bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Determination of Angular Diameters of Stars
Authors: Davis John 
Published: March 2016

Ideally the determination of the angular diameter of a star would include the measurement of the distribution of intensity across the stellar disc. However, direct methods of measuring angular diameters have so far lacked adequate ‘signal to noise’ ratio to measure the intensity distribution and it has been the custom, in the first instance, to express the measured angular diameter in terms of the angular diameter of the equivalent uniform disc (θUD). Subsequent use of the angular diameter involves the assumption of a limb-darkening law and the application of an appropriate correction to θUD to find the ‘true’ angular diameter (θLD) of the star (e.g. Hanbury Brown et al., 1967). In this article we will discuss the determination of θUD for single stars and we will not refer further to the more difficult problems of determining intensity distributions involving limb-darkening and rotational effects and of measuring the angular parameters of binary systems.By itself the angular diameter of a star has no intrinsic value but when it is combined with other observational data it enables basic physical properties of the star to be determined. It is then possible to make a direct comparison of the observed properties of the star with the predictions of theoretical models of stellar atmospheres and interiors. For example, the combination of an angular diameter with the absolute monochromatic flux received from the star (ƒν), corrected for interstellar extinction, yields the absolute emergent flux at the stellar surface (). If the spectral energy distribution for the star is known it can be calibrated absolutely by and hence the effective temperature (Te) of the star can be found (this is equivalent to knowing the bolometric correction for the star and using it with the angular diameter to find Te). In addition to leading to the determination of Te, the absolute surface flux distribution may be compared directly with the predicted flux distributions for theoretical model stellar atmospheres (e.g. Davis and Webb, 1970). For O and early B. type stars a large fraction of the emergent flux is in the far ultra-violet and the effective temperatures cannot be determined from the, at present, incomplete empirical flux curves. In these cases it is possible to obtain an estimate of the effective temperatures by using the values of to calibrate a grid of model atmospheres which have Te as a parameter. In this way, by measuring the angular diameters of stars of different spectral types, it is possible to establish an effective temperature scale.
The Occurrence of Lithium in Stars
Authors: Herbig G. H. 
Published: March 2016

My task is to outline our present observational knowledge of the occurrence of lithium in stellar atmospheres. On account of the limited time, I shall not attempt to include a description of the situation in post-main sequence stars. Also for shortness of time, forgive me if I do not stop at each point to give due credit to the astronomers who have contributed to that topic.The youngest stars we know which are cool enough to exhibit neutral Li are the T Tauri stars, which are in the early stages of contraction toward the main sequence. All these objects that have been adequately observed are very abundant in Li: the range is between 50 and 400 times the so-called ‘solar abundance’, a convenient unit which corresponds to a H/Li ratio by number of atoms of about 1011. It is significant that the average Li content of chondritic meteorites, which are often considered to be samples of the non-volatile, unprocessed material of the original solar nebula, is about 150 on this system, within the range observed in the TTauri stars.
Precession and Galactic Rotation on the Basis of Various Proper Motion Systems
Authors: Fricke W. 
Published: March 2016

Previous determinations of precession on the basis of proper motions resulted in corrections to Newcomb’s value of lunisolar precession in the interval per century. From a compilation of several determinations and their reduction to the average of the systems of FK3 and N30 Morgan and Oort (1951) obtained:per century, where A and B are Oort’s constants. Values in the system of FK4 were derived by Bošniakovich (1965), who applied differential corrections to previous results on the basis of the systematic differences FK4–FK3.
Relative Star Positions from Overlapping Photographic Plates
Authors: Clube S. V. M. 
Published: March 2016

