bluuebunny/crossref_metadata_2025 huggingface dataset:
Vertical Motion and the Thickness of Hi Disks: Implications for Galactic Mass Models
Authors: van der Kruit P. C. , Shostak G. S.
Published: June 2019

Most studies of the mass distribution in spiral galaxies have been based on the observed rotation curves. A serious ambiguity in this approach has always been that the rotation curve contains in itself no information on the mass distribution in the direction perpendicular to the galactic plane. The usual assumption has been that the mass in late type galaxies is distributed as the light, namely outside the central bulge in a highly flattened disk. In recent years it has been found that the rotation curves decline little or not at all, indicating large increases in the local value of M/L with increasing galactocentric radius (e.g. Bosma and van der Kruit, 1979). On the basis of dynamical arguments involving stability it has been suspected that the material giving rise to the large values of M/L - the “dark matter” - is distributed in the halos of these galaxies, so that the assumption of a flat mass distribution would have to be wrong.
Dynamical Mechanisms for Discrete Unstable Spiral Modes in Galaxies
Authors: Bertin Giuseppe
Published: June 2019

Progress in understanding physical mechanisms for the excitation and maintenance of spiral structure has considerably benefited from investigations of tightly wound spiral density waves (e.g., see Bertin 1980). These studies have identified the existence of four basic kinds of density waves (trailing and leading waves, and in each case short and long waves) with different propagation properties. In addition, they have led to the conclusion that some realistic galaxy models can support self-excited global normal spiral modes. These owe their maintenance to the presence of trailing waves with opposite propagation properties and are excited mostly as a result of a WASER (superreflection) mechanism at corotation. In discussing the dynamics of spiral structure and in comparing theory with observations a number of important issues should be kept in mind (Lin and Bertin 1981). Here we just recall that the calculation of spiral modes is being pursued by many researchers, using different methods. In general the structure and the growth rates of the dominant modes are determined by the radial distributions of the active disk density, the differential rotation, and the dispersion speed through the dimensionless functions εo, j, and Q (Haass, Bertin and Lin 1982).
Spiral Instabilities and Disc Heating
Authors: Carlberg R. G. , Sellwood J. A.
Published: June 2019

Many simulations of disc galaxies exhibit a succession of transient spiral patterns when sufficient halo is included to suppress the global bar instability (Hohl 1970, Hockney and Brownrigg 1974, Sellwood and James 1979). This behaviour causes the velocity dispersion to rise secularly which in turn reduces the intensity of the patterns. Spiral activity usually ceases altogether as Q tends asymptotically to a value somewhat greater than 2.
Conflicts and Directions in Spiral Structure
Authors: Norman C. A.
Published: June 2019

This is a critical and selective review of current theoretical and observational work related to spiral structure in disk galaxies. Productive future areas of research are suggested.
Numerical Experiments on the Response Mechanism of Barred Spirals
Authors: Thielheim K. O. , Wolff H.
Published: June 2019

As a generating mechanism of spiral structure, we have recently studied the driving of density waves in the stellar component of disk galaxies by growing barlike perturbations or oval distortions. Numerical experiments (Thielheim and Wolff 1981, 1982) as well as analytical calculations using the first-order epicyclic approximation (Thielheim 1981; Thielheim and Wolff 1982) have been performed, demonstrating that this mechanism is capable of producing two-armed trailing spiral density waves in disks of noninteracting stars. These regular, global spiral structures are similar to those found in N-body experiments on self-consistent stellar disks that show bar instabilities which are weak enough to allow spiral patterns to persist (e.g., Hohl 1978; Berman and Mark 1979; Sellwood 1981). On account of this similarity, we take the view that the spiral structure observed in N-body experiments is primarily not an effect of the self-gravity of the stellar disk but a response phenomenon, caused by the formation of a weak central bar and its subsequent growth due to angular momentum extraction by interaction with the spiral as described by Lynden-Bell and Kalnajs (1972).
Observations of the Neutral Hydrogen in the Barred Spiral Galaxies NGC 3992 and NGC 4731
Authors: Gottesman S. T. , Ball J. R. , Hunter J. H.
Published: June 2019

We report observations of the atomic hydrogen properties of the barred spiral galaxies NGC 3992 and NGC 4731. These systems were observed in 1980 and 1981 with the VLA telescope of the National Radio Astronomy Observatory. In Table 1 we list the systemic parameters of interest.
X-Ray Line Emission from Supernova Remnants and Models for Nonequilibrium Ionization
Authors: Sarazin Craig L. , Hamilton Andrew J. S. , Chevalier Roger A.
Published: June 2019

An extensive grid of nonequilibrium ionization models for the X-ray spectra of adiabatic supernova remnants (SNRs) is described. The models are compared to the SSS spectra of remnants in the LMC, the Tycho SNR, and SNR 1006. In Tycho, we show that the observed spectrum requires significantly enhanced abundances of Si and S, and that this conclusion is independent of the detailed ionization and thermal structure in the remnant. We find that the SSS spectrum of SNR 1006 can be fit reasonably by a thermal emission model with abundances of about one half solar. In this model, the weak line emission results from the very low ionization state in the remnant, and not because the X-ray emission is non-thermal. We argue that the failure to detect strong Fe line emission in young Type I SNRs poses a severe problem for models of Type I SN, which predict that most of the ejecta be iron. Finally, the results of UV observations of a star behind SNR 1006 are mentioned; these observations show that the remnant contains a large amount of rapidly moving, cold iron.
A Theoretical Model of the Cygnus Loop and its X-ray Emission
Authors: Falle S. A. E. G. , Garlick A. R.
Published: June 2019

