bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic Catastrophes in Stars
Authors: Perdang J.
Published: March 2016

The invariance of the equations of linear stability of a physical system under a change of sign of the dependent variables implies that we cannot foresee, once an instability is detected, in which direction the system will evolve. In the context of radial dynamical stability this means that the linear analysis does not allow us to discriminate between a subsequent contraction or expansion. To fix up the arrow of the evolution beyond the onset of the instability, a nonlinear analysis is required.
The Rayleigh-Taylor Instability Overturn of Supernova Cores during Bouncing and Resulting Neutrino Energy Release
Authors: Colgate S. A. , Petschek A. G.
Published: March 2016

We show that Rayleigh-Taylor convective overturn of the dynamically formed lepton-trapped core of a supernova is a likely outcome of three sequential events: (1) The bounce or weak reversal shock; (2) the diffusive and convective lepton release from the neutrino-sphere during a fraction of the reversal time (≌ 100 ms); and (3) the rapid (≤ 10 ms) Rayleigh-Taylor growth of the l = 2 mode of an initial rotational perturbation. The overturn releases gravitational energy corresponding to a differential trapped lepton pressure energy of 30 to 50 MeV/nucleon by P dV work in beta equilibrium in a fraction of a millisecond. The resulting kinetic energy of ≌ 7 × 1052 ergs is more than adequate to cause the observed supernova emission. Also, the sudden release of ≌ 7 × 1051 ergs of ˜ 10 MeV neutrinos from the neutrinosphere will cause adequate mass and energy ejection.
Mass Loss and Stellar Wind in Massive X-Ray Binaries
Authors: Henrichs H. F.
Published: March 2016

A number of massive stars of early type is found in X-ray binary systems. The catalog of Bradt et al. (1979) contains 21 sources optically identified with massive stars ranging in spectral type from 06 to B5 out of which 13 are (nearly) unevolved stars and 8 are supergiants. Single stars of this type generally show moderate to strong stellar winds. The X-rays in these binaries originate from accretion onto a compact companion (we restrict the discussion to this type of X-rays).

We consider the compact star as a probe traveling through the stellar wind. This probe enables us to derive useful information about the mass outflow of massive stars.

After presenting the basic data we derive an upper limit to mass loss rates of unevolved early type stars by studying X-ray pulsars. Next we consider theoretical predictions concerning the influence of X-rays on the stellar wind and compare these with the observations. Finally, using new data from IUE, we draw some conclusions about mass loss rates and velocity laws as derived from X-ray binaries.
X-Ray Studies of Quasars and Active Galaxies with the Einstein Observatory
Authors: Ku William H.-M.
Published: March 2016

The launch of the Einstein Observatory has added a new and exciting dimension to the study of active galaxies. Not only have a large number of optical and radio active galaxies been detected, but many new examples of high energy activity have been found. The ease with which a large number of quasars may now be studied in the X-ray regime out to a redshift of at least four promises to improve our understanding of the nature of these tremendous powerhouses and the evolution of the universe.

The Columbia Astrophysics Laboratory (CAL) is carrying out an extensive program to study active galaxies with the imaging proportional counter (IPC) on board the Einstein Observatory (Giacconi et al. 1979). These observations have already yielded a large number of positive detections including four Seyferts, five N galaxies, seven BL Lacs, and 17 quasars. Upper limits were obtained for eight additional quasars. Six new Seyfert I and/or quasars have been identified from X-ray observations (Chanan 1979). Preliminary results from the first six months of the CAL survey of active galaxies will be presented below. A few representative objects of interest will be discussed briefly. Simple statistical tests will be applied to determine whether X-ray properties can be used to understand the differences and similarities between the various classes of active galaxies. Particular emphasis will be placed on the quasars in our sample. Our results for the quasar survey will be compared with those discussed by Tananbaum et al. (1979). Finally, the implications of the discovery of a large number of quasars will be briefly discussed. (Cosmological parameters of qo = 0 and Ho = 50 km (s Mpc)-1 are used throughout.)
X-Ray Spectra of Clusters of Galaxies
Authors: Mushotzky Richard F. , Smith Barham W.
Published: March 2016

Now that the predominantly thermal nature of the X-ray emission from clusters of galaxies is well established (see e.g. Mushotzky et al. 1978, Smith et al. 1979), it is appropriate to ask what further information can be gleaned from X-ray spectroscopic observations. While the above publications made use of data from the OSO-8 satellite, two newer experiments have been flown by our group on the HEAO-1 and HEAO-2 (Einstein) satellites. The first of these, the A-2 experiment, was comprised of gas proportional counters like our OSO-8 detectors, while the second, the Solid State Spectrometer (SSS), is a cooled Si detector with narrow field of view and heightened energy resolution.
Constraint on Quasar Number Counts from Their Contribution to The X-Ray Background
Authors: Henry J. P. , Soltan A. , Tananbaum H. , Zamorani G.
Published: March 2016

Observations with the Einstein X-ray telescope show that quasars, as a class, are luminous X-ray emitters. By coupling our X-ray observations with quasar optical number counts, we can show that the quasars contribute significantly to the diffuse X-ray background. In fact, the X-ray data strongly suggest that somewhat above 20m the slope of the optical counts must flatten.
The U. S. Space Telescope: Astrometric Capabilities
Authors: Jefferys William H.
Published: March 2016

