bluuebunny/crossref_metadata_2025_split huggingface dataset:
JET-X A Joint European X-ray Telescope for SPECTRUM-X
Authors: Wells A. , Lumb D.H. , Pounds K.A. , Stewart G.C. , Aschenbach B. , Brauninger H. , Hasinger G. , Trumper J. , Citterio O. , Scarsi L. , Peacock A. , Taylor B. 
Published: July 2016

AbstractThe Joint European X-Ray Telescope, JET-X, is one of the core instruments of the scientific payload of the USSR SPECTRUM-X astrophysics mission due for launch in 1993. The JET-X instrument concept is described and its scientific performance and capability discussed.
Thermal Detectors for X-ray Astronomy
Authors: Holt Stephen S. 
Published: July 2016

AbstractSpectroscopy is traditionally characterized by the sacrifice of quantum efficiency for high spectral resolution. Since X-ray astronomy is a photon-limited discipline, the choice between high resolution for very few sources versus much lower resolution for many more has not always been an easy one. The development of new thermal detectors offers the opportunity to “have one’s cake and eat it, too.”
Cometary Activity, Discrete Outgassing Areas, and Dust-Jet Formation
Authors: Sekanina Z. 
Published: July 2016

Abstract.Major progress has recently been achieved in the understanding of the coma morphology of dust comets. The conceptual models for the various types of feature observed in the coma (jets, spirals, halos, fans, etc.), their computer simulation, and the hydrodynamic models for jet formation are critically reviewed, and evidence for anisotropic, strongly collimated flows of ejecta emanating from discrete active regions (vents) on the rotating cometary nuclei is presented. The techniques employed to generate synthetic comet images that simulate the observed features are described and their relevance to the primary objectives of coma-morphology studies is exemplified. Modelling of temporal variations in the water emission from discrete active regions suggests that production curves asymmetric with respect to perihelion should be commonplace. Critical comparisons are offered with the activity profiles of Encke’s comet and with light curves of disappearing comets and comets that undergo outbursts. Discussed next is the potential evolutionary significance of nuclear shape, the fractional area of the surface that is active, the vents’ erosion rate, and available information on their activation, aging, dormancy, migration, rejuvenation, and extinction. Recent developments in the understanding of the processes that cause the nongravitational perturbations of cometary motions are reviewed and the observed discontinuities are identified with birth of new sources and/or deactivation of old vents.
The Electrodynamics of Charged Dust in the Cometary Environment
Authors: Horanyi M. , Mendis D.A. 
Published: July 2016

AbstractDust in the plasma and radiative environment of a comet is necessarily electrically charged. This charging has both physical and dynamical effects on the dust, being particularly important on the smallest particles observed in the dust size spectrum. In this paper, these dynamical effects are reviewed and the pertinent observations are discussed.
The Global Interaction of Comets with the Solar Wind
Authors: Flammer K. R. 
Published: July 2016

AbstractThe global interaction of the solar wind with a comet as it orbits the Sun is reviewed. After a brief survey of the flow transition regions observed at comet Halley is presented, theoretical models are given for the cometocentric distance of the bow shock, the cometopause, and the ionopause. In addition, predictions are made as to what heliocentric distance these boundaries should form at. The results of these models are compared with the in situ observations at comet Halley.
Historical Comets Over Bavaria: The Nuremberg Chronicle and Broadsides
Authors: Olson R. J. M. , Pasachoff J. M. 
Published: July 2016

Abstract.The first widely distributed printed comet images appear in the Nuremberg Chronicle, whose Latin edition appeared in 1493, followed closely by a German edition. In the first section, we begin our consideration with the comet image that has frequently been cited as a representation of the A.D. 684 apparition of Comet P/Halley. To better understand this image, we present a thorough survey of the 13 comet images that appear in the Chronicle, all reproduced from four woodblocks, representing 14 apparitions between A.D. 471 and A.D. 1472. In the second part, we present an analysis of the unpublished preparatory drawings for the comet images in the handwritten Exemplars (manuscript layout dummies) for both the Latin and German editions in the Stadtbibliothek, Nuremberg. Finally, in the third part, we demonstrate how the Chronicle presaged the proliferation of broadsides--woodcut prints that functioned like tabloids of the sixteenth and seventeenth centuries. We examine broadsides recording historical comets over such Bavarian cities as Nuremberg and Augsburg. In spite of their superstitious, hysterical journalism, fed by turbulent political and religious upheavals, these broadsides reveal a nascent scientific attitude.
Compatibility issues in VLBI
Authors: Wietfeldt R. D. , D’Addario L. R. 
Published: July 2016