An account is given of an investigation to obtain accurate relative star positions from overlapping photographic plates. The method described is quite general, but in the first instance is to be applied to plate material obtained with the new Cape astrometric camera.The Cape 4-component f/10 20-cm astrometric camera produces a field of 5° square with a scale of 100″ per mm in the focal plane. The telescope is fitted with a yellow filter 12 mm from the focal plane, which in combination with the Ilford-R40 emulsion used, transmits more or less uniformly in the range 5300<λ<6500 and little elsewhere. Within this spectral range, the maximum longitudinal aberration is only 0·75 mm and the images sensibly round out to the edge of the plate. The instrument is currently being used to obtain a complete coverage of the Southern hemisphere down to mv~11·5 such that each plate, effectively 4° square, has overlapping plates centred at each corner and at the middle of each side. In practice, this system of overlapping is applied to a number of zones in declination each about 10° broad. Each plate is taken within 30m of the meridian and is given two 3m exposures above and below the 1950 standard positions, the geometrical plate centre being estimated to be within 20″ of the tangential point. At the time of writing, coverage of the zone −30° to −52° is almost complete, and preparations for the zone 0° to −30° well advanced.
Asymmetry in Solar Spectral Lines
Authors: Magnan C. , Pecker J. C. 
Published: March 2016

AbstractAfter reviewing observations of the spectral solar features originated either in the chromospheric layers or in the photospheric layers, from the point of view of the observations, and after having shown the strikingly discrepant set of interpretations that can be found currently in literature, a numerical experiment is performed in a case not too different from the solar case. It is shown that the use of the line bisector to determine, from the asymmetry of a single line, the trend of the velocity field might be considerably misleading, a fact which explains partly the results published in literature.
Properties of Radio Sources in Clusters of Galaxies
Authors: Harris Daniel E. 
Published: March 2016

“The properties of radio sources in clusters” presupposes that we know something about radio sources out of clusters, or that we even know whether a radio source is “in” or “out” of a cluster. Thus we are faced with the problem of defining what we mean by a cluster. Most of us use Abell’s catalogue of RICH clusters and assume that we are really “in” a cluster. However, most radio sources are identified with faint, distant objects and it is often difficult to know whether the remark “galaxy in a group” or “galaxy in a cluster” indicates a cluster such as the Coma Cluster, a cluster similar to an “open” Zwicky cluster, or a group of galaxies which may be gravitationally bound.This uncertainty must not be forgotten, and in the following discussion, we will try to limit the effects of this by concentrating on catalogued clusters; ignoring most distant radio galaxies, many of which may be in rich clusters; and also by neglecting quasars, some or all of which may be in clusters.
Spectral Classification with Objective-Prism Spectra from Skylab
Authors: Henize K. G. , Parsons S. B. , Wray J. D. , Benedict G. F. 
Published: March 2016

During the Skylab I, II and III missions, ultraviolet spectra were obtained in 188 fields with a. 15 cm aperture objective-prism spectrograph. The instrument has been described by Henize, Wray, Parsons, Benedict, Bruhweiler, Rybski and O’Callaghan (1975; hereafter referred to as Paper I).The spectra cover the wavelength region from 1300 to 5000 Å and have a resolution of 2 Å at 1400 Å and 12 Å at 2000 Å. Absorption and/or emission lines of C II λ 1335, Si IV λ 1394, 1403 and C IV λ 1549 are visible in more than one hundred stars. The lines of Si IV and C IV are found to be particularly sensitive to stellar temperature and luminosity. Since these lines are visible in spectra of moderate to low resolution it is clear that they should be of special interest in any UV classification system for faint stars. This paper investigates the correlation of the intensities of the C IV and Si IV lines with MK spectral type, and presents a preliminary classification scheme for 04 to B2 stars based on these lines.
Atomic and Molecular Gas in the Inner Regions of the Milky Way and Other Galaxies
Authors: Liszt H. S. 
Published: March 2016