This paper describes the Space Telescope and the instruments it will carry which will be used for astrometry. Particular attention is paid to the two imaging cameras and to the Fine Guidance Sensors. A brief outline of the kinds of programs which are to be carried out is also given.
Red Giants in Globular Clusters and Large-Scale Variations in The Galaxy
Authors: Pagel B. E. J.
Published: March 2016

This review concerns recent work on the determination of overall metallicities [Fe/H] in a number of globular clusters and the systematics of mixing effects displayed (usually) by weak CH and strong CN. Special attention is given to the globular cluster ω Centauri, where both metal abundance variations and mixing effects occur and are closely intertwined. Recent observations carried out at the Anglo-Australian Telescope by E.A. Mallia and D.C. Watts have revealed large variations in the strength of metallic lines across the red giant branch of this cluster.
X-Ray Observations of Stellar Coronae and Winds
Authors: Swank Jean H.
Published: March 2016

Since the discovery in 1974 by Heise et al. (1975) with the ANS satellite of X-ray flares from YZ CMi and UV Cet, only a few attempts to observe X-rays from flare stars have succeeded. On the other hand, the discovery of X-ray emission from Capella by Catura, Acton and Johnson (1975) in a rocket flight has been followed by so many detections of RS CVn binaries by the low energy detectors (0.15 - 3 keV) of the HEAO A2 experiment that, while Catura et al. estimated that many variable soft X-ray sources probably exist, Walter, Charles and Bowyer (1978) could identify the RS CVn systems as a class of quiescent sources. They have higher temperatures than at first predicted, so they are ideal for detection in the energy range ¼ keV to a few keV, and the high space density provides many close candidates. Further information on these sources is now available from the GSFC Solid State Spectrometer Experiment on the Einstein Observatory, which with energy resolution of 140 eV can resolve the major complexes of line emission from Si, S, Fe and less abundant elements that are an important part of the emission of 10 million degree plasmas. The imaging experiments on the Einstein Observatory have detected X-ray emission from subsets of all types of stars, and results on the luminosities, variability and temperatures are just beginning to come out.
Mass Transfer between Binary Stars
Authors: Modisette Jerry L.
Published: March 2016

The transfer of mass from one component of a binary system to its companion can involve ejection of the mass by radiation pressure, by Roche lobe overflow, by centrifugal action through the coupling of the upper atmosphere to the stellar rotation through magnetic fields, and by the dynamics of the atmospheric fluid under the action of an energy source at the photosphere or lower chromosphere. I will primarily address the latter mechanism, which commonly goes under the title of stellar winds. The major determinant of a stellar wind is the energy source. In the case of a binary system, the dynamic effects of the system on the flow pattern, particularly for close binaries, is a second major determinant, once an energy source is available to drive the wind.

In addition to ejection, the transfer of mass requires capture of the ejected matter by the companion. The first issue is free particle vs. continuum flow. I will only consider continuum flow, as I doubt the applicability of the free particle concept to any binary system with significant mass transfer, because of the scale height and because of the likely presence of magnetic fields in the interstellar plasma. For more distant binaries, rather simple considerations show that the capture of matter for continuum flow can be enhanced over that for free particle flow.
Astronomy in Ancient Greece
Authors: Hoskin Michael
Published: March 2016

Science has spread from western Europe where it developed into recognisably-modern form in the seventeenth century, stimulated by Copernicus’s claim that the Earth is a planet. Copernicus however was an astronomer in the Greek tradition, whose task was to reproduce the planetary paths by geometrical constructions using uniform circular motions. Eudoxus’s attempt to do this with nests of concentric spheres had been superseded by the use of the more flexible techniques necessary to meet the observational standards of the Hellenistic era. Ptolemy’s Almagest synthesised the Greek achievement but its shortcomings led Copernicus to make the Earth a planet.
Modern Cosmology
Authors: Zeldovich Ya. B.
Published: March 2016

It is a qreat honour to pronounce this lecture before the General Assemhly of United astronomers of all the world.

In fact, the astronomers are really more united, than the representatives of manv other sciences. Perhaps this unity is connected with the noble spirit of inquiry into the immense distant worlds, characteristic for astronomers. And of all the problems and all the systems investigated the biagest one is surely the Universe as a whole i.e. the tonic of cosmoloay.

This branch of astronomy is the greatest challenge, one is always confronted with the danaer of being captured by prejudices.

In the last decades the situation chanaed. At least cosmology has transformed into a respectable science, which it was not say 50 or 60 years ago.

There remain deep unsolved questions beginning with the very existence and/or birth of the Universe.

But there are also in modern Cosmology definite successes, there are achievements in the understanding of the Universe. The successes are due to the work of many people, they are due to an international collective effort.

In a talk in the Odeon, an ancient theatre of Patrai, Greece and also in this high-light paper it seems me unappropriate to give precise citations and to divide priorities. Therefore no names will be called except a handful of great men gone of us - Einstein, Rubble, Friedmann, Jeans, Planck.
Chemical Composition of Halo Field Stars and the Chemical Evolution of the Halo
Authors: Spite Francois
Published: March 2016

The abundance of the elements in the halo are mostly known from analysis of stellar atmospheres. Data from evolved stars and planetary nebulae have to be used with some caution. I will therefore rely primarily on unevolved cool dwarfs, using other information only as a complement.