In this paper we present a theoretical model of the Cygnus Loop which, with only a small number of free parameters, accounts for most of the gross features of the observations. We believe that none of the models described in the literature can explain the observations in such a simple way.
Kinematics of Supernova Remnants in the Galaxy and LMC
Authors: Shull Peter
Published: June 2019

The optical emission lines of six SNRs have been observed at very high angular and kinematic resolutions. Kinematic ion temperatures were derived, and evidence was found in shocked regions for Maxwellian microturbulence on scales ≤ 0.01 pc, and for non-Maxwellian macroturbulence on scales > 0.1 pc. The widths of shocked regions in the Cygnus Loop and the existence of three types of spectral feature in the LMC remnants are discussed in terms of SNR evolution in cloudy interstellar media.
Supernova Remnants Resembling the Crab Nebula
Authors: Weiler K. W.
Published: June 2019

While reviewing and systematizing the properties of the class of supernova remnants resembling the Crab Nebula it has been found that supernova remnants can be split into three morphological groups – Class S (shells), Class P (plerions), and Class C (combinations) – where the Class C objects appear to represent a new and especially interesting classification. In this overview, the identifying properties of all three classes are defined. Because the large Class S has been studied in detail many times previously, it is not discussed further here. For the smaller Classes P and C, the individual members and suspected members are presented and their properties reviewed. Finally, an origin and evolution for each class is suggested.
Observations of G320.4–1.2 and Speculations on Supernova Remnant Morphology
Authors: Manchester R. N. , Durdin J. M.
Published: June 2019

The galactic radio source G320.4–1.2 (MSH15–52) consists of several components, the most prominent of which is situated in the north-west quadrant and is associated with the Hα nebula RCW89. Caswell et al. (1981) mapped the source at 1.4 GHz with a resolution of 50″ arc and concluded that it was a single supernova remnant with all components having spectral index α ≈ −0.34. This SNR has become more significant with the recent discovery (Seward and Harnden, 1982) of an X-ray pulsar of period 150 ms at the position (1950) R.A. 15h09m59s.5, Dec. −58°56′57″ near the centre of the remnant and the detection of this pulsar at radio frequencies (Manchester et al., 1982). The pulsar has some similarities to the Crab pulsar in that its period derivative is extremely high and hence its characteristic age low, ∼1570 years, comparable to that of the Crab pulsar. Timing observations (Manchester and Durdin, unpublished) indicate that the pulsar is not a member of a binary system and hence that the pulsed X-ray emission is powered by rotational energy, as in the Crab pulsar.
Soft X-ray Observation of Supernova Remnants in the Small Magellanic Cloud
Authors: Inoue H. , Koyama K. , Tanaka Y.
Published: June 2019

The Small and Large Magellanic Clouds (SMC and LMC) are the nearest neighbouring galaxies. Their proximity enables us to investigate these galaxies in X-rays in fair detail.
The e+–e− Annihilation Line and the Cosmic X-Ray Background
Authors: Kazanas Demosthenes , Shafer Richard A.
Published: June 2019

The possibility that the processes responsible for the Cosmic X-ray Background (CXB) would also produce an e−-e+ annihilation feature is examined. Under the assumption that these processes are thermal, the absence of a strong e−-e+ annihilation feature places constraints on the compactness (L/R ratio) of these sources. Observations favor sources of small compactness ratio.
Studies of Faint Field Galaxies
Authors: Ellis Richard S.
Published: June 2019

Although claims are often made that photometric surveys of faint field galaxies reveal evidence for evolution over recent epochs (z<0.6), it has not yet been possible to select a single evolutionary model from comparisons with the data. Magnitude counts are sensitive to evolution but the data is well-mixed in distance because of the width of the luminosity function (LF). Colours can narrow the possibilities but the effects of redshift and morphology can only be separated using many passbands.
Survey of the Bootes Void
Authors: Kirshner R. P. , Oemler A. , Schechter P. L. , Shectman S. A.
Published: June 2019

Radial velocities have been measured for 231 galaxies chosen by apparent magnitude from 282 small fields spanning the area on the sky thought to contain the Bootes void. The galaxy distribution exhibits a spherical volume 6000 km/s in diameter in which no objects are found. The rms velocity difference of close pairs in our sample is less than 180 km/s.
Filaments
Authors: Gott J. Richard
Published: June 2019

I would like to make a brief comment on some work that John Moody, Edwin Turner and I have done on filamentary structure in the Shane-Wirtanen galaxy counts. The picture of the Shane-Wirtanen galaxy counts to the 19th magnitude appears quite filamentary to the eye. The existence of such filaments is quite important theoretically, since filaments could easily be Zeldovich pancakes seen edge-on, and filamentary structure is expected in general when the perturbation spectrum is cut off at small scales. So if the filaments are real, they are telling us something quite important about the universe. The experience of the Martian “canals” reminds us, however, that the eye is very good at picking linear features in random data, and here we are dealing with a clumpy distribution where such effects may be enhanced.
Perturbations in the Universe by Massive “…INOS”
Authors: Bisnovatyi-Kogan G.S. , Lukash V. N. , Novikov I. D.
Published: June 2019