It seems likely that we inhabit a galactic system which in its gross characteristics exemplifies the present meaning of the word “normal”. The general framework for interpreting the distribution and kinematics of the observed gas consists of a well-defined rotation curve, applicable over the great majority of a thin, nearly flat disk; the stars and gas within this disk are generally quite cold, having little energy in random as opposed to well-ordered motions. In fact, neutral gas in the galactic disk is sufficiently regular in its structure that no grand pattern of spiral arms stands out in sharp relief.As one moves inward in the Galaxy from the location of the Sun, this regularity persists to within about 4 kpc of the center, at which distance the abundance of gas in either atomic or molecular form drops fairly abruptly. Inside this region, the character of the gas distribution undergoes a marked change and the behaviour observed there, while it may be common to even the best-ordered systems, represents a rather spectacular departure from the organization at larger galacto-centric radii. As the gas abundance increases once more toward the galactic center it is seen to reside in a multitude of individual features having disparate spatial and kinematic properties. Many of these have large and/or obvious components of non-circular motion, usually taken as indicating expulsion of matter from the galactic center region at both large and small angles with respect to the rotation axis of the galaxy at large. Others, while occurring at velocities whose sign is consistent with rotation motion alone, have contradictory changes in velocity with position. Line profiles taken in the inner regions of the Galaxy are generally decomposed according to catalogs of classifiable features but no generally accepted interpretative framework exists as a means of relating the wide variety of observable phenomena.
Kinematics of Nuclei of Sc Galaxies
Authors: Rubin V. C. 
Published: March 2016

For a sample of 21 Sc galaxies with a wide range of luminosities, of radii, and of masses, W. K. Ford and I have obtained spectra and determined rotation curves. By their kinematical behavior in their central regions, the Sc’s can be separated into two groups. Some galaxies, generally small and of low luminosity, have shallow central velocity gradients, reflecting their low central masses and densities. Other galaxies, most often large ones of high luminosity, have steep central velocity gradients. One reason this separation by central velocity gradients is of interest is because these galaxies exhibit other significant spectral differences which go hand-in-hand with the kinematical differences.The small, low luminosity galaxies show emission lines of Hα and [NII], with nuclear Ha sharp and stronger than [NII], and little or no stellar nuclear continuum, just as conventional HII regions. In contrast, the high luminosity galaxies show broad nuclear emission, with [NII] stronger than Ha. These galaxies have a strong red stellar continuum, arising from a red stellar population. The cause of the Hα[NII] intensity reversal in the nuclei of some galaxies remains unknown. However, the strong [NII] emission in generally high luminosity galaxies with massive nuclei, nuclei which show strong red continua, suggests that [Nil] intensity correlates with nuclear luminosity, and in turn with the density and velocity properties of the nuclear populations. We would expect high velocity dispersions and high bulge luminosities for galaxies with strong nuclear [NII] and steep central velocity gradients.
Theoretical Considerations on the Dynamics of Normal Galactic Nuclei
Authors: Sanders Robert H. 
Published: March 2016

I want to discuss the origin of non-circular gas motions observed in the nuclei of normal spiral galaxies and the possibility that recurring violent activity in normal nuclei excites such motion. But first, let us review several basic aspects of the nearest normal galactic nucleus — the nucleus of our own Galaxy.The rotation curve as observed in the 21-cm line of neutral hydrogen gives some indication of the form of the gravitational field in the central region of the Galaxy. Figure 1 is a smooth fit to the rotation curve in the inner few kiloparsecs (solid line) taken essentially from the data of Rougoor and Oort (1960) and Simonson and Mader (1973). This rotation curve, within 1 kpc of the centre, is completely accounted for by the mass distribution implied by the extended 2.2-μ emission (Becklin and Neugebauer 1968, Oort 1971). Moreover, there is little doubt that this centrally condensed mass distribution should be identified with the bulge or spheroidal component of the Galaxy, because the spatial distribution of the 2.2-μ intensity is practically identical to the distribution of visible starlight in the bulge of M31 (Sandage, Becklin, and Neugebauer 1969). The conclusion is that the bulge overwhelmingly dominates the gravitational field inside of 1 kpc.
Ultraviolet Spectra of Planetary Nebulae and their Central Stars
Authors: Seaton M. J. 
Published: March 2016