I will designate these unevolved cool dwarfs indifferently by the words halo dwarfs or Population II dwarfs; such stars were formerly often called subdwarfs for historical reasons. Remaining in the field of vocabulary, I will call underdeficient an element which is deficient, but by a factor smaller than the general deficiency factor for the other elements. Finally, let us recall the classical notation:
The Stellar Content of Elliptical Nuclei
Authors: Faber S. M.
Published: March 2016

New developments in understanding the stellar populations of elliptical nuclei are discussed. Variations in iron-peak spectral features have recently been detected, which are much smaller than changes in CN, Mg, and Na. A study is presently in progress to determine whether this pattern is consistent with lockstep enhancements in all heavy elements. Present evidence on the ultraviolet spectra of ellipticals now suggests that the source of the excess UV light is not young OB stars. A new correlation is also pointed out between UV excess and mean line strength. If real, this trend is further evidence against significant numbers of young OB stars in elliptical galaxies.
Summary
Authors: Freeman Kenneth C.
Published: March 2016

The old populations of galaxies are particularly vital for understanding the formation and chemical evolution of galaxies. As in any field, ideas come and go, and today’s discussion has given us a valuable assessment of current knowledge and thinking on many topics.

The problems of star formation and the IMF are basic to many aspects of work on old populations. The discussion included some interesting ideas on star formation, and a useful warning that there are probably different modes of star formation which may give different IMFs: for example the mode of star formation that is widely studied now (in spiral arms) is probably different to the mode of star formation that was active in forming the stars of the galactic bulge.
Distance Indicators in the Magellanic Clouds
Authors: Graham J.A.
Published: March 2016

In talking about the overall distance scale of the Universe and the Hubble Constant, the Magellanic Clouds are good places to start. They are stellar systems large enough to contain stars, clusters and nebulae of all types, covering a wide age range. With modern telescopes and detectors, we are able to observe stars from the very bright down to those fainter intrinsically than our own Sun. From comparative studies, we may thus establish our basic calibrations of bright objects before moving out to measure the Universe at large. At the same time, the fact that both Magellanic Clouds are independently evolving galaxies, enables us to separate the effects of stellar age and chemical evolution on the calibrations that we make.
Globular Cluster Ages
Authors: Carney Bruce W.
Published: March 2016

Within the framework of a model cosmology, the ages of the globular clusters should provide upper limits to the value of Hubble’s constant. Should both the ages and H be known to high accuracy, we may ask if standard comsological models are adequate. This paper will summarize recent results obtained by many astronomers for the ages of the globular clusters, how large the uncertainties are thought to be, and will suggest some further work.
The Value of Ho
Authors: Tammann G. A. , Sandage A.
Published: March 2016

A brief discussion of the systematic effects of selection bias on the extragalactic distance scale is given. Distance indicators with intrinsic scatter yield only upper limits to Ho, unless the true intrinsic scatter is either small or well determined. Several distance indicators (luminosity index, diameters, globular clusters, 21 cm line widths) are discussed. It is concluded that type I supernovae, calibrated through brightest M supergiants, yield presently the most reliable determination of the large-scale value of the Hubble constant, i.e. Ho(cosmic) = 50 ± 7 km s–1 Mpc–1.
Five Crucial Tests of the Extragalactic Distance Scale Using the Galaxy as Fundamental Calibrator
Authors: de Vaucouleurs G.
Published: March 2016

The traditional approach to the extragalactic distance scale has been through several levels of primary, secondary and tertiary indicators of increasing range and decreasing precision. This piecemeal approach, fraught with the danger of cumulative errors, has led in recent years to two widely divergent scales, the “long” scale of Sandage and Tammann (1974–1982), leading to ≃ 50 km s-1 Mpc-1, and the “short” scale of the author (1978–79), leading to Ho ≃ 100 (de Vaucouleurs and Bollinger 1979; de Vaucouleurs and Peters 1981; de Vaucouleurs et al. 1981). The divergence, increasing from ~ 0.4 mag in the Local Group to ~ 1.2 mag at the distance of the Virgo Cluster, indicates that at least one of the scales is not linear (de Vaucouleurs 1981).
Comets: Nature, Evolution and Decay
Authors: Whipple Fred L.
Published: March 2016

A brief summary is given of the current concepts of the icy conglomerate cometary nucleus and of the origin of comets. Evidence that the cores of comets may contain less than average volatile material, whether in formation or by radiative heating, raises the question of why at least two very faint short-period comets suddenly experienced violent outbursts (~4000 times in brightness). A preliminary study of close double comet nuclei as affected by differential nongravitational forces shows that a collision of a cometary satellite with its primary is a likely outcome. Thus double nuclei may possibly explain these rare but extreme outbursts. Statistics suggest, however that most comet splitting and comet outbursts represent intrinsic activities in extremely non-homogeneous nuclei.
Some Considerations Relating to an Interstellar Origin for Comets
Authors: Clube S.V.M. , Napier W.M.
Published: March 2016

The idea that the Solar System possesses a primordial isotropic cloud of comets has been with us in quantitative form for about thirty years (Oort 1950). A considerable edifice has been built on this proposition (e.g. Weissmann 1982), which has proved durable despite indications that the cloud may in fact be significantly non-thermal and display Galactic alignments (Tyror 1957, Richter 1963, Oja 1975, Yabushita et al. 1979, Radzievsky 1981). However with the discovery in recent years of a system of massive molecular clouds in the Galactic disc, it has become apparent that the environment in which the Oort cloud has to survive is very different from that envisaged in 1950; a re-appraisal of the standard picture is therefore called for.