Weak interacting particles (WIPs): neutrinos (νe, νμ, ντ), the hypothetical photino (), the gravitino (), etc., may have nonzero rest mass. This fact is extremely important for cosmology. WIPs do not annihilate in the very early Universe and their number is preserved. If they have a rest mass, their mass density may dominate in the Universe (1).
Phase Transitions of Cosmological Vacuum and Primordial Black Holes
Authors: Kardaskhev N. S. , Novikov I. D.
Published: June 2019

We propose to discuss here the possibility of the origin and existence of primordial black holes and their expected parameters in the scenarios of the early Universe in which vacuum phase transitions play a decisive role.
A Survey of Magellanic Cloud Clusters for RR Lyrae Stars
Authors: Graham J. A. , Nemec J. M.
Published: June 2019

Preliminary results are presented of a search for RR Lyrae variable stars in old clusters in the Magellanic Clouds. RR Lyraes have been found in NGC 1786 and NGC 2210. With the exceptions of NGC 339 (SMC) and NGC 2019 and Hodge 11 (LMC), all clusters classified as type VII by Searle, Wilkinson and Bagnuolo (1980) contain RR Lyrae variables. On the other hand, none have yet been found in clusters of type VI.
RR Lyrae Stars and Novae in the Magellanic Clouds
Authors: Graham J. A.
Published: June 2019

RR Lyrae variable stars and classical novae can be used very effectively for locating and studying the old stellar populations in the Magellanic Clouds. RR Lyrae stars are found in large numbers in both Clouds at about 19m. Apart from the concentrated searches within Cloud clusters, intensive surveys for field variables have so far been made in four areas, each about 1° square. Novae have the advantage of being as bright as 10m at maximum light. They can be detected with small telescopes and surveys have covered almost the entire area of each Cloud. Needs and prospects for future surveys are discussed. Both types of object are suitable for investigating the early chemical composition of the Clouds and its subsequent enrichment with heavy elements. Studies of the old populations in the Clouds are reinforcing the view that, while some stars in the Magellanic Clouds are as old as any in our Galaxy, the major bursts of star formation came along comparatively recently.
Magellanic Cloud Cepheids: Intrinsic and Extrinsic Properties as Inferred from Numerical Simulations of Two-Colour Photometry
Authors: Stift M. J.
Published: June 2019

Controversy is raging over the question which of the relations linking periods, luminosities and colours of classical cepheids should be used to the greater advantage of cosmologists in desperate need of a reliableprimary distance indicator. Adopting a cepheid evolutionary scenario of utmost simplicity, Sandage and Tammann (1969) (=ST) have introduced the concept of an universal period-luminosity-colour (PLC) relation which is expected to minimize the Malmquist bias by virtue of its small intrinsic spread; the PLC relation constitutes the only primary pillar of their controversial distance scale. More recently, Stift (1982) and Clube and Dawe (1983) have shown that there are good reasons to consider a more realistic and hence more complex picture of cepheid evolution and photometric behaviour.
Fine Structure of Photospheric Faculae
Authors: Muller R.
Published: June 2019

Properties of the photospheric bright points associated with magnetic flux tubes are reviewed both in faculae (facular points) and in the photospheric network (network bright points - NBPs) out of active regions. A special attention is given to their size distribution, to their location relative to the granular, mesogranular and supergranular patterns, and to their relation with the small scale magnetic features, both in active and quiet regions. In particular a new granulation movie reveals that NBPs form in large intergranular spaces, compressed by the surrounding granules.

At the center of the solar disk, bright points are much brighter than the mean photosphere; their contrast increases toward the limb up to μ = 0.3 − 0.2 and then decreases to the limb, as it is now widely accepted. But, all the published contrasts are of little significance because of center-to-limb selection effects. New center-to-limb contrast variations of individual network bright points are presented, which take into account the selection effects.
High-Resolution Observations of Emerging Magnetic Fields and Flux Tubes in Active Region Photosphere
Authors: Tarbell T. , Ferguson S. , Frank Z. , Shine R. , Title A. , Topka K. , Scharmer G.
Published: June 2019

On 29 September 1988, filtergrams of the solar photosphere with excellent resolution (0.3 to 0.5 arcsecond) were obtained at the Swedish Solar Observatory on La Palma, Canary Islands. An outstanding 2.5 hour run of digital filtergram observations was obtained, looking at a small area within an active region near disk center. On 6 August 1987, an 80 minute run of similar observations was obtained at the Vacuum Tower Telescope of the National Solar Observatory at Sacramento Peak. Digital and video movies have been made of Dopplergrams, magnetograms, line center, continuum, and white light images. Several examples of magnetic field emergence and formation of flux tubes can be studied in detail in the movies. The relationship between photospheric bright points, “filigree”, the line center brightness, and the magnetic field has been established for individual images in analysis to date.
Comparative Analysis of Physical Conditions in the Solar and Procyon Atmospheres
Authors: Atroshchenko I.N. , Gadun A.S. , Kostik R.I. , Pikalov K.N.
Published: June 2019

An analysis is made of the velocity field determined from spectral absorption line profiles as well as inhomogeneous element sizes obtained from stochastic theory. The results of three-dimensional simulations of convective motions in the photospheres of the Sun and Procyon are presented. It is concluded that penetrative convection covers a considerable fraction of the Procyon photosphere.
Dust Formation in WC-Star Shells
Authors: Sedlmayr E. , Gass H.
Published: June 2019

