bluuebunny/crossref_metadata_2025_split huggingface dataset:
Oh as a Constituent of the Interstellar Medium
Authors: Turner B. E. 
Published: March 2016

AbstractOH is at present the most ubiquitous molecule known in the interstellar medium. It is primarily associated with galactic continuum sources, both thermal and nonthermal. The fraction of all sources showing OH decreases monotonically with galactic longitude away from the Galactic Center region. However, neither the projected density of OH nor the abundance relative to hydrogen or other molecules seems to depend on location within the galactic plane. OH also occurs in many dark dust clouds, where its density is typically 10−5—10−4 cm−3 and its abundance relative to hydrogen is very high. OH is not found in the heavily reddened stellar clusters of Reddish, in Wolf Rayet stars, in planetary nebulae, or in globules. Recent surveys have indicated that OH both in emission and in absorption, is highly correlated with H2CO in direction and in velocity. OH emission, but not absorption, is also highly correlated with anomalous H2O emission. These relationships are important in deciding between various processes for interstellar molecule formation. On present information, it appears that more than one such process is operative in different regions of the interstellar medium.
The Relative Density of H, OH and H2CO in Interstellar Clouds
Authors: Davies R. D. 
Published: March 2016

I have attempted to obtain an answer to the question “In which gas (neutral hydrogen) clouds do we find molecules?” By limiting the investigation to those clouds which might be considered normal (i.e. specifically excluding the class of clouds which emit by maser action) it is possible to obtain a definitive answer to the question namely, those clouds which have the highest gas concentration. Further data on the correlation between the distribution of neutral hydrogen and of different molecules can be found.The clouds which are seen in absorption against background radio sources are most ideally suited for this purpose and I have used the absorption spectra of neutral hydrogen (21 cm wavelength), of the OH radical (18 cm, the 1667 MHz line) and the formaldehyde molecule (6 cm). Only the absorption spectra of the three strongest non-thermal sources (Cas A, Cyg A, and Tau A) were used because only these have unambiguous neutral hydrogen absorption spectra and also because they, unlike the HII regions, do not have 1667 MHz OH spectra which may be confused by anomalous emission. In the absorption spectra of these three sources individual clouds at specific velocities can be recognized on the H, OH and H2CO spectra. We can then investigate the OH and H2CO content of each cloud relative to its neutral hydrogen content.
Atomic Data of Importance for Ultra-Violet and X-Ray Astronomy: A Review of Theory
Authors: Seaton M. J. 
Published: March 2016

In the present review I will discuss certain rather general aspects of the theory, and will leave it to other speakers to describe the results of detailed calculations. Further references to recent work are given in the Report of IAU Commission 14.1. Atomic Wave Functions and Energy LevelsIn order to calculate data of importance for ultra-violet and X-ray astronomy - wavelengths, transition probabilities, collision cross-sections, ionization and recombination rates, and line-broadening parameters - it is necessary to begin with the calculation of atomic wave functions. The interest is mainly in highly ionized systems.
New Results on Electron Broadening of Some UV Lines of NII, CII/IV and SiII/III/IV
Authors: Sahal-Brechot S. , Segre E. 
Published: March 2016

AbstractThe electron damping constants of some UV lines of astrophysical interest are calculated within the impact and semiclassical approximations and are compared with experimental results where these are available (NII, UV(1) and CII, UV (1)).
Absolute Magnitudes of RR Lyrae Stars
Authors: Christy Robert F. 
Published: March 2016