One corollary of the discovery of the molecular clouds is that the traditional objections to an interstellar comet cosmogony, such as the lack of observed hyperbolic comets and the difficulty of growing and capturing them, may no longer apply (see for example the recent review by Clube & Napier 1982a, hereinafter CN). Another consequence, reviewed here, is that tidal effects due to molecular clouds may be so large that the Oort cloud is frequently disrupted (CN; Napier & Staniucha 1982). In this situation frequent replenishment is implied and it is therefore necessary to discriminate between the primordial and the observed Oort cloud and to consider possible alternative sources for the latter. Such an enquiry would provide constraints on the as yet unsolved problem of comet formation.
Dynamical Evolution of the Oort Cometary Cloud
Authors: Weissman Paul R.
Published: March 2016

The dynamical evolution of comets in the Oort cloud under the influence of stellar perturbations has been modeled using Monte Carlo techniques. It is shown that the cloud has been depleted over the history of the solar system. Comets are lost from the cloud by direct ejection due to close stellar encounters, diffusion of aphelia to distances beyond the sun’s sphere of influence, or diffusion of perihelia into the planetary region where Jupiter and Saturn perturbations either eject them on hyperbolic trajectories or capture them to short-period orbits. The population of the cloud is estimated to be 1.0 – 1.5 × 1012comets and the total mass is on the order of 1.9 earth masses. In addition to random passing stars, less frequent encounters with giant molecular clouds may play a significant role in randomizing the orbits of comets in the cloud and reducing the effective radius of the sun’s sphere of influence.
Origin and Evolution of Saturn’s Rings
Authors: Morfill G.E.
Published: March 2016

The formation of planetary rings is discussed in the context of formation theories of the gaseous planets. The subsequent evolution of Saturn’s ring system, both dynamically and mechanically, is described, and the consequences are compared with observations.
Active Nuclei and Redshift Problems
Authors: Burbidge Geoffrey
Published: March 2016

More than 20 years ago V. A. Ambartsumian proposed that much of the activity in galaxies was dominated and even generated by their nuclei. Subsequent observational work in radio, optical and x-ray frequencies has borne out his prophecy, and major interest has centered about the nature of the machine in the galactic nucleus. The major characteristic of this machine is that it releases energy rapidly and often spasmodically by processes which are not thermonuclear in origin.

The original studies which led to the conclusion that nuclei were all important were observations of the powerful radio sources and Seyfert galaxies, and evidence for the ejection of gas from galaxies of many types. The realization that the synchrotron mechanism was the dominant radiation mechanism and the later studies of Compton radiation were fundamental in leading to the conclusion that large fluxes of relativistic particles must be generated in galactic nuclei.
Mass Loss Phenomena: Hot Stars – Recent Observations and Theoretical Implications
Authors: Lucy L.B.
Published: March 2016

In the last decade or so, significant progress has been made in developing the theory of the winds of early-type stars. This theory, as is by now well known, is built on the basic assumption that selective radiation pressure, acting in general through a multitude of lines, is responsible both for the expulsion of matter from these stars' atmospheres and for its subsequent acceleration to beyond escape velocity (Lucy and Solomon 1970; Castor, Abbott and Klein 1975; Weber 1981). In order to investigate the effects presumed to be dominant as well as to keep within reasonable bounds the effort required to compute complete models and work out their observational implications, contributors to this line of research have concentrated on a simplified version of the general problem. Thus, for the gas dynamic part of the problem, spherically symmetric flow has usually been assumed and time-dependent effects neglected while, for the radiative transfer part, the Sobolev approximation has usually been adopted and multiple scattering effects neglected. The validity of these assumptions and approximations, and indeed of the whole enterprise, is of course ultimately a matter of observational test.
Stellar Winds and the Interstellar Medium
Authors: McCray Richard
Published: March 2016

Theoretical models for the interaction of stellar winds and the interstellar medium are described. Stellar wind energy sources are listed, and observations of expanding shells around stars and star clusters are reviewed. Giant expanding shells, or “superbubbles,” with radius > 100 pc, are formed by the combined action of stellar winds and supernovae from clusters of OB stars. Smaller expanding shells around Wolf-Rayet stars are probably formed by the interaction of the WR stellar wind with matter ejected in a previous red supergiant phase of stellar evolution. Stellar wind activity is evident in regions of star formation in dense molecular clouds.
The Interaction of Quasar Winds and the Circumquasar Environment
Authors: Dyson J. E.
Published: March 2016