Antediluvian UV observations of planetary nebulae were made using rockets and the ANS, TD-1 and Soyuz-13 satellites: this work is reviewed by Gurzadyan (1978) and Pottasch (1978); see also Bohlin et al. (1978), Pottasch et al. (1978) and Lutz and Carnochan (1979).With IUE came the deluge. To my knowledge, IUE observations have been made of 54 planetaries, and work continues by many groups, on making observations and reducing and interpreting the data. The present report describes some preliminary results.
Secular Instabilities
Authors: Paczyński B. 
Published: March 2016

The relation between a linear series and thermal (i.e. secular) stability of stellar models is discussed. Models of accreting degenerate dwarfs are considered as an example of transition from instability to stability. This transition occurs while the accretion rate increases. Such models are relevant for novae and symbiotic stars.I shall limit this presentation to the secular i.e. thermal instabilities of stellar models in the hydrostatic and thermal equilibria. The time dependence of a small perturbation of an equilibrium model is assumed to be exponential, but the eigenvalues do not have to be real. In general they are complex. That means that thermal instability, if present, may either lead to an exponential growth of a perturbation, or may lead to oscillations with a growing amplitude. The complex nature of the eigenvalues makes a search for them difficult and no general prescription is available for a search to be complete and successful. However, a search for real eigenvalues is much easier. Some information about the presence of purely exponential modes may be achieved even without searching for them by means of calculating a linear series of stellar models. I shall describe this concept in this communication.
Origin and Development of Solar Flares
Authors: de Jager C. 
Published: March 2016

AbstractA review is given of knowledge on solar flares with particular emphasis on progress made during the international Solar Maximum Year (1979-1981).The pre-flare structure is described by a flux tube model or a circuit model. The instability leading to a flare may occur by a disturbance of the field topology (mostly: field emergence) followed by field-line reconnection (circuit coupling). In the first, impulsive, phase of a flare this causes jets of energetic electrons, originating near the top of the flux tube, to bombard lower chromospheric regions: footpoint heating. In the second (‘gradual’ or ‘diffuse’) phase heated gas from the footpoints ascends convectively upward producing a large cloud of hot gas. Consequent shock wave phenomena cause moving fronts and associated waves in the high parts of the corona; these show up in coronagraphic or radio-observations. In some cases it happens that several hours after a large flare extended loop-like structures appear of fairly high temperature (> 6 MK), emitting a very faint X-ray flux. They are the basic structures of a more extended configuration, visible on metric radio waves, and pointing out from the area where the flare occurred. They may extend to distances of ≈ 106 km or more from the solar surface.
Nuclear Constraints on the Age of the Universe
Authors: Schramm David N. 
Published: March 2016

In this paper a review will be made of how one can use nuclear physics to put rather stringent limits on the age of the universe and thus the cosmic distance scale. As the other papers in this session have demonstrated there is some disagreement on the distance scale and thus the limits on the age of the universe (if the cosmological constant Λ = 0.) However, the disagreement is only over the last factor of 2, the basic timescale seems to really be remarkably well agreed upon. The universe is billions of years old - not thousands, not quintillions but billions of years. That our universe has a finite age is philosophically intriguing. That we can estimate that age to a fair degree of accuracy is truly impressive.No single measurement of the time since the Big Bang gives a specific, unambiguous age. Fortunately, we have at our disposal several methods that together fix the age with surprising precision.
Star Formation Activity in Giant HII Complexes
Authors: Heidmann J. 
Published: March 2016

With the advent of powerful instruments in various spectral domains many scientists became involved in the study of giant HII regions - i.e. regions as important as 30 Doradus in the Large Magellanic Cloud or NGC 604 in Messier 33 - and of HII complexes still more active such as isolated extragalactic HII regions, some galactic central regions, chains in large spirals and clumpy irregular galaxies.In view of the importance of these investigations for the problems of formation and evolution of stars and of galaxies, we organized a Meeting at thelAU XVIIIth General Assembly on”Giant HII complexes outside our Galaxy”with the following program:
a-Observational results and their primary interpretations in the X-ray, UV, visible, IR and radio domains.b-Confrontation with related investigations in our Galaxy and general interpretations.
UV Observations of Close Binary Stars
Authors: Polidan R. S. 
Published: March 2016