AbstractNew VLBI systems are under development in many countries in preparation for large dedicated arrays, including orbiting radio telescopes to be launched in the 1990s. In particular, several new recording systems and correlators are being constructed: the VLBA in the U.S., the S2 in Canada, and the K4/VSOP in Japan. This paper reviews the issues of technical compatibility among these systems in an effort to facilitate global VLBI experiments.
Opacity at the Owens Valley Millimeter Array
Authors: Scott Stephen L. , Woody David P. 
Published: July 2016

AbstractWe have measured the opacity at the Owens Valley Millimeter Array with tip curves at frequencies from 95 to 248GHz. These measurements have been interpreted using a model and then scaled to a standard frequency of 100GHz. The unique aspect of this data is the wide range of frequencies covered and the use of receivers with different sideband gain ratios.
The Bispectrum analysis technique in millimeter interferometry
Authors: Morita Koh-Ichiro 
Published: July 2016

AbstractWe propose an application of the bispectrum analysis technique in millimeter interferometry, which is used in the speckle masking technique. For the millimeter interferometry, in which atmospheric phase fluctuations are very serious, not only bispectrum phase but also bispectrum amplitude is good observable. We have developed an algorithm to reconstruct images directly from bispectrum. We have made good maps with this algorithm in the case of both simple and compact sources.
Phase calibrators for MERLIN
Authors: Patnaik A.R. , Browne I.W.A. , Wilkinson P.N. , Wrobel J.M. 
Published: July 2016

AbstractThe VLA was used to derive a catalogue of phase–reference sources for MERLIN. These sources can also be used for other interferometer arrays such as the VLBA. The positions of the sources in the catalogue are repeatable to about 5 milliarcsec rms.
The Multi-resolution CLEAN
Authors: Wakker B.P. , Schwarz U.J. 
Published: July 2016

AbstractWe describe a modification to CLEAN which alleviates some problems for extended sources. This is accomplished by combining the results of a number of conventional CLEAN operations, each done at a different resolution. The algorithm is called “Multi-Resolution Clean” or “MRC”. Experiments on model sources have shown that it works well even when the source is so extended that the usual CLEAN becomes impractical. For extended sources, MRC enhances the signal-to-noise ratio, resulting in an easier definition of the area of signal. Moreover, MRC is in principle faster than a standard CLEAN because less δ-functions are needed. This work was published in Astr. Ap., 200, 312.
A new method of analysis and reduction of VLBI observations
Authors: Arias E. F. , Biasi M. S. de 
Published: July 2016

AbstractIn this paper we develope an inertia! system of coordinates introducing a new observable: the arc between a pair of radio sources. We shall discuss how the conditional equation involving all the parameters is written in diferent systems when the observations are performed with a two and a three-element interferometers.
Aperture Synthesis imaging from the Moon
Authors: Burns Jack O. 
Published: July 2016

AbstractFour candidate imaging aperture synthesis concepts are described for possible emplacement on the Moon beginning in the next decade. These include an optical interferometer with 10 μarcsec resolution, a submillimeter array with 6 milliarcsec resolution, a Moon- Earth VLBI experiment, and a very low frequency interferometer in lunar orbit.
Imaging with a Homogeneous Millimeter Array
Authors: Holdaway M.A. 
Published: July 2016

AbstractPresented here are the results of simulations of the effects of various errors on the imaging capabilities of the compact (72 m) configuration of a 39 element homogeneous millimeter array using the mosaic algorithm to image sources larger than the primary beam. Included in the errors investigated to date are systematic and random pointing errors, Gaussian receiver and atmospheric noise, and primary beam errors. Various methods of quantifying the quality of the reconstructed image are also discussed.
Accretion Disk Instabilities
Authors: Mineshige S. 
Published: July 2016