High speed winds from quasars interact with the gas and stars in a surrounding galaxy and produce a variety of effects. The winds sweep up the general interstellar matter into fast moving shells of cool gas which can produce lower velocity (≲, 0.01c) sharp absorption line systems. The impact of the wind on dense interstellar clouds may contribute to the narrow emission line region. Finally, supernovae or stellar winds near the QSO set up shock waves in the outflow. Shocked QSO wind material is responsible for the broad emission line material.
UV Observations of Giant Extragalactic H II Regions
Authors: Benvenuti P.
Published: March 2016

UV observations of extragalactic H II regions were pioniered by the ANS Satellite which collected spectra of 30 Dor in the LMC (Koorneef 1977, 1978, Israel & Koorneef 1979), of NGC 604 in M 33 (Israel et al. 1982) and of the brightest regions in M 101 (Blitz et al. 1981). It was however the International Ultraviolet Explorer, IUE, which allowed to extend the UV spectroscopic observations to a larger number of giant H II complexes with a combination of spectral and spatial resolution and sensitivity not available before. It should be noted that IUE, together with the data on the extragalactic H II regions, has provided a wealth of UV spectra of all class of astrophysical objects. These archive data are easily accessible and being all processed and calibrated in the same way, represent a unique tool for the interpretation of a composite spectrum such as the one obtained from extragalactic H II regions. For this reason the present review mainly deals with new IUE data.
Ultraviolet Observations of Binaries with Luminous Cool Primaries and Hot Secondaries
Authors: Parsons Sidney B.
Published: March 2016

Unresolved, spectrum or spectroscopic binaries containing F or G giant or supergiant primaries are being observed with the IUE satellite. In most cases the secondary is a B or early A star probably still on the main sequence. The temperature class of the secondary is determined accurately by UV spectral classification and/or analysis of the composite flux distribution. The flux analysis uses a large grid of intrinsic colors and yields a best trial-and-error fit for the spectral types of primary and secondary, their V magnitudes, and the interstellar reddening. From the temperature class of the secondary, its probable main-sequence luminosity leads to a luminosity estimate for the primary with an uncertainty of about one magnitude. Several single-lined spectroscopic binaries are also being observed at high dispersion (in collaboration with Ake, Fekel, Harvel, and Kondo), to get the differential velocity between components from which mass ratios can be determined and masses estimated for the supergiants.
IUE Studies of Luminous X-Ray Binaries
Authors: Hutchings J.B.
Published: March 2016

IUE has been used to study 11 high luminosity X-ray binaries, of which 3 are in the Magellanic Clouds. In the supergiant systems, X-ray ionisation bubbles have been found in most cases, leading to a greater understanding of the winds and accretion processes. Further studies of precessing objects such as LMC X-4 with IUE and ST are clearly of considerable interest, relating to X-ray heating and blanketing. Detailed studies of the Cyg X-l ionisation bubble may resolve the long standing puzzle of its orbit inclination and masses. UV continua have furnished valuable information on extinction, temperatures and luminosities, and the presence of non-stellar (i.e. disk) luminosity. Here too, more detailed studies are clearly indicated for the future. A unique object of interest is the LMC transient 0538-66 whose UV spectrum has quasarlike lines and luminosity which varies oppositely to the visible. This may be a case of supercritical accretion generating an optically thick shell (“disk”) about the pulsar.
The Geometrical Structure of Activity in the AR Lac System
Authors: Basri Gibor , Gibson David , Walter Fredrick
Published: March 2016

Although solar activity is seen to exhibit a very inhomogeneous and convoluted set of structures, and we have good reason to expect that stellar activity is also very structured, it is naturally quite difficult to resolve such structures on other stars. One of the best opportunities for doing so is afforded by an active eclipsing system of short period, in which both the eclipses and stellar rotation serve to cover and uncover features on reasonable time scales. AR Lac is the nearest and most suitable such system, it is an RSCVn system comprised of an active G dwarf and active K subgiant with a two day period. We report here on simultaneous X-ray and UV coverage of an eclipse in this system obtained with the Einstein and IUE observations. More details are given in reports in IAU Coll. No. 71 and IAU Symp. No. 102.
Optical Giant and Supergiant Interstellar Shells
Authors: Meaburn John
Published: March 2016

Giant (20 – 260 pc) and supergiant (600 – 1400 pc) filamentary ionized shells are found in several galaxies. Some giant shells are collisionally ionized supernova remnants but many surround ≃ 5 – 50 OB stars and, if approximately spherical, could be formed by the combined effects of successive, 1050 erg, supernova explosions, pressure of the Lyman photons and stellar winds. Other mechanisms are required for their supergiant counterparts for perhaps they are toroidal rings rather than radially expanding spherical shells.
Hydrodynamic Models of the Cometary Atmosphere
Authors: Mendis D. A.
Published: March 2016

As a cometary nucleus, regarded as an admixture of ices and dust (Whipple, 1950), approaches the sun, the ices sublimate and the resulting gases freely expand dragging along some of the dust.