The conditions of dust formation in the winds of WC-stars are discussed. Simple analytical estimations for the existence of neutral carbon zones where carbon nucleation might occur are given. It is shown that efficient dust formation can take place in the CI region if the effective temperature of the star is below ~ 25000 K and the density at the base of the wind is larger than ~ 1011 cm–3.
Abundances in WR Nebulae
Authors: Vilchez J.M. , Esteban C.
Published: June 2019

Within an ongoing program of long slit spectroscopy of the known WR nebulae, we show here some results for eight objects of the sample selected from the master list of Chu et al. (1983). We present an analysis of their ionisation structure and determine abundances of oxygen, nitrogen and helium in as many positions as possible. The implications of the abundance results for the chemical enrichment of the Interstellar Medium and stellar nucleosynthesis are briefly reviewed.
Recurrent Dust Formation by Wolf-Rayet Stars
Authors: Williams P.M. , Van Der Hucht K.A. , Thé P.S. , Bouchet P. , Roberts G.
Published: June 2019

A number of Wolf-Rayet stars show variations of up to a factor of ten in their infrared emission on timescales of months to years while their photospheric luminosities remain unchanged. This can be interpreted in terms of variation in the rates at which dust grains form in their stellar winds. Our data show variable circumstellar dust emission from WR 70 (HD 137603), with an episode of enhanced dust formation in early 1989, and fading of emission by dust formed around WR 48a (in 1979) and WR 19 some time before our first observation in 1988. We consider their relation to WR 140 (HD 193793), which forms dust in its wind for a few months at intervals of 7.94 years.
R127 and its Surrounding Star Cluster
Authors: Nota Antonella , Leitherer Claus , Clampin Mark , Gilmozzi Roberto
Published: June 2019

The LMC star R127 (=HDE 269858) has originally been classified as a late WN type (Walborn 1977). Since then, R127 has developed from spectral type B to A (Wolf et al. 1988) and is now categorized as a Luminous Blue Variable. R127 is embedded in gaseous material and a star cluster which are barely resolvable even on the highest-quality images available. Stahl (1987) studied the circumstellar material using narrow-band CCD images centered on nebular emission lines. No systematic census of the stars in the immediate vicinity of R127 has yet been published.
Absolute Magnitudes and Colours of Field Giant Branch Stars
Authors: Neckel H. , Klawitter P.
Published: June 2019

Concerning the absolute magnitudes of the giant stars contained in the Mt. Wilson catalogue of Adams et al., the following facts are pointed out:

(1)

There is a linear relation between the Mt. Wilson magnitudes and the absolute magnitudes derived on account of the Wilson-Bappu effect (Figure 1). The scale factor is 2.3.

(2)

The accuracy of the properly scaled Mt. Wilson magnitudes follows from the scatter in relation (1) and the accuracy of the Wilson-Bappu magnitudes (± 0.6). It is ± 0m.6 (mean error).

(3)

The assured reliability of the corrected Mt. Wilson magnitudes allows a subdivision into bright, intermediate and faint giants (Figure 2). The 3 subgroups exhibit different 2-colour relations (Figures 3, 4, 5).

(4)

The large number of giant stars present in the Mt. Wilson catalogue allows a trustworthy derivation of the mean absolute magnitudes of giant branch stars (Figure 6) and an estimate of its dispersion as well (Figure 7).
On the Kline Width-Absolute Magnitude Relation
Authors: Bappu M. K. V.
Published: June 2019

Bright fine mottles seen on the solar disk are identified as the agency that locates the Sun on the K-line width-absolute magnitude relation of Wilson and Bappu. The contribution of the supergranular network emission and the plage emission from centres of activity tends to upset the uniqueness of the relation. The line width-absolute magnitude relation is thus a characteristic of the convection zone underlying the stellar atmosphere.
The Use of Model Atmospheres for Temperature-Gravity Calibration
Authors: Pecker J. C.
Published: June 2019

Regardless of the degree of elaboration of series of models, just how can they be used for calibration purposes? And how much is this calibration sensitive to the quality of the model theory? These two questions are the basis of our discussion, which covers : I – The general principles of the use of model atmospheres in stellar calibration (1 – The two dimensional classifications; 2 – The use of the total luminosity; 3 – The cases of Vega and Sirius; 4 – The calibration of ST – Teff relation); II – The failures of the two parameters model atmospheres (1 – The observational need for more-than-two-parameters classification; 2 – The abundance of elements, the line formation, and the model atmospheres; 3 – Various sources of unadequacy of models; 4 – Envelopes or shell features; their influence on model-building; 5 – The case of HD 45677. Diagnostic of early-type stars; 6 – Various unexplained spectral features); III – The present state of the model factory (1 – The classical models; 2 – New concepts in the description of a stellar atmosphere; 3 – New approaches in model making; 4 – Conclusions).
Monolithic Si Bolometer Arrays: Detectors for Far Infrared and Submillimeter Detection
Authors: Moseley Harvey
Published: June 2019

The improvement of photoconductors and photovoltaic detectors for λ < 200 μm has displaced bolometers as detectors of choice for many applications requiring high sensitivity. Continued development of bolometers for operation at low temperatures (T < 0.1 K) has resulted in significant improvements in their sensitivity, making them excellent choices for many broad band applications at λ > 200 μm, cryogenic spectrometer applications in the submillimeter, and applications requiring extreme stability and ease of calibration.

