmin double structure (3.0 Mpc, Ho=75 kms−1 Mpc−1) in which the SE lobe is narrow and edge-brightened and the NW lobe more diffuse and centre-brightened (Barthel et al., 1985). About half the flux density comes from a steep spectrum (α = −0.7) radio core of overall size 1.3 arcsec (2.2 kpc) located in the centre of a 17m elliptical galaxy with redshift 0.0988.
Observations of the cores of extended quasars
Authors: Hooimeyer J. R. , Barthel P. D. , Schilizzi R. T.
Published: June 2019

A critical test of the simple relativistic beaming models is provided by the search for structural changes in the weak cores of lobe-dominated quasars. In these sources, which are expected to be close to the plane of the sky, superluminal motion should be comparatively rare. We are engaged in the study of a complete sample of 30 extended quasars (Barthel et al. 1984), ranging in projected size from 40h−1 to 200h−1 kpc (Ho = 100h, qo = 0.05). High resolution multi-epoch observations have been obtained for two objects, the quasars 1721+343 and 0742+318, at 5 GHz (1982.3, 1983.3, 1986.5) and 10.7 GHz (1986.2, 1986.5). Superluminal expansion with transverse velocities of 3.1h−1 c has been detected in the core of 1721+343, which is the largest quasar known (Barthel et al., these proceedings). No superluminal motion was found in 0742+318. The large and small scale structure of 1721+343 are shown in fig.1a,b. The spectra of the individual components point to component A as the core
18 cm High Resolution Observations of 15 Extragalactic Radio Sources
Authors: Waak J. A. , Spencer J. H. , Simon R. S. , Johnston K. J.
Published: June 2019

On 8–12 October 1981 a 5-station MkII VLBI experiment at 1660 MHz was performed using antennas at Westford, Maryland Point, Green Bank, Fort Davis, and Owens Valley. Fifteen extragalactic sources (Table 1) were observed with an average synthesized beam of roughly 10 mas. The data were processed using the NRAO correlator, globally fringe-fit, and mapped with the AIPS package. In addition to the usual self-calibration techiques, corrections were made to eliminate baseline-dependent calibration errors. Some of the sources, for example 1641+399 (3C345), display extended components not before observed at the dynamic range of the maps, which on the average is roughly 200:1.
Global Fringe Fitting for Polarization VLBI
Authors: Brown L. F. , Roberts D. H.
Published: June 2019

We present a global fringe fitting technique for the polarized fringes in VLBI. The standard search method imposes a signal-to-noise (SNR) limit on usable data. In our method the search procedure is circumvented and the SNR limitation removed.
The Physical Nature of Spurious Polarization Transformed from the Background Nonpolarized Radiation of Cosmic Extended Radio Sources
Authors: Han-Shu Chu
Published: June 2019

Polarized radiation of cosmic radio sources carries the important information of the radiation mechanism and magnetic field structure of the sources. The background nonpolarized radiation of extended radio sources may also be partly transformed into spurious polarization. No comprehensive theory concerning the whole physical process of radiation transfer has been developed.
The Brightness and Polarization Structure of Compact Radio Sources
Authors: Rusk Raymond
Published: June 2019

New VLA polarization data for 140 radio sources have been used to study the correlation between VLBI structural position angle and radio and optical polarization position angles reported in Rusk and Seaquist (1985). We confirm that there is a strong tendency, in active galaxies and quasars, for the radio E vector (of the core component) to lie normal to the VLBI structural axis. However, in BL Lac objects we find a tendency for the core radio E vector to be aligned parallel to the VLBI structural axis.
VLBI Observations of Supernova 1987A
Authors: Shapiro I. , Bartel N. , Preston R. , Jones D. , Kemball A. , Nicolson G. , Jauncey D. , Reynolds J. , Whitney A. , Rogers A. , Phillips R. , Clark T. , Robertson D.
Published: June 2019

VLBI observations at 2.3 GHz of SN1987A on 28 February 1987 yielded no fringes, implying, for an optically thin shell, a lower bound on the (outer) diameter of 1.9 mas. From the comparison of the VLBI and optical results, we infer that the radiosphere of SN1987A was either about equal to, or larger than, the photosphere of the supernova five days after the explosion.
Shell-like Structure in 41.9+58, A Powerful SNR in M82
Authors: Wilkinson P. N. , de Bruyn A.G.
Published: June 2019

We have studied the brightest of the many compact radio components, designated 41.9+58, in the central region of M82 (Unger et al. 1984; Kronberg et al. 1985) with the EVN at frequencies of 1.6 and 5 GHz (Wilkinson and de Bruyn, 1984; 1987).
Circular Polarization of Astrophysical Masers
Authors: Deguchi S. , Nedoluha G. , Watson W. D.
Published: June 2019