For a typical medium bright comet (e.g., P/Halley) at 1 AU, the cometary atmosphere is collision dominated within a radius Rc, within which the collision mean free path is equal to the radial distance itself. Within this radius, the cometary atmosphere may be considered as a continuum, and described by hydrodynamic equations. Admittedly, this definition of the regime of hydrodynamic flow is rather crude, and what one has is a gradual transition from pure hydrodynamic flow when r << Rc to slip and transitional flow when r ≈ Rc, to free molecular flow when r > Rc. Depending on whether one considers neutral-neutral collisions or neutral-ion collisions Rc ~ 5 × 103 – 104 km for a medium bright comet at a heliocentric distance of 1 AU.
Review of Spectroscopic & Spatial Observations of Cometary Comae
Authors: Barker Edwin S.
Published: March 2016

During the past few years the field of comet spectroscopy has greatly expanded with observations of comets having nuclear magnitudes in the range of 10-19th magnitude. Although the total integrated magnitudes of several of the comets were brighter than 10th magnitude, the surface brightness of the coma of a comet is low and, with few exceptions, the night sky line at 5577A is brighter than any cometary emission line. The observational programs on which I will concentrate had their start under the P/Tempel 2 and P/Encke observing programs set up by R. Newburn at JPL.

Fortunately, the groups listed in Table 1 are regularly scheduled on large telescopes to observe comets. The regularly scheduled telescope time has enabled these groups to study systematically the activity of the available comets brighter than 20th magnitude. Observations have been made with respect to heliocentric distance and distance from the nucleus. The goal of the systematic study of comets is to acquire a uniform data set on comets. The great variety of comet spectra in the past did not provide a useful data base for statistical studies. These older observations ranged from high resolution spectra of brighter comets, to sporadic medium and low resolution spectra and filter photometry of the 10th magnitude comets, to nonexistent spectra of fainter comets.
On Cometary Dust and Gas Dynamics
Authors: Wallis Max K.
Published: March 2016

I select three topics of current interest (and some current confusion) to complement the previous paper of Asoka Mendis. The topics are relevant particularly to the studies developing models and predictions of the comet Halley environment for the forthcoming missions to that comet. The idea that electromagnetic forces are important for the dynamics of submicron dust grains and so should produce fans and structures in the coma indicates that correlations with solar wind conditions should be sought.
3C293: Beams in a Rotating Gaseous Disk
Authors: van Breugel Wil
Published: March 2016

The radio source 3C293 has a bright, steep spectrum (α ≃ 0.7; Sν ∝ ν-α ) radio core with kiloparsec-sized structure (Bridle, Fomalont & Cornwell 1981, (BFC)). The core is misaligned by ~35° with respect to the extended radio lobes and both the core and lobes have a Z-shaped morphology. To explain this misalignment and morpholoqy. BFC propose a model in which beams ‘refract’ (buoyantly deflect) in a static hot gaseous halo. Alternatively, Wilson & Ulvestad (1982) have proposed that the source structure may be caused by ram pressure deflection of beams propagating through a rotating gaseous disk. Our optical imaging and spectroscopic observations reported here support the latter model.
Needs for Atomic Data for Quantitative Analysis of Stellar Spectra
Authors: Baschek B.
Published: March 2016

The goal of a quantitative analysis of a stellar spectrum is to derive the physical and chemical state of the stellar atmosphere, i.e. by definition the region emitting the spectrum. Of particular interest are the element abundances, they have to be determined together with the temperatures and densities (pressures) in the atmosphere. A detailed analysis usually is an iterative procedure: a model atmosphere is constructed from reasonable starting values of the parameters (effective temperature, surface gravity, element abundances,...) and used to calculate a theoretical, “synthetic” spectrum. By comparing the observed with the theoretical spectrum, improved stellar parameters are gained for the next iteration step.

Ideally, all atomic data entering the analysis should be known with sufficient accuracy, i.e. errors in the analysis should be due to uncertainties in the assumptions of the models, in the treatment of the velocity fields etc., and not due to insufficient atomic data. In the last decade, the ultraviolet portion of the spectrum below λ ≲ 3200 Å has become accessible by satellites such as Copernicus and the International Ultraviolet Explorer (IUE) with high spectral resolution. Studies of stellar spectra in this new range have revealed the needs for a large amount of atomic data required for the analyses.
Cross Sections and Line-Broadening
Authors: Dalgarno A.
Published: March 2016

Stimulated mostly by the needs of the fusion energy programs, data centres have been established from which information can be obtained on photoionization and recombination processes, electron and ion Impact excitation, ionization and charge transfer cross sections and on electron and proton line-broadening parameters. Regular reports are issued by most of the centres. A partial list follows:

(1)

Atomic Data for Fusion, Ed. C.F. Barnett, D.H. Crandall and W.L. Wiese: Oak Ridge National Laboratory, P.O. Box X, Bldg, 6003, Oak Ridge, Tenn. 7830, USA.

(2)

International Bulletin on Atomic and Molecular Data for Fusion, Ed. K. Katsonls: Atomic and Molecular Data Unit, Nuclear Data Section, International Atomic Energy Agency, Wagramerstrasse 5, P.O. Box 100, A-1400 Vienna, Austria.

(3)

Research Information Center, Institute of Plasma Physics, Nagoya University, Nagoya, Japan, Ed. Y. Itikawa.

(4)

JAERI Data Center, Division of Plasma Physics, JAERI, 319-11, Tokai-Mura, Japan. K. Ozawa.