AbstractBasic properties of accretion disk instabilities are summarized. We first explain the standard disk model by Shakura and Sunyaev. In this model, the dominant sources of viscosity are assumed to be chaotic magnetic fields and turbulence in gas flow, and the magnitude of viscosity is prescribed by so-called a model. It is then possible to build a particular disk model. In the framework of the standard model, accretion disks are stationary, but when some of the basic assumptions are relaxed, various kinds of instabilities appear. In particular, we focus on the thermal limit-cycle instability caused by partial ionization of hydrogen (and helium). We demonstrate that the disk instability model well accounts for the basic observed features of outbursts of dwarf novae and X-ray nova. We then introduce other kinds of instabilities based on the α viscosity model. They are suspected to produce time variabilities observed on a wide range of timescales in close binaries and active galactic nuclei.
Period Analysis of theδScuti Star HD93044
Authors: Zong-Li Liu 
Published: July 2016

AbstractHD93044 was observed electrophotometricaly on April 21, May 1, 2 and 4, 1991 at Xinglong Station of Beijing Astronomical Observatory. Combining the data with Li Zhiping’s data together and a period analysis was completed using a program which consists of a combination of Fourier transforms of prewhitened data and the multifrequency least squares of brightness residuals (LSR). Three pulsation frequencies (11.90809, 16.79553, 22.44827 cycles per day) with visual amplitudes between 0.0056 and 0.0203 mag were found. The solution fits the observations to +0.0071 mag which is equal to the mean square deviation of observations. The first frequency (11.90809 cycles per day) must be the right value of the fundamental frequency of HD93044.
Observation and Statistical Analysis of ZZ Piscium
Authors: Satoh H. , Saijo K. , Yanagita T. 
Published: July 2016

AbstractTo examine the non-linear oscillation and period change of white dwarf variables, ZZ Psc is discussed observationally. As a result of data reduction we found two major periods, 680 seconds and 860 seconds, but could not find chaotic behavior of ZZ Psc.
Bifurcation in Hydrodynamic Models of Stellar Pulsation
Authors: Aikawa T. 
Published: July 2016

AbstractPhenomena of bifurcation in hydrodynamic stellar models of radial pulsation are reviewed. By changing control parameters of models, we can see qualitatively different pulsation behaviors in hydrodynamic models with transitions due to various types of bifurcation.In weakly dissipative models (classical Cepheids). the bifurcation is induced by modal resonances. Two types of the modal resonances found in models are discussed: The higher-harmonic resonances of the second overtone mode in the fundamental mode pulsator and of the fourth overtone mode in the first overtone pulsator are relevant to observations. The subharmonic resonance between the fundamental and first overtone modes is confirmed in classical Cepheid models.In strongly dissipative models (less-massive supergiant stars), the bifurcation of nonlinear pulsation is induced by the hydrodynamics of ionization zones as well as modal resonances. The sequence of the bifurcation sometimes leads to chaotic behaviors in nonlinear pulsation. The transition routes from regular to the chaotic pulsations found in models are discussed with respect to the theory of chaos in simple dynamical systems: The cascade of period-doubling bifurcation is confirmed to cause chaotic pulsation in W Virginis models. For models of higher luminosity, the tangent bifurcation is found to lead intermittent chaos.Finally, hydrodynamic models for chaotic pulsation with small amplitudes observed in the post-AGB stars are briefly discussed.
Nonlinear RR Lyrae Models with New Livermore Opacities
Authors: Guzik J. A. , Cox A. N. 
Published: July 2016