I will describe the development of bolometers over the past decade, with particular emphasis on the detectors we have developed at GSFC. Detectors have been constructed in 36 element arrays with NEP ∼ 5 × 10−18 W/√Hz and a response time of ten ms. Such detectors are very useful for many current problems in Cosmic Microwave Background studies and submillimeter spectroscopy. We will describe the current state of development of the detectors and the improvements we are pursuing.

There has been significant progress in the development of superconducting tunnel junction detectors for operation in the far infrared and submillimeter spectral range. Though at an early stage, these detectors have great promise of excellent sensitivity and ease of array fabrication. I will discuss the current developments in this area.
PtSi IR Array in Mosaic Configuration
Authors: Ueno Munetaka , Tsumuraya Fumiaki , Chikada Yoshihiro
Published: June 2019

The rapid progress in the focal plane technology enables us to use large format infrared sensors, such as 256 × 256 InSb/HgCdTe and 1040 × 1040 PtSi arrays. Infrared two-dimensional sensors make possible not only the imaging observations but a deep detection limit. The development of a large format infrared array is one of the most important breakthroughs in observational astronomy.

We propose to build a mosaic infrared camera for the SUBARU 8-m telescope. The SUBARU telescope is designed to reach a diffraction limited image at infrared wavelengths with a wide field of view (six arcsec at the Cassegrain focus). The camera is designed to cover the entire field of view with PtSi infrared sensors and to employ a weighted shift-and-add operation and a real-time image processing. The efficiency of the mosaic infrared camera and power of the 8-m telescope have a strong potential to meet challenging problems. Most of the regions of the near infrared sky are not covered with enough sensitivity. It is essential to conduct infrared deep and wide surveys.
The Large-Field Bright-Star High-Precision CCD Photometer of BAO
Authors: Jiang Shiyang
Published: June 2019

Time-series high-accuracy photometry is very important for research in stellar variability. For a long time photometry made by a photomultiplier was the only instrument for high precision stellar photometry. To overcome the atmospheric variation and instrumental problems, we must choose at least one stable star as a comparison star and move the telescope quickly between the targets. So the real efficiency is very low and one only can do it on photometric nights. To overcome this limitation, since 1989 we began to cooperate with the team of the STEPHI network. We used the Chevreton four-channel photometer which can observe the variable, two comparison stars and a chosen sky background simultaneously. The multi-channel photometer is much better than normal single channel photometer as we can see from the several STEPHI results. Now the very high quantum efficiency CCD becomes more and more popular, so we are trying to change to use CCDs. Here we give some general description of a large field high accuracy bright star CCD photometer being prepared for the Beijing Astronomical Observatory (BAO).
A Unified Treatment of the Filament and Flare Instabilities
Authors: van Hoven Gerard
Published: June 2019

Filaments and flares occur in sheared magnetic structures as a result of radiative cooling and resistive reconnection, respectively. A new integrated theory of these two unstable processes is described, which includes the relevant effects of magnetohydrodynamics and energy transport. The normally dissociated thermal and tearing phenomena are coupled together by a temperature–dependent Coulomb resistivity. As a result, the filamentation and flaring instabilities of a sheared field may coexist, as is familiar from the solar example.

The growth rates and spatial structures of these two modes are detailed here. The much faster radiative instability is shown to provide significant magnetic reconnection, particularly at shorter wavelengths. The long–wavelength reconnection mode is found to be abetted by the resistivity increase caused by the dominance of cooling at the X point, in contrast to its nonradiative behavior. Implications of these results for the development of coronal activity are described.
Velocity Shear Instabilities in the Anisotropic Solar Wind
Authors: Migliuolo Stefano
Published: June 2019

The linear and quasilinear theory of perturbations in finite-β (β is the ratio of plasma pressure to magnetic energy density), collisionless plasmas, that have sheared (velocity) flows, is developed. A simple, one-dimensional magnetic field geometry is assumed to adequately represent solar wind conditions near the sun (i.e., at R ≃ 0.3 AU). Two modes are examined in detail: an ion-acoustic mode (finite-β stabilized) and a compressional Alfven mode (finite-β threshold, high-β stabilization). The role played by equilibrium temperature anisotropies, in the linear stability of these modes, is also presented. From the quasilinear theory, two results are obtained. First, the feedback of these waves on the state of the wind is such as to heat (cool) the ions in the direction perpendicular (parallel) to the equilibrium magnetic field. The opposite effect is found for the electrons. This is in qualitative agreement with the observed anisotropies of ions and electrons, in fast solar wind streams. Second, these quasilinear temperature changes are shown to result in a quasilinear growth rate that is lower than the linear growth rate, suggesting saturation of these instabilities.
A New Technique for the Analytical Determination of a Fundamental System of Positions and Proper Motions
Authors: Schwan Heiner
Published: June 2019

A survey of the various tasks involved in the construction of the Fifth Fundamental Catalogue (FK5) is given. One of these tasks is the determination of the FK5 system which will be performed with the aid of new analytical methods developed at Heidelberg. The basic characteristics of these methods are described, and information on the most significant errors in the FK4 system is given.
Application de la Methode par Deconvolution Integree Au Cas des Occultations D'etoiles Doubles
Authors: Froeschle M. , Meyer C.
Published: June 2019

The most usual method for the determination of the angular separation ρ and the magnitude difference Δm of the two components is the direct comparison of the observed diffraction pattern with a theoretical model. However, it is possible to use a deconvolution process based, for a single star, on the equation I′(x) = F′(x) * O(x), involving both the derivative of the Fresnel diffraction function for a point-like source, and that of the observed intensity. O(x) is the brightness distribution function of the occulted source.