In addition, Daresbury Laboratory, Science and Engineering Research Council, Daresbury, Warrington WA4 4AD, England, Eds., W. Eissner and C.J. Noble, publishes an Information Quarterly for Atomic Processes and Applications, which reports continuing developments particularly on calculations of electron impact cross sections.
Some Atomic Parameters Pertinent to Studies of Nebular Ionized Plasmas
Authors: Aller Lawrence H.
Published: March 2016

In order to predict emission line and continuum intensities and assess plasma diagnostics for gaseous nebulae, it is necessary to have accurate atomic parameters. In some Instances the requirements for accuracy are very severe. A variety of data is required:

1)

Coefficients of continuous absorption;

2)

Direct and dielectronic recombination coefficients;

3)

Transition probabilities for permitted lines, so we can accurately predict both recombination line intensities and fluorescent phenomena (e.g., OI λ8446);

4)

Transition probabilities for forbidden and intercombination lines;

5)

Collision cross sections for levels which give rise to:

a)

resonance transitions,

b)

intercombination lines,

c)

forbidden lines;

6)

Charge-exchange cross sections.
Report of the IAU WGAS Sub-group on Standard Procedures
Authors: Fukushima T.
Published: March 2016

The given mission of the subgroup on Standard Procedures (SGSP) of IAUWGon Astronomical Standards (IAU/WGAS) is to prepare report “on standard procedures needed in fundamental astronomy, which a) should have a maximum degree of compatibility with the IERS Standards, b) should include the implementations of procedures in the form of tested software and/or test cases, c) should be available not only in written form, but also in machine-readable form” as described in the third item of Recommendation VIII of the IAU Resolution A4 (1991). After some general discussions, in 1992 we issued a questionnaire on the image and mechanism on the supposed set of standard procedures (IAU/WGAS Circulrar 51.2.1) and sent it to over 200 scientists in the world. Within a few months, 16 answers were sent back. The detailed answers and the questionnaire was distributed (IAU/WGAS Circular 54.2.1). As a simulation of supposed mechanism, in 1993 we did a test campaign to collect computational procedure(s) to compute the present IAU precession formula (Lieske et al., 1979) in FORTRAN (IAU/WGAS Circular 59.2.1). Within a few months, 5 submissions from 5 countires (Japan, China, USA, Russia, and UK) reached. The analysis of submitted routines was circulated (IAU/WGAS Circular 65.2.1). Based on these responses, we concluded that the establishment of a mechanism to provide such procedures to the community of astronomy is quite useful. To realize this, we will recommend some action items to the IAU as is described in the next Section. Its main purpose is to establish a mechanism by the combination of a comiittee of board and a center, which are to operate within a year after the recommendations are adopted by the IAU. Section 3 will describe some guidelines for the committee and center to perform the tasks to create and maintain standard procedures. Of course, these are to be consulted if necessary and are not to restrict or regulate the activity of the committee and the center.
Standard Software for Astronomy - the Starlink SLALIB Example
Authors: Wallace Patrick
Published: March 2016

Starlink’s SLALIB library is a collection of algorithms used in astronomical position and time applications. After describing the library in the context of Starlink as a whole, we look at more general questions about the provision of such software.
Determination of Precession and Nutation from Lunar Laser Ranging Analysis
Authors: Williams J.G. , Newhall X.X. , Dickey J. O.
Published: March 2016

From the analysis of 24 yr of lunar laser ranges, the correction to the IAU luni-solar precession constant is found to be −3.2 ± 0.3 mas/yr for a total value of 50.3846˝/yr at J2000. The size of the 18.6 yr nutation of the pole needs to be increased by about 2 mas over the 1980 IAU theory.
Observations of the Celestial Ephemeris Pole
Authors: Gross R.S.
Published: March 2016

Space-geodetic measurement systems are capable of determining: (1) a terrestrial, body-fixed reference frame defined in practice by the stated positions and secular motions of a set of observing stations, (2) a celestial, space-fixed reference frame defined in practice by the stated locations of celestial objects, and (3) the rotation parameters linking these two frames together. Five parameters are conventionally used to specify the orientation of the terrestrial frame with respect to the celestial frame: two nutation parameters, two polar motion parameters, and one spin parameter. The celestial ephemeris pole (CEP) is defined as the north pole of that axis about which the spin parameter (UT1) is measured. The two nutation parameters locate the CEP in the celestial frame, and the two polar motion parameters locate the CEP in the terrestrial frame. By examining the frame transformation matrices, an expression relating the location of the rotation pole to that of the CEP can be derived. In order to compare theoretical predictions with observations, results of models for the effect on the nutations of geophysical excitation processes such as diurnal oceanic current and sea level height variations should not only be given in terms of the location of the CEP (rather than of the rotation pole), but must also account for the resonance effects of the free core nutation.
Time Ephemeris
Authors: Fukushima Toshio
Published: March 2016

The location-independent part of TCB–TCG, the difference between the two new time scales adopted by the IAU (1992), was integrated numerically for three JPL planetary/lunar ephemerides; DE102, DE200, and DE245. The differences among these three integrations were mostly explained by the difference in the adopted constants of the ephemerides. It was shown that the post-Newtonian correction and the perturbation by asteroids are negligible except for the mean rate, LC The comparison of these numerical integrations with the analytical formulas of Hirayama et al. (1987) and Fairhead and Bretagnon (1990) as well as their extended versions lead to the best estimate of LC as