AbstractA. N. Cox recently showed that a 20% opacity decrease in the 20,000–30,000 K region as indicated by the new Livermore OPAL opacities reconciles the discrepancy between pulsation and evolution masses of double-mode RR Lyrae variables. Nonlinear hydrodynamic calculations were performed for RR Lyrae models of mass 0.75M⊙, 51L⊙, andZ= 0.0001 (Osterhoff II type) including this opacity decrease. The Stellingwerf periodic relaxation method was used to converge the models to a limit cycle, and the Floquet matrix eigenvalues calculated to search for a tendency of the fundamental mode to grow from the full-amplitude overtone solution, and the overtone mode to grow from the full-amplitude fundamental solution, thereby predicting double-mode behavior. Models ofTeff< 7000 K with the opacity decrease have positive fundamental-mode growth rates in the overtone solution, in contrast to earlier results by Hodson and Cox (1982), and models withTeff> 7000 have positive 1st overtone growth rates in the fundamental-mode solution, but double-mode behavior was not found.
On The Nature of Interstellar Organic Chemistry
Authors: Charnley Steven B. 
Published: July 2016

AbstractA theory for the origin of all organic molecules observed in regions of massive and low-mass star formation, as well as in dark molecular clouds is described. On dust grains, single atom addition reactions and stability of the intermediate radicals, mechanisms similar to those believed to form the organic component of the Murchison meteorite, lead to a very limited number of mantle compositions depending upon the degree of hydrogenation. The key step in the theory is the formation of the formyl radical by H atom addition (by quantum tunnelling) to CO. Subsequent H atom additions lead to formaldehyde and methanol, as previously suggested; C, N, and O atoms can also undergo additions to HCO. For increasing hydrogenation, the mantle types include one in which there is little contribution from formyl-initiated chemistry; one in which an acetylenic chain develops through C atom additions; and others where the acetylenic chain is increasingly hydrogenated to form aldehydes and alcohols. Following evaporation of grain mantles, such as occurs in protostellar «hot cores», these molecules can form new organics, for example, by alkyl cation transfer from alcohols. In dark clouds, different mantles lead to different gas phase organics. This scenario accounts naturally for the formation of many interstellar organics for which none presently exists, predicts observable correlations between specific interstellar molecules, indicates the presence of many new organic molecules and why several others are not observed.
Catastrophic Impacts and the Drake Equation
Authors: Chyba Christopher F. 
Published: July 2016

AbstractOver the past quarter century, our understanding of the impact history of the Solar System has greatly improved. This chapter considers how this increased knowledge affects our evaluation of the chances for other intelligent communicative civilizations in the Galaxy. In addition, the role of impacts is examined for insight into the long-standing debate over whether the evolution of technical intelligence is contingent upon extremely special circumstances, or might instead be a likely outcome of many different but parallel evolutionary pathways.
Organic Syntheses During the Impact of a Comet with a Gaseous Planet
Authors: Guaita Cesare , Crippa Roberto , Manzini Federico 
Published: July 2016

AbstractA large amount of CO has been detected above many SL9/Jupiter impacts. This gas was never detected before the collision. So, in our opinion, CO was released from a parent compound during the collision. We identify this compound as POM (polyoxymethylene), a formaldehyde (HCHO) polymer that, when suddenly heated, reformes monomeric HCHO. At temperatures higher than 1200°K HCHO cannot exist in molecular form and the most probable result of its decomposition is the formation of CO. At lower temperatures, HCHO can react with NH3 and/or HCN to form high UV-absorbing polymeric material. In our opinion, this kind of material has also to be taken in to account to explain the complex evolution of some SL9 impacts that we observed in CCD images taken with a blue filter.
Formation, Frequency and Spacing of Habitable Planets
Authors: Lissauer Jack J. 
Published: July 2016

AbstractModels of planet formation and of the orbital stability of planetary systems are described and used to discuss estimates of the abundance of habitable planets which may orbit stars within our galaxy. Modern theories of star and planet formation, which are based upon observations of the Solar System and of young stars and their environments, predict that most single stars should have rocky planets in orbit about them. Terrestrial planets are believed to grow via pairwise accretion until the spacing of planetary orbits becomes large enough that the configuration is stable for the age of the system. Giant planets orbiting within or near the habitable zone could either prevent terrestrial planets from forming, destroy such planets or remove them from habitable zones. The implications of the giant planets found in recent radial velocity searches for the abundances of habitable planets are discussed.
From Brown Dwarfs to Planets
Authors: Mayor Michel , Queloz Didier , Udry Stéphane , Halbwachs Jean-Louis 
Published: July 2016