We describe here a new method, using the integrated deconvolution process (Froeschlá, Meyer, 1983), in which we determine the variations of the apparent surface of the two sources of a double star while it is being occulted. The two sources are supposed to be of the same diameter. This method has been applied to several cases of theoretical diffraction curves with different values of ρ and Δm. Noise has been added and we have studied the effect of the signal to noise ratio on determination of the parameters ρ and Δm of the pair. A good accuracy for Δm is obtained for |Δm| ≦ 1 and noise ≦ 10%. The determination of ρ is well achieved for noise level reaching 15 %. The technique has been at last applied to the observation of the occultation of SAO 95166 made by Africano et al.(1977) leading to nearly similar results.
Timing and Data Acquisition System for a Field Astrolabe
Authors: Lukac Carl F. , Wheeler Paul J. , Keating Richard E. , Clarke Randolph T.
Published: June 2019

The Danjon Astrolabe at the Naval Observatory has been traditionally used to determine Universal Time and improve the systematic accuracy of star positions. During the past year, it has been used to determine latitude and longitude at remotely scattered sites for geodetic purposes. Operating this instrument away from the Observatory necessitated a mobile support and timing system rugged enough to operate dependably in ever changing, and sometimes harsh, environmental conditions.

This paper describes the performance of the astrolabe timing and data acquisition system, gives engineering design considerations, and describes the equipment and instrumentation.
Connection Between the HIPPARCOS Catalogue and the FK5
Authors: Röser S.
Published: June 2019

Two methods are presented which can be applied to determine the rotation of the Hipparcos system of stars with respect to the FK5. These methods are presently tested in numerical simulation runs.
Milliarcsecond Polarization Measurements
Authors: Roberts David H. , Potash Robert I. , Wardle John F. C. , Rogers Alan E. E. , Burke Bernard F.
Published: June 2019

Linear polarization measurements at milliarcsecond resolution have been made at λ6 cm using four stations of the U. S. VLBI Network and the Mark III recording system. Calibration of the cross-polarization contamination of the feeds was done using the cross- to parallel-hand fringe ratios of the essentially unpolarized source 0Q 208, and is believed accurate to 0.5%. Other sources, such as AO 0235+164 and 0106+013, which are nearly unresolved in the unpolarized fringes, show clear signs of resolution in their polarized fringes. A preliminary map of the BL Lacertae object OJ 287 is compatible with a weakly-polarized optically-thick core and a highly-polarized optically-thin jet about 5 mas in length.
Superluminal Motion in BL Lacertae: 10.65 GHz VLBI observations from 1981.7 to 1982.7
Authors: Mutel R. L. , Phillips R. B.
Published: June 2019

We report here VLBI observations at four epochs of the source BL Lacertae made with the U.S. VLBI Network (plus Bonn) at 10.65 GHz which show clear evidence of uniform superluminal expansion at v/c = 4.4 ± 0.2 (H0 = 55 km s−1 mpc−1). The maps, which were made every three months from 1981.76 to 1982.75 (Figures 1, 2), were part of an ongoing program of VLBI observations of BL Lacertae at 5 and 10.65 GHz starting in 1980.4, near the beginning of a series of violent flux outbursts (Aller, Hodge, and Aller, 1983; Aller and Aller, 1984). Previous results from earlier epochs have been reported by Mutel, Aller, and Phillips (1981) and Phillips and Mutel (1982).
Dynamic Spectra of Low-Frequency Variables
Authors: Dennison Brian , Broderick J. J. , O'Dell S. L. , Mitchell K. J. , Altschuler D. R. , Payne H. E. , Condon J. J.
Published: June 2019

A wide range of models have been proposed to account for low-frequency (≲ 1 GHz) variability in cosmologically-distant radio sources. To address these models we (Payne et al. 1982) are monitoring the 0.3–1.4 GHz spectra of such sources. Results from three years of monitoring indicate that two distinct types of low-frequency variables may exist.
The Galactic Center
Authors: Lo K. Y.
Published: June 2019

The center of our Galaxy contains an extremely compact nonthermal radio source. For the first time, elongation in the source structure has been detected. The long axis is nearly aligned with the minor axis of the Galaxy. Recent high resolution observations of the ionized gas within the central 3 parsecs suggest that matter may be falling in towards the center. This has interesting implications on the processes within our Galactic nucleus.
Mark III VLBI Observation of Pulsars
Authors: Bartel N. , Cappallo R. J. , Ratner M. I. , Rogers A.E.E. , Shapiro I. I. , Whitney A. R.
Published: June 2019

Mark III VLBI observations of the pulsars PSR 0329+54 and PSR 1133+16 were made at 2.3 GHz using antennas with diameters and locations as follows: 100m, Effelsberg, West Germany (but only for SPR 0329+54); 43m Green Bank, WV, USA; and 40m, Big Pine, CA, USA. The Mark III processor at the Haystack Observatory was “gated” to compute visibility amplitudes and phases as a function of pulsar longitude. This method allowed a) an improvement of the signal to noise ration, by as much as a factor of ten in the case of PSR 1133+16, and b) an interferometric investigation of the pulse structure.
Anomalous Ejection in SS433
Authors: Spencer R. E. , Waggett P.
Published: June 2019