Results of further calculations are presented to explore the non-linear, Zeeman overlap effect as the cause for the circular polarization of astrophysical masers. Emphasis is placed on the regime in which the Zeeman splitting is small and on the variation of the polarization with maser saturation.
The Ground State Hydroxyl Masers Associated with OH 340.78–0.10
Authors: Gaume Ralph A.
Published: June 2019

The dynamics of the molecular envelopes surrounding ultracompact H II regions has been the subject of much debate. Since the envelopes often exhibit molecular line emission, line observations have been the primary method of probing these circumstellar shells. The existence of expansive motions in the molecular clouds surrounding massive newly formed stars has been firmly established. Proper motion studies of galactic water masers indicate general expansion, whereas high resolution CO studies often show collimated bipolar outflows centered on continuum H II regions. Maps of the OH maser emission, often associated with these regions, show the masers scattered, apparently randomly, in both position and velocity across an area a few arcseconds in size. Typical velocity ranges for the associated OH maser emission are on the order of 10 km s−1. A number of models of the dynamical and physical conditions of the masing envelope have been developed to explain the properties of specific regions, e.g., collapsing remnant accretion envelopes, rotating disks or tori of neutral material, and expanding shells of shocked material. There is evidence from comparing the velocities of the OH masers to that of recombination lines associated with the underlying H II regions that in most sources the OH masers are formed in material still accreting toward the central object (Garay et al., 1985) (however see Welch and Marr, 1986). Mirabel et al., 1986, have reported the detection of (thermal and subthermal) high velocity OH outflow in several regions of star formation, demonstrating that OH molecules, at least in some instances, are also involved in collimated flows from central stellar objects.
VLBI and VLA Observations of Sco X-1
Authors: Geldzahler B. J. , Fomalont E. B. , Cohen N. L.
Published: June 2019

We have measured the absolute and relative positions of the components of Sco X-1 (Fig. 1) with the VLA from 1980–1986 (Geldzahler, Fomalont, Hilldrup, and Corey, 1981, A.J., 86, 1036; Fomalont, Geldzahler, Hjellming, and Wade, 1983, Ap.J., 275, 802; Geldzahler and Fomalont, 1986, Ap.J., 313, 815). We have used these observations to test the dual-beam model of radio sources proposed by Blandford and Rees (1974).
Variations of VLBI Structure in UX Ari
Authors: Catarzi M. , Felli M. , Massi M. , Palagi F. , Pallavicini R. , Tofani G.
Published: June 2019

Five stellar binary systems of RS CVn and Algol type have been observed on June 12, 1986, with the EVN (Effelsberg, Jodrell, Medicina, Onsala, Westerbork), and two US antennas (Green Bank, Haystack). Observations were carried out at 5 GHz, with 13 min of integration and with MARK III mode B.
Limits to VLBI Resolution Due to Solar Coronal Scattering
Authors: Gorgolewski Stanislaw
Published: June 2019

Electron irregularities in the solar corona scatter the radiation from radio sources. During more than 30 years of coronal electron irregularity investigations, a very complex temporal and spatial behavior of the solar corona have been observed. In particular, the scattering dependence on distance from the Sun varies with both the phase of the cycle and the different solar maxima. The influence of scattering on the highest resolution observations in radio astronomy, i.e., those made with VLBI, is not negligible. It should be taken into account in planning of both the global VLBI as well as the future orbital VLBI stations such as the QUASAT and RADIOASTRON missions. The “normal” scattering dependence on the distance from the Sun is close to the inverse square law, but substantial deviations from this dependence have been found.
A Spaceborne Multi-Arm Interferometer for VLF Gravitational Wave Detection. (The SMILE Project)
Authors: Anderson Allen Joel
Published: June 2019

This project would be the next step in our ability to detect very low frequency (VLF) gravitational waves and the first committed spaceborne designed experiment. Present Deep Space spacecraft tracking experiments are severely limited in their detection capability. It is proposed to construct a spaceborne multi-arm microwave interferometer using current elements of design applicable for the detection of VLF gravitational waves. The elements are outlined with particular emphasis placed on the utilization of small inexpensive get away special (GAS) modules currently under development at JPL for launch in the 1990's.
Astrometry of Millijansky Sources Using A Phase Reference VLBI Technique
Authors: Lestrade Jean-François , Rogers Alan E. E. , Niell Arthur E. , Preston Robert A.
Published: June 2019