In discussing the absolute magnitudes of RR Lyrae stars, I concur in the importance of the question but I would, at the outset, like to insert a word of caution: It seems to me most likely that the Mb of RR Lyrae stars is not an immutable constant but depends on the original composition of the star. I will come back to this point later.The principal point of view which I shall emphasize is rather different from that of most observers. The observer is usually interested in using the RR Lyrae stars as a means to calibrate distances and thereby looks out from the star to the Galaxy and then to the Cosmos. In contrast, the interests of the theorist look inward and he views the RR Lyrae stars as convenient fixed points for the comparison of theory with observation.At present, the theory of stellar structure is tied to fitting the Sun. We all know the difficulties in the solar neutrino experiment which casts some doubt on whether we even understand the Sun. But we have few if any cases of evolved stars where we know M, L, R, to check our stellar evolution calculations.We are now in desperate need of new fixed points where we can compare observation and the theory of stellar structure and evolution. The RR Lyrae or Cepheid type variables provide excellent examples for this comparison of observation and calculation. They are in late stages of evolution where tests are needed, they are readily identifiable, and well observed, and already the knowledge of the period provides a very precise mass-radius relationship so that a complete determination of the model will be provided by only a few additional measures.
Review of Observational Data on RR Lyrae Stars
Authors: van Herk G. 
Published: March 2016

The data on which my work on the secular parallaxes of RR Lyrae stars is based (Bull. Astron. Inst. Neth.18) were in many respects so incomplete that I have tried to interest astronomers to get a more complete set of data. The number of stars for which I had a proper motion was only 168, and for which a radial velocity was known, was 180, with an overlap of only 138 stars. The accuracy of the proper motions was certainly unsatisfactory for 43% of the total. The greatest trouble in dealing with such insufficient numbers arises when one wants to subdivide the material into groups which are homogeneous from a physical point of view. Many subdivisions, in making up my paper, were not tried at all, simply because the material was inadequate.In recent years plenty of work has been done by various investigators, of which I will at this point only mention the work on proper motions done at the Leander McCormick Observatory, and the great number of radial velocities determined by Dr. Clube and his associates. I do not, however, believe we are yet in a position to consider the whole subject as finished. Discussions about space motions, as given by Professor Oort in the book Stellar Structures, Volume V, will, at this time, be hardly improved upon. I still feel we should increase the number of stars substantially in order to get a better statistical discussion possible. This means we have to go to fainter stars. Plenty of these stars will be found on the plates which have served to make the Charts of the Carte du Ciel, which means we have at least one old position available for proper motions. At Leiden we are now engaged in the determination or redetermination of the proper motions of 430 RR Lyrae stars.
The Cosmic X-Ray Background
Authors: Weymann R. J. 
Published: March 2016

Evidence for a diffuse, isotropic non-terrestrial component of X-ray radiation has been accumulating since the very beginnings of X-ray astronomy. This evidence now seems secure beyond reasonable doubt, especially in the light of recent experiments, e.g. one in which a very clean separation from the cosmic ray and terrestrial background has been achieved (Pounds, 1967), and another in which a counting rate proportional to the solid angle of the sky subtended to three different detectors was found (Matsuoka et al., 1967).In this very brief review I wish simply to discuss the current ideas on the mechanisms for the production of the background and the extent to which the present observations can discriminate between them and to consider the information obtainable from the very soft X-ray region, which so far has been relatively little exploited.
What can We Learn from Li and Be Stellar Observations?
Authors: Reeves H. 
Published: March 2016

I want to state a few questions which are raised by the observations of stellar Li and Be, and to see how far we can go in giving answers.I do not believe that we can, at the present time, give very definite answers to most of these questions and build up a unique model in which all observations are properly understood. We are still faced with different possibilities which can not be excluded, but can only be classified according to their degree of ‘plausibility’. Although we are led to choose one of these possibilities in the light of present data, it seems wise to keep the other possibilities in the back of our mind, and to assess their merit. Considering this, a few alternate models will be described and discussed.
Presentation and Interpretation of High Resolution Infrared Spectra of Late-Type Stars
Authors: Thompson R. I. 
Published: March 2016