AbstractThe burst of discoveries of planets and brown dwarfs during the past year has revealed an unexpected diversity of planetary systems. 51 Pegasi, the first extrasolar planet detected in orbit around a solar-type star appears to be the prototype of a new family of Jovian planets, the «Hot Jupiters». Though the physical mechanisms underlying the orbital decay of protoplanets are already understood, we are far from having a general view of planetary formation and of the diversity of planetary systems. If the orbital decay of protoplanets is sometimes so efficient, what is the frequency of planetary systems like ours, what is the frequency of telluric planets, what is the limit between planets and brown dwarfs?
The Lick Observatory Planet Search
Authors: Butler R. Paul , Marcy Geoffrey W. 
Published: July 2016

AbstractWe have constructed a precision Doppler technique with which we have detected 6 extrasolar planets to date. Doppler precision is achieved by inserting an iodine absorption cell in the telescope, providing a fiducial wavelength scale against which to measure Doppler shifts. Our current precision is 3 m s−1, which corresponds to one part in one hundred million in wavelength, or 1/1000 th of a pixel on the CCD detector. As of 1996 August, our survey of 120 stars has revealed six stars that show velocity variations consistent with Jupiter-mass companions in Keplerian orbits. These objects span a greater range of orbital radii and eccentricity than the planets in our own solar system. Three of these objects are «51 Peg-type» planets with circular orbits having radii 0.15 AU or less. The other three objects have masses between 1.7 and 6.5 MJUP, eccentricities between 0.01 and 0.6, and semi-major axes between 0.4 and 2.1 AU. We have not found any objects having masses between 10 and 80 MJUP, the domain usually associated with brown dwarfs. These six new companions represent a new population of objects having extremely low masses, similar to that of Jupiter. We have begun a survey of an additional 400 stars using the Keck 10-m telescope, which will allow the detection of Neptune-mass planets.
Prebiotic-Related Chemistry in the Present Outer Solar System
Authors: Raulin François , Coll Patrice , Gazeau Marie-Claire , Bruston Paul 
Published: July 2016

AbstractThere are numerous places in the outer solar system where the formation of the starting ingredients which are involved in the early terrestrial prebiotic chemistry is currently going on. Organic chemistry is largely present in the outer planets, particularly in Titan. Titan has a dense atmosphere, mainly composed of N2and CH4and very rich in organic compounds, both in gas and aerosol phases. Because of the low temperature of Titan’s environment, liquid water is currently absent from the satellite and compounds of low stability at the (Earth) laboratory temperature, and very reactive, are still or may be present. However, Titan study should provide information on prebiotic chemistry – at least prebiotic chemistry in absence of liquid water. This quasi-planet thus appears as a natural laboratory enabling to study prebiotic evolution toward complex organic systems in a planetary environment over a long time scale. A detailed study of such a natural prebiotic laboratory is precisely one of the main objectives of the Cassini-Huygens mission. With the sending of the Cassini orbiter around Saturn and the Huygens probe in the atmosphere of Titan, this mission, due to be launched in October 1997, for a Saturn arrival in 2004, will offer a unique opportunity to study in detail extra-terrestrial prebiotic processes, together with important implications in the field of bioastronomy and the origins of life.
Molecular Evolution of Aminoacyl tRNA Synthetases and Origin of Universal Genetic Code
Authors: Ishigami Masahiro , Ihara Hideshi , Shinoda Hiroyuki 
Published: July 2016

AbstractIt is thought that living things first appeared on the primitive earth 35 hundred million years ago. At that time, a primitive protein synthesis mechanism was thought to have been established, a genetic code system evolved, and a universal genetic code system developed. Aminoacyl tRNA synthetase must have evolved with the genetic code system. The aim of the present study is to clarify the evolution of aminoacyl tRNA synthetase and the process and era of the establishment of the universal genetic code system.