A series of MERLIN observations at λ6 cm of the peculiar object SS433 were made during the spring/summer of 1982. The maps obtained by hybrid mapping show that an elongated structure was formed, extending to ~1 arcsec in length before evolving into two symmetrically placed knots of radio emission straddling a central unresolved core. The structures can be compared with the loci of radio emission expected on the twin jet model for SS433. The knots C and D clearly visible on the map of 820514 (= JD2445104, Figure 1) fit the locus well and were ejected on JD2445085 ±2 at the same time as a strong outburst in the X band total power (Johnston et al. 1983). Knots A and B were ejected earlier but do not lie on the expected locus, even if allowance for the nodding motions proposed by Katz et al. 1982 is made.
The Use and Abuse of Standard Stars
Authors: Garrison R. F.
Published: June 2019

Most systems of classification or quantitative measurement depend on standards. It is of the greatest importance for the user to be aware of the mandate of a particular system. If the mandate is not understood by casual users, a system can be either underutilized or abused.

In the particular case of the MK system of spectral classification, types are defined by the standard stars. They can be calibrated, and the calibration may evolve with time, but the types are relatively stable because they are defined by the standards. The autonomy of this powerful system is crucial to its success, but some astronomers do not understand the importance of this distinction. Recent suggestions to change the spectral type of the Sun show an ignorance of the way the system works.

Precautions in the use of standard stars and the frequency of their use depend on the particular system and on its mandate.
The Influence of an Outflowing Gaseous Stream on the Determination of Masses of Semidetached Binary Systems
Authors: Chochol D. , Vittone A.
Published: June 2019

A general model of gaseous streams in semidetached systems is proposed from the study of the eclipsing binary symbiotic stars CI Cyg and V 1329 Cyg. The influence of gaseous streams on the determination of the masses of semidetached systems is shown.
The Determination of Extinction and Temperature for the Central Star of the Planetary Nebula NGC 40
Authors: Bianchi L. , Grewing M.
Published: June 2019

NGC 40 is an extended, very inhomogeneous planetary nebula, with a WC8 central star. Very discrepant determinations exist in the literature for the extinction towards this object and the temperature of its nucleus. We review here the various methods which can be used to derive these quantities, discussing the assumptions underlying each method and their inherent limitatons, and the uncertainties that arise when they are applied to this particular object. The results are compared to the values which we derive from far UV spectrophotometry: E(B-V) = 0.50 and T* ≍ 90000K.
Molecular Clouds at High z
Authors: Blitz Leo
Published: June 2019

The evidence for the existence of molecular clouds at large distances from the Galactic plane is reviewed. The molecular clouds at high Galactic latitudes are shown to be largely confined to the Galactic plane. There is evidence for one giant molecular cloud as much as four scale heights from the Galactic plane, but given the sample size from which the cloud is drawn, it is reasonable to suppose that it is part of the tail of the thin disk population. There is weak evidence that one star-forming molecular cloud may have originated in the Galactic halo. On the basis of kinematic evidence however, it is shown that there are three molecular clouds identified at high galactic latitude that, if not at high z, are likely to have resulted from interaction with gas in the halo. Understanding how these clouds have formed is likely to be an important key to understanding how the halo interacts with the disk gas.
Galactic Worms
Authors: Koo B.C. , Heiles C. , Reach W. T.
Published: June 2019

We have found and cataloged over 100 vertical structures in H I, infrared, and radio continuum emission. These correspond to the H I worms detected by Heiles (1984). The infrared and the radio continuum properties of worms suggest that some worms have associated ionized gas. The area filling factor of superbubbles in the inner Galaxy is estimated to be greater than ~0.1.
Grain Evolution in the Framework of Disk-Halo Interactions
Authors: Ferrini F.
Published: June 2019

The presence of dust grains at high galactic latitude as well as in the Halo of external galaxies has received substantial observative support in the last few years. Besides intense hydrodynamics stirring phenomena, like supernovae expanding shells, stellar winds and Galactic fountains, the removal of dust from the Disk can be ascribed to the global galactic radiation field. The continuous sputtering in the hot Halo gas may explain the large scale height found for refractory elements (Edgar and Savage, 1989).
Nonlinear Evolution of the Parker Instability
Authors: Matsumoto R. , Shibata K.
Published: June 2019

Two-dimensional MHD simulations are performed to study the nonlinear evolution of the Parker instability in galactic gas disks. When the most unstable mode grows, magnetic field lines kink across the equatorial plane of the disk and thin spur-like structures are formed above dense regions in magnetic pockets. In low β (= ρgas/ρmag < 3) disks, shock waves are produced at the footpoint of magnetic loops, while in high β (> 3) disks, nonlinear oscillations are excited and the loop length increases with time up to λc ≃ (3.5β + 6)H, where H is the half-thickness of the disk.
Chaotic Layers in Resonance Problems
Authors: Henrard Jacques , Moons Michèle , Morbidelli Alessandro
Published: June 2019