Current interest in stellar evolution is concentrated on the life of a star after it has left the main sequence. Of particular interest are the red giant or supergiant periods during the hydrogen and helium shell burning phases. Convective mixing during these stages can mix nuclear processed material to the surface where it may be viewed by spectroscopic methods. It is imperative that this rare chance to view processed material be exploited fully to increase our knowledge of stellar evolution.The observation and interpretation of cool star spectra has its own particular set of problems and advantages. A particular difficulty is the formation of molecules at the low temperatures which occur in the atmospheres of late stars. Not only must the particularly complex spectra of molecules be dealt with but the problem of chemical equilibrium in the atmosphere must be solved accurately before quantitative analysis may be performed. The formation of molecules, however, has one advantage in that it very dramatically separates those stars with carbon to oxygen ratios greater than one from those with ratios less than one. It is the very high dissociation energy of 11.1 eV for the CO molecule which performs this separation. If carbon is less abundant than oxygen all of the carbon is tied up in CO and only oxides are formed in the stellar atmosphere which produce typical M star spectra. If, however, carbon is more abundant than oxygen then carbon compounds such as C2 are formed in place of the oxides and a carbon star spectrum is formed. One of the great advantages of infrared stellar spectra is that it is the only ground based technique for observing CO in stellar atmospheres.
The Movement of Small Particulate Matter in the Early Solar System and the Formation of Satellites
Authors: Gold T. 
Published: March 2016

Satellites are a common feature in the solar system, and all planets on which satellite orbits would be stable possess them. (For Mercury the solar perturbation is too large, and the retrograde spin of Venus would cause satellites to spiral in to the planet through tidal friction.) An explanation of the formation of satellites must hence be one which makes the phenomenon exceedingly probable at some stage in the solar system formation processes, and very improbable processes like a capture cannot be the answer in most cases.Small particulate matter must have been very abundant in the early solar nebula. Such particulate matter must have existed both from the first condensation of the low vapor pressure components of the gas in the first round, and it must also have been composed of material scattered from impacts after some major bodies had begun to form, frequently finding themselves no doubt on collision orbits.
X-Ray Bursts of Nuclear Origin?
Authors: Maraschi L. , Cavaliere A. 
Published: March 2016

The fate of nuclear energy carried by matter accreted onto a neutron star was considered e.g. by Rosenbluth et al. (1973). They examined pycnonuclear reactions on an originally cold star, and found that the whole star is thereby heated up to 106−107 °K. The thermonuclear reactions that can be ignited then, have been studied by Hansen and Van Horn (1975), who computed stationary burning shells, finding, however, that in most cases the shells are thermally unstable.We consider here the possibility that X-ray bursts are due to instabilities of this type. The observational constraints fix the parameters uniquely.
The Distribution of Field Horizontal–Branch Stars in the Galactic Halo
Authors: Philip A. G. Davis 
Published: March 2016

Since the time allowed per paper at this joint discussion is short only one aspect of the stellar distribution at high galactic latitudes will be presented here, namely the distribution of field horizontal-branch stars (FHB) in the galactic halo. First, the method by which FHB stars are found will be described. Second, the density distribution of FHB stars will be compared with that of the RR Lyrae stars.As part of a general program to study the stellar density distribution perpendicular to the galactic plane a number of possible FHB stars has been discovered. In each survey area objective prism plates (at a dispersion of 280 Å/mm. to a limiting magnitude of V = 14) are taken with the Michigan Curtis Schmidt telescope at Cerro Tololo Inter-American Observatory or the Schmidt telescope at the Warner and Swasey Observatory. A set of direct plates are taken also to obtain photographic magnitudes for the stars with spectral classifications.
Small Scale Solar Magnetic Fields: Theory
Authors: Weiss N. O. 
Published: March 2016

One of the most exciting developments in solar physics over the past eight years has been the success of ground based observers in resolving features with a scale smaller than the solar granulation. In particular, they have demonstrated the existence of intense magnetic fields, with strengths of up to about 1600G. Harvey (1976) has just given an excellent summary of these results.In solar physics, theory generally follows observations. Inter-granular magnetic fields had indeed been expected but their magnitude came as a surprise. Some problems have been discussed in previous reviews (Schmidt, 1968, 1974; Weiss, 1969; Parker, 1976d; Stenflo, 1976) and the new observations have stimulated a flurry of theoretical papers. This review will be limited to the principal problems raised by these filamentary magnetic fields. I shall discuss the interaction of magnetic fields with convection in the sun and attempt to answer such questions as: what is the nature of the equilibrium in a flux tube? how are the fields contained? what determines their stability? how are such strong fields formed and maintained? and what limits the maximum field strength?
Introductory Remarks to the Joint Discussion on Nuclei of Normal Galaxies
Authors: Burton W. B. 
Published: March 2016