Combining this with the recent value of the geoid potential in Bursa et al. (1992), we estimated the value of LB, the scale difference between TCB and TT, as
The Magnetic Structuring of the Solar Corona
Authors: Hood A.W.
Published: March 2016

The magnetic structuring of the solar corona can be seen during a solar eclipse. The X-ray corona has been photographed during recent satelite missions and balloon flights and it is made up of a few main building blocks including (i) coronal loops and (ii) X-ray bright points. These appear to be connected with interacting loops and magnetic cancellation. Much cooler structures, called, quiescent prominences, are also visible in the corona and these are supported against gravity in a stable magnetic configuration.
Magnetic energy conversion in the Corona and Magnetosphere
Authors: Haerendel Gerhard
Published: March 2016

Five different categories of magnetic energy conversion will be discussed, and their manifestations in the ionosphere-magnetosphere and chromosphere - corona will be compared. They are: (1) Ohmic dissipation of d.c. currents, (2) Damping of Alfvén waves, (3) Magnetic pumping, (4) Reconnection, and (5) Electrostatic acceleration parallel B. Manifestations are: (1) Ionospheric heating by Peder sen currents and generation of chromospheric faculae and plages by dissipation of field-aligned currents; (2) Generation of spicules by collisional damping of Alfvén-waves and Alfvén-wave damping in the corona by resonant absorption; (3) Drift resonance acceleration of radiation belt particles and stochastic acceleration of high-energy flare particles by Alfvén waves; (4) Reconnection at the magnetopause and in the tail, plasmoid formation and a wide variety of configuration changes and mhd instabilities in the corona for which reconnection plays a decisive role; (5) Acceleration by field-aligned potential drops as origin of auroral arcs and of > 10 MeV electron and ion beams in solar flares. Some of these mechanisms, e.g. plages, spicules, nanoflares and field-aligned electrostatic accelerations will be selected for more detailed discussion.
Introduction
Authors: Däppen W.
Published: March 2016

Since the early 1960s the surface of the Sun has been know to be in a regular pulsating motion with periods of about 5 minutes. While at the beginning various explanations were offered, only in the 1970s it was recognized that these so-called solar oscillations are manifestations of global motions of the Sun about its equilibrium. Helioseismology is the name of the branch of astrophysics that deals with deciphering these data, that cover the whole range of spherical harmonics from l = 0 (radial) to very high angular order (above l = 1000). Thanks to observational data of superb quality (each of the oscillation frequencies is measured accurately to better than one part in ten thousand), our knowledge of the Sun has leap-frogged in the last 20 years. For instance, we now know the run of temperature inside the Sun, or have good information about the internal solar rotation. In the solar neutrino problem the data from solar oscillations have become a compulsory testing stone for any model proposed to explain the discrepancy between observed and theoretically predicted solar neutrinos.
Determining the solar structure from oscillation frequencies
Authors: Basu S.
Published: March 2016

The inverse problem of finding the structure of the solar interior from the observed frequencies can be written as

where, δωi is the difference in frequency of the ith mode between the solar data and the reference model, f1 and f2 are an appropriate pair of model parameters (e.g. sound speed squared c2, and density ρ), Ei is the mode kinetic energy, K(1) and K(2) are known functions of the reference model, and F(ω) is the unknown function added to account for uncertainties associated with the physics of the surface layers.
Solar-like oscillations in η Boo
Authors: Kjeldsen H. , Bedding T.
Published: March 2016

We have observed evidence for p-mode oscillations in the G0 IV star η Boo (V = 2.68). This represents the first clear evidence of solar-like oscillations in a star other than the Sun. We used a new technique in which we measure fluctuations in the temperature of the star via their effect on the equivalent width of the Balmer lines. The observations were obtained over six nights with the 2.5 m Nordic Optical Telescope on La Palma and consist of 13000 low-dispersion spectra. The upper part of Fig. 1 shows the power spectrum of the equivalent-width measurements (the inset shows the window function).
roAp stars
Authors: Kurtz D. , Martinez P.
Published: March 2016

Among the A stars there is a subclass of peculiar stars, the Ap stars, which show strongly enhanced spectral lines of the Fe peak, rare earth and lanthanide elements. These stars have global surface magnetic fields several orders of magnitude larger than that of the Sun, 0.3 to 30 kGauss is the measured range. For stars with the strongest magnetic fields, the spectral lines are split by the Zeeman Effect and the surface magnetic field strength can be measured. Generally, though, the magnetic fields are not strong enough for the magnetic splitting to exceed other sources of line broadening. In these cases residual polarization differences between the red and blue wings of the spectral lines give a measure of the effective magnetic field strength - the integral of the longitudinal component of the global magnetic field over the visible hemisphere, weighted by limb-darkening. In the Ap stars the effective magnetic field strengths vary with rotation. This is well understood in terms of the oblique rotator model in which the magnetic axis is oblique to the rotation axis, so that the magnetic field is seen from varying aspect with rotation.