The recent numerical simulations of Tittemore and Wisdom (1988,1989,1990) and Dermottet al.(1988), Malhotra and Dermott (1990) concerning the tidal evolution through resonances of some pairs of Uranian satellites have revealed interesting dynamical phenomena related to the interactions between close-by resonances. These interactions produce chaotic layers and strong secondary resonances. The slow evolution of the satellite orbits in this dynamical lanscape is responsible for temporary capture into resonance, enhancement of eccentricity or inclination and subsequent escape from resonance. The present contribution aims at developing analytical tools for predicting the location and size of chaotic layers and secondary resonances. The problem of the 1:3 inclination resonance between Miranda and Umbriel is analysed.
Comet Showers
Authors: Fernández Julio A.
Published: June 2019

Variations in the influx rate of incoming Oort cloud comets, leading to the occurrence of comet showers, are reviewed with special emphasis on the dynamical processes that produce them. It is found that comet showers as intense as 10-100 times the background comet flux may occur at average intervals of several 107 years, being very close stellar passages at distances ≈ 104AU and, perhaps, penetrating encounters with intermediate-size molecular clouds the trigger mechanisms. Penetrating encounters with giant molecular clouds and stellar passages at distances of a few 103AU may trigger comet showers about 102 − 103 times more intense than the background comet flux at average intervals of a few 108 years. The impact cratering record on the Earth does not show any clear evidence of past comet showers. Some orbital properties of new and dynamically young comets suggest that the influx rate of Oort cloud comets is currently near its background level.
Dynamics of Comets
Authors: Carusi A. , Valsecchi G.B.
Published: June 2019

The gravitational processes affecting the dynamics of comets are reviewed. At great distances from the Sun the motion of comets is primarily affected by the vertical component of the galactic field, as well as by encounters with stars and giant molecular clouds. When comets move in the region of the planets, encounters with these can strongly affect their motion. A good fraction of all periodic comets spend some time in temporary libration about mean motion resonances with Jupiter; some comets can be captured by this planet as temporary satellites. Finally, there is a small number of objects with orbital characteristics quite different from those of all other short-period comets.
Morphology and Kinematics of Planetary Nebulae
Authors: Reay N. K.
Published: June 2019

The application of monochromatic images and internal kinematic data to the construction of 3-dimensional models for the HII regions of planetary nebulae is discussed, and the role of models in investigating envelope evolution is commented upon.
High Resolution Maps with the VLA
Authors: Bignell R. C.
Published: June 2019

The Very Large Array is a radio picture making instrument operated by the National Radio Astronomy Observatory in Socorro, New Mexico. The array, which has been in full time operation for more than 2 1/2 years, operates at four main wavelengths, 1.3, 2.0, 6.0 and 20 cm with achievable resolutions of .05, .08, .25 and 0.8 arc seconds respectively.
Observations of Dust in Planetary Nebulae
Authors: Barlow M. J.
Published: June 2019

This review summarises those observations which relate to the existence of dust in planetary nebulae. The most direct evidence for dust, namely infrared emission by grains, is considered in Section 2. Section 3 describes a variety of optical and ultraviolet observations which bear more indirectly on the properties of the dust, while Section 4 discusses the power sources of the observed infrared emission. The peculiar nebulae A 30 and A 78 are considered in Section 5, and Section 6 concludes with a discussion of the various infrared features which have been observed in emission from planetary nebulae.
Radiative Transfer Problems in Planetary Nebulae
Authors: Hummer David G.
Published: June 2019

In view of the enormous importance of the UV observations of planetary nebulae made possible by the IUE, this review will concentrate primarily on the formation of resonance lines in nebulae; an important special case is that of He II Lyα and its role in the Bowen mechanism. Special attention is given to the effects of dust on the line and continuum formation.
The Effects of Mass and Metallicity upon PN Formation
Authors: Papp K. A. , Purton C. R. , Kwok S.
Published: June 2019

We construct a parameterized function which describes the possible dependence of planetary nebulae formation upon metal abundance and stellar mass. Data on galaxies in the local group compared with predictions made from the parameterized function indicate that heavy element abundance is the principal agent influencing the formation of planetary nebulae; stars which are rich in heavy elements are the progenitors of planetary nebulae. Our analysis, when compared with the observations, argues for a modest degree of pre-enrichment in a few of the sample galaxies. The heavy element dependence of planetary nebulae formation also accounts for the deficit of planetary nebulae in the nuclei of NGC 221 and NGC 224, and in the bulge of our galaxy.
Optical and UV Nebular Spectra of NGC 40
Authors: Peimbert M. , Torres-Peimbert S. , Clegg R.E.S. , Seaton M.J.
Published: June 2019

The planetary nebula NGC 40 has a WC8 central star. From analysis of IUE spectra of the star, Benvenuti, Perinotto and Willis (1981, IAU Symposium 99) obtain the remarkable abundance ratio C/He ~ 0.2 and probably no hydrogen. The question arises as to whether the nebular shell has abnormal abundances.
Near-Infrared Sources in the Complex H II Region NGC 6357
Authors: Persi P. , Ferrari-Toniolo M. , Roth M. , Tapia M.
Published: June 2019

We have searched for the near-infrared emission from selected areas of the star forming region NGC 6357. Six fields of 90x90 arcsec2 centered on the far-IR peaks G353.19+0.91, G353.22+0.67, G353.13+0.64, G353.05+0.56, on the 6-cm radio continuum peak G353.035+0.78, and on the water maser source H20353.27+0.64, were scanned through the K(2.2 μm) filter at the 2.1-m telescope of the Observatorio Astronómico Nacional at S. Pedro Mártir, Baja California, México.