The papers which follow here were given at the Joint Discussion on Nuclei of Normal Galaxies, held in Montreal on August 21, 1979, at the XVIIth General Assembly of the International Astronomical Union. This Joint Discussion was jointly sponsored by IAU Commission 28 on Galaxies, by Commission 33 on Structure and Dynamics of the Galactic System, by Commission 34 on Interstellar Matter and Planetary Nebulae, and by Commission 40 on Radio Astronomy. The scientific organizing committee consisted of W. B. Burton (Minnesota), chairman, R. D. Ekers (Groningen), H. Okuda (Kyoto), D. E. Osterbrock (Lick), V. I. Pronik (Crimea), and D. W. Weedman (Pennsylvania).While preparing for this Joint Discussion, the organizing committee struggled with the problem of finding a suitable definition for the notorious word “normal” as applied to the enormously complicated phenomena of galaxies. What has become clear is that galactic nuclei of all sorts share an involvement in a continuous hierarchy. Thus the lower extent of the energy range exhibited by nuclei of Seyfert galaxies overlaps the upper extent of the energy range of galactic nuclei which most of us would classify as normal. Likewise, many agreeably-normal nuclei show signs of disruptive activity, high velocity dispersions, morphological asymmetries, and ejection. It has also become clear that study of nuclear phenomena must extend to include study of the entire bulge or core region. These matters are reflected in the papers which follow. These papers also collectively emphasize that galactic nuclei reveal different properties at different wave-lengths; presented at the Joint Discussion were results derived from optical and infrared data, as well as from millimeter and longer wavelength radio data. The papers also serve to put work on the nucleus of our own galaxy, which shows many of the same puzzling properties found in other normal galactic nuclei, into a pleasantly wider perspective.
Tri-Axial Dynamics in the Core of Normal Galaxies
Authors: Schwarzschild M. 
Published: March 2016

Please let me for fifteen minutes completely ignore all gas in the cores of galaxies. Clearly the core gas exhibits fascinating, varied and consequential phenomena. Still it would seem to me a plausible assumption that while the gas might be decisively affected by the stars, this gas might not have major effects on the stars, at least in their dynamical behavior. Accordingly I shall here consider purely stellar systems.Obviously in the core of a galaxy, for dynamical consideration we have to distinguish sharply between the nucleus of a galaxy and the larger components of the galaxy co-occupying the core. Following Baade’s usage the nucleus is a stellar system of at most a few parsecs in radius and of a density substantially higher than the central densities of the other components. That the nucleus of a galaxy, at least in its equilibrium state -though not necessarily in its origin- is a distinct, self-contained stellar system seems to me strongly indicated by the rotation velocity measurements in the nucleus of the Andromeda Nebula (Lallemand et al. 1960; Walker, 1974) as well as by the photometric profile determination of the same nucleus (Light et al. 1974). For a nucleus of 108 stars the relaxation time for stellar encounters is, according to Spitzer (1971), comparable to a Hubble time while stellar collisions are ignorable. On the other hand, for a nucleus containing 1010 stars, which may be the right order of magnitude for more pronounced nuclei, the relaxation time may be comfortably long enough so that encounters may be ignored, but the frequency of collisions is not ignorable. In both cases therefore it would seem that in the centermost portions of nuclei of galaxies either encounters or collisions play a decisive dynamical role but the same may not be true in the outer portions of these nuclei. For the larger subsystems occupying the core of a galaxy, obviously neither encounters nor collisions play a significant role. To stay within the limits of my knowledge, I shall limit myself here to encounterless and collisionless stellar systems, a marginal approximation for the nuclei proper.
Nova Modeling
Authors: Sparks Warren M. , Kutter G. Siegfried , Starrfield Sumner , Truran James W. 
Published: March 2016