Ultraviolet spectroscopic observations made with the Copernicus and IUE spacecraft have been analyzed. The observations represent a collaborative effort with G. J. Peters.Observational evidence supporting the existence of a turbulent accretion layer has been discussed earlier by Peters. Ionization equilibrium calculations suggest the following physical conditions in the layer: T ~ 125000 K°, Ne ~ 109 cm-3; composition: Nitrogen rich, carbon poor. Observations also show the region to be turbulent and not co-rotating with the accreting star. We conclude that the region is formed by new material arriving from the secondary and that the accretion process is turbulent.
X-Ray Superbubbles
Authors: Cash Webster 
Published: March 2016

AbstractFour regions of the galaxy, the Cygnus Superbubble, the ƞ Carina complex, the Orion/Eridanus complex, and the Gum Nebula, are discussed as examples of collective effects in the interstellar medium. All four regions share certain features, indicating a common structure. We discuss the selection effects which determine the observable x-ray properties of the superbubbles and demonstrate that only a very few more in our galaxy can be detected in x-rays. X-ray observation of extragalactic superbubbles is shown to be possible but requires the capabilities of a large, high quality, AXAF class observatory.
The Soft X-Ray Background
Authors: Kraushaar W. , Burrows D. , McCammon D. , Sanders W. 
Published: March 2016

AbstractMaps in three energy bands from the recently-completed Wisconsin survey of the soft X-ray sky are presented. The lowest energy data require emission, almost certainly from hot interstellar gas, from regions within 100 pc of the sun. The data do not require diffuse emission from beyond the neutral galactic gas but are compatible with such emission under certain assumptions.
Local Cells of the Universe: Two Voids of Diameter 200 MPC
Authors: Einasto J. , Corwin H.G. , Huchra J. , Miller R.H. , Tarenghi M. 
Published: March 2016

Recent studies of the spatial distribution of galaxies and of clusters indicate that practically all clusters and a vast majority of galaxies are concentrated into supercusters. The space between superclusters has no rich clusters and very few galaxies. The whole structure is cellular, with cell walls formed from sheetlike superclusters and the empty cell interiors being huge voids.In this presentation we show how the observed distribution of clusters, and the Local Supercluster in particular, are part of a cellular structure and suggest that the entire pattern of superclustering is cellular. Our study is based on the recently completed CFA redshift survey. All other available redshifts are also used, including cluster redshifts where available.
Atomic Energy Levels and Spectra: Astronomical Needs
Authors: Cowley Charles R. 
Published: March 2016

The astronomer’s requirements for atomic data are so massive that it is not easy to approach this subject in a way that will sound responsible to someone who is not thoroughly familiar with the situation. The simple truth is that there is hardly an atomic species, neutral or ionized, for which additional data is not urgently needed in some domain of astronomy. This comes about partly because of the high temperatures and very long path lengths in astronomical objects which can make them extremely powerful light sources. More generally, we can say that conditions in the astronomer’s violent universe manage to render large volumes of phase space available to the atoms which occupy it.While the truth is that we need to know “everything,” one task of the present paper is to cast this need in a pragmatic form. This will be done by reviewing certain problems in the very recent history of astronomical spectroscopy, where real progress has come about as a result of new work on atomic spectra. Many of the illustrative problems are taken from the spectroscopy of upper main sequence, chemically peculiar or CP stars.
The High Energy Frontier
Authors: Rubbia Carlo 
Published: March 2016

AbstractThis is a review of the present status of particle physics and the main scientific goals in our work at the “High Energy Frontier” with specific emphasis on connections to Cosmology. Based on an extraordinarily successful Standard Model, our field is exploring the fundamental questions such as the nature of mass and the unification scheme which only a few decades ago seemed to belong mostly to the realm of dreams. Neutrino masses and oscillations, the key to CP-violation and proton decay are actively searched for at accelerators and underground experiments.