Atmospheric fluctuations and hydrogen maser clock instabilities limit the coherence time (Tcoh) and hence the signal-to-noise ratio of VLBI data , to a few minutes at GHz frequencies. However, Tcoh can be increased up to many hours if a phase reference is established throughout the experiment by using a strong source observed alternatively with the program source. Weak celestial radio sources with flux densities in the milliJansky range at centimeter wavelengths, such as radio stars, fast pulsars and faint components of extragalactic sources, can then be reliably detected by this VLBI technique, simultaneously providing accurate astrometry relative to the reference source.
VLBI Positions of Eight Stellar Systems
Authors: Niell A. E. , Lestrade J.-F. , Preston R. A. , Mutel R. L. , Phillips R. B.
Published: June 2019

Radio positions of eight stellar systems have been derived with accuracies of 3 to 300 milliarcseconds from MkIII VLBI observations conducted with multi-station arrays. The best accuracy was obtained during a relatively strong outburst of the RS CVn system HR 5110. The epoch J2000.0 positions are obtained in the Jet Propulsion Laboratory VLBI reference frame of extragalactic radio sources.
Radio Reference Frame Stability from VLBI Data
Authors: Sovers O. J. , Treuhaft R. N.
Published: June 2019

Intercontinental radio interferometric measurements between NASA's Deep Space Network stations yield a catalog of positions of 106 extragalactic radio sources, uniformly distributed between −45° and +85° declination. Almost all of the source positions have formal uncertainties between 0.5 and 3 milliarcseconds. Estimates of three independent sets of 32 source coordinates at average epochs 1980.0, 1983.2, and 1984.5 assess the level of stability of the reference frame over a five-year period. Comparisons of the three sets of source coordinates show a number of cases of > 2σ differences, particularly betwen the 1984–85 and 1981–83 periods. No sources, however, show 2σ shifts among both pairs of epochs. Upper limits of the order of 1 mas/yr can be placed on the time rates of change of the 32 source coordinates.
NASA/Crustal Dynamics Project Results: Tectonic Plate Motion Measurements with Mark-III VLBI
Authors: Ryan J. W. , Clark T. A.
Published: June 2019

The NASA Crustal Dynamics Project (CDP) has been using VLBI on intercontinental baselines to measure tectonic plate motions since 1979. We report on measurements between sites on the North American plate (Haystack/Westford, MA; Owens Valley and Mojave, CA; Ft. Davis, TX and Gilmore Creek, AK), the Eurasian plate (Onsala, Sweden; Wettzell, West Germany, and Shanghai, China), the Pacific plate (Kauai, HI; Kwajalein in the Marshall Islands, and Vandenberg AFB, CA), the African plate (Hartebesthoek, RSA), and Japan (Kashima).
Geodesy by Radio Interferometry: Determination of Vector Motions for Sites in the Western United States
Authors: Gordon David
Published: June 2019

Since the late 1970's, NASA's Crustal Dynamics Project has been using Mark III VLBI to study plate tectonic motion, plate boundary deformation and earth dynamics. One major thrust has been the study of crustal motions in the western continental United States. For this effort, several relatively large, fixed antennas (Mojave, Owens Valley, Hatcreek, Ft. Davis and Vandenberg) have been used along with two small, highly mobile VLBI systems which have periodically visited sites of tectonic interest.
First Results of the VLBI-Investigation of Sources from Geodetic IRIS-Experiments
Authors: Schalinski C. J. , Alef W. , Witzel A. , Campbell J. , Schuh H.
Published: June 2019

The geodetic IRIS-network is a transatlantic 5-station VLBI-array dedicated to monitor variations of the earth's rotation (e.g. Carter et al., 1985). In a 5 day cycle 14 extragalactic radio sources are observed in regular intervals, covering a GST-range of up to 24 hours, using the MkIII-system in Mode C at 3.6cm (X-band) and 13cm (S-band) wavelength. As this database provides a great potential to monitor source structures and their changes with mas-resolution (the sample contains at least 7 “superluminals”), we have initiated a project to obtain radio maps at both frequencies and to determine, how accurate physical parameters like component flux densities and separations can be measured. Early results are published in Schalinski (1985) and Schalinski et al. (1986). In addition to astrophysical applications, we investigate the influence of structure phases on phase- and group-delay corrections (s. Campbell et al., this vol.).
Plate Motion and Earth Orientation
Authors: Mallama A. , Kao M.
Published: June 2019

Earth orientation series are linked to the terrestrial reference frame in which the observing site locations are measured. The effect of tectonic motion is a simple rotation for any given plate, but the overall effect depends on the distribution of sites. The magnitude of this motion is large enough to be evident in the data. For example, the coefficient of rotation for the North American plate around the Earth's Y-axis is −0.8 millarcseconds per year in the AMO-2 plate motion model of Minster and Jordan. The VLBI analysis system at NASA/GSFC for computing earth orientation series has recently been enhanced by including the Minster and Jordan model for a priori tectonic effects. Tests indicate that the weighted-root-mean-square residual of observations to the solution is decreased by using this model.
Distribution of Relative Plate Motion Along the Pacific-North American Plate Boundary Determined from Mobile VLBI Measurements
Authors: Kroger P. M. , Lyzenga G. A. , Wallace K. S. , Davidson J. M.
Published: June 2019