The mass and energy of nova ejecta are 10-5 - 10-4M⊚ and 1044 - 1045 ergs, respectively (Payne-Gaposchkin 1957). Comparison of these quantities with the mass (1 M⊚) and binding energy (1051 erg) of the erupting white dwarf implies that the nova outburst is a surface event. From an average of two or three novae detected each year, it is estimated that the rate of novae is 40-50 per year in our galaxy (Payne-Gaposchkin 1954) . Comparing this rate with a white dwarf birth rate of 2 per year in our galaxy (Weidemann 1968), we conclude that the nova outburst is a recurrent phenomenon (cf. Ford 1978). The recurrent nature also implies that the white dwarf cannot be drastically altered from event to event, thus giving further evidence for a surface event. The argument of recurrency become even stronger when it is realized that observations strongly indicate a close binary structure for the nova candidates—the white dwarf and a red companion (Kraft 1964).Kraft (1963) proposed the following hypothesis. The red companion overflows its Roche lobe and supplies hydrogen-rich material to an accretion disk around the white dwarf. This material eventually accretes onto the white dwarf, forming an hydrogen-rich envelope whose base is electron-degenerate. As the accretion proceeds, the temperature at the base of this envelope increases. This has little effect on the pressure, but greatly increases the thermonuclear energy generation. The thermonuclear energy generation, in turn, increases the temperature. This positive feedback loop leads to a thermonuclear runaway, which Kraft proposed as the cause of the nova outburst. A large number of theoretical studies based on this model have been carried out. These studies are presented in order of increasing realism and complexity in the following sections.
Chromospheres, Coronae, and Mass Loss in Stars Hotter than the Sun
Authors: Snow Theodore P. 
Published: March 2016

AbstractReviews of the mass-loss characteristics of OB stars have been published recently, and the present review therefore emphasizes the A and F stars and very recent results on O and B stars. For the F stars, chromospheric indicators are present in the form of emission lines, seen in visible and ultraviolet wavelengths. Winds are present in A supergiants, but not in main sequence stars, although at least a few of the latter are X-ray sources, indicating the possible existence of coronae. Most OB supergiants are X-ray sources as well, indicating, along with the presence of super-ionization, that these stars have coronae. On the main sequence, the O stars and some B stars (including Be stars in many cases) have mass loss with highly-ionized species in the wind. The winds in the O and B stars are commonly variable. The mass-loss rates do not show a simple dependence on luminosity, contrary to the predictions for radiatively-driven winds.
Small-Scale Dissipative Processes in Stellar Atmospheres
Authors: Leibacher J. W. , Stein R. F. 
Published: March 2016

AbstractThe outer atmospheres of stars must be heated by some non-thermal energy flux to produce chromospheres and coronae. We discuss processes which convert the non-thermal energy flux of organized, macroscopic motions into random, microscopic (thermal) motions. Recent advances in our description of the chromosphere velocity field suggest that the acoustic waves observed there transmit very little energy, and hence are probably incapable of heating the upper chromosphere and corona. The apparent failure of this long held mechanism and the growing appreciation of the importance of strong magnetic fields in the chromosphere and corona have led to hypotheses of heating by the dissipation of currents (both oscillatory and quasi-steady). This follows discoveries in laboratory and ionospheric plasmas and work on solar flares, that instabilities can concentrate currents into thin high current density filaments where they dissipate rapidly.
Spectroscopy-What are The Needs Once Space Astrometry has Given us a New Data Base
Authors: Jaschek C. 
Published: March 2016

I think the best way to start my talk is to recall what kind of spectroscopic, information is available at present. Spectroscopic information can be of various kinds; starting from the simplest it may be:a- a spectral type (unidimensional)b- a spectral type plus a luminosity class (bi-dimensional type)c- a description-more or less detailedd- a quantitative analysis, which can be more or less sophisticated