The problem of understanding the deformation occurring along the Pacific-North American plate boundary in the western United States depends upon understanding the forces which drive the plates in this region. One of the primary sources of our knowledge concerning these forces lies in their manifestation as relative displacements which occur throughout the broad zone of deformation surrounding the San Andreas fault system. It is information concerning the spatial and temporal distribution of these motions which will be of greatest benefit in helping to determine which of several possible mechanisms is responsible for driving contemporary plate motions in this region.
Earth Rotation from the IRIS Project
Authors: Robertson D. S. , Carter W. E. , Fallon F. W.
Published: June 2019

Project IRIS (International Radio Interferometric Surveying) was set up under the IAG and COSPAR to provide an operational system that would employ Very-Long-Baseline Interferometry (VLBI) techniques to monitor variations in the rotation of the Earth. Currently the IRIS-A network, with stations at Westford, MA, Ft. Davis, TX, Richmond, FL, and Wettzell, FRG, conducts 24-hour observing sessions every five days to produce determinations of pole position, UT1, and nutation in addition to other parameters of geophysical and astrometric interest. The resulting Earth orientation parameters (EOP) have been shown to have an accuracy of 1 to 2 milliseconds of arc in pole position, and 0.05 to 0.1 milliseconds of time in UT1. In order to observe the relatively large higher frequency variations in UT1, daily 45-minute observing sessions are conducted using the single baseline between Westford and Wettzell. Intercomparison of the UT1 values from the daily and the 5-day series indicates that the accuracy of the daily values is better than 0.1 millisecond of time.

The longer term objectives of the IRIS project include improving the monitoring of Earth orientation by increasing the sampling rate and accuracy of the observations. In April, 1987, the IRIS-P network, with stations in Kashima, Japan, Fairbanks, AK, Ft, Davis, TX, and Richmond, FL began monthly 24-hour observing sessions, and a second series of daily UT1 observing sessions was begun using the stations in Richmond and Bologna, Italy. The additional networks will provide redundancy that will improve the reliability of the system and allow the accuracy of the EOP values to be estimated.

The IRIS UT1 time series provides, for the first time, sufficient accuracy and temporal resolution to look for the few percent increase in k/C caused by the anelastic response of the mantle. Initial results presented here suggest that improved methods of accounting for the dynamics of the oceans and atmosphere may be required before the intertwined variations in UT1 can be fully separated.
Atmospheric Effects on Earth Rotation and Polar Motion
Authors: Salstein David A.
Published: June 2019

The variability in the earth's rotation rate not due to known solid body tides is dominated on time scales of about four years and less by variations in global atmospheric angular momentum (M), as derived from the zonal wind distribution. Among features seen in the length of day (Δl.o.d.) record produced by atmospheric forcing are the strong seasonal cycle, quasi-periodic fluctuations around 40–50 days, and an interannual signal forced by a strong Pacific warming event, known as the El Niño. Momentum variations associated with these time scales arise in different latitudinal regions. Furthermore, winds in the stratosphere make a particularly important contribution to seasonal variability.

Other related topics discussed here are (i) comparisons of the M series from wind fields produced at different weather centers, (ii) the torques that dynamically link the atmosphere and earth, and (iii) longer-term non-atmospheric effects that can be seen upon removal of the atmospheric signal. An interesting application for climatological purposes is the use of historical earth rotation series as a proxy for atmospheric wind variability prior to the era of upper-air data. Lastly, results pertaining to the role of atmospheric pressure systems in exciting rapid polar motion are presented.
On the Rotation Model in VLBI Geophysical Analysis
Authors: Xia Y. F.
Published: June 2019

The basic observables of VLBI for geophysics are the delay and the delay rate. These two basic observables depend on relative geometric relations between the baseline vector and the source vector. The transformation of baseline orientation from the terrestrial coordinate system, in which the stations are located, to the celestial coordinate system, in which VLBI observables are calculated, involves a number of models. One of them is the rotation model.
Effects of Source Structure on Astrometry and Geodesy
Authors: Ulvestad James S.
Published: June 2019

VLBI hybrid maps of a number of radio sources have been used to make calculations of the effects of source structure on group delay and phase delay for baselines of different lengths. The effects on the two types of delay are of comparable magnitude and have a slightly stronger than linear dependence on baseline length. For sources selected to have only modest structure, the r.m.s. delay error caused by structure at 8.4 GHz is on the order of 30 to 50 picoseconds for intercontinental baselines.