bluuebunny/crossref_metadata_2025_split huggingface dataset:
Turbulence in the Atmosphere of B-Type Stars
Authors: Kodaira Keiichi 
Published: July 2016

AbstractThe stationary turbulent surface layer, whose depth is of the order of the pressure scale height in the subphotospheric layer, was investigated for B-type stars, using the momentum and the continuity equations with the inertia term neglected but the turbulence-viscosity term included. The mean velocity field is dominated by the horizontal component of the meridional circulation, driven by the pressure-density unbalance in the radiative envelope of the rotating star, and the differential rotation induced by the Coriolis force.The model calculation for a B3IV-V star with the equatorial rotational velocity of 200 km/s led to the conclusions that the velocity field due to the differential rotation is of the order of 0.1-1 km/s, the velocity field of the meridional circulation itself is negligible, the velocity field of the three-dimensional shear turbulence is of the order of 0.1 km/s, and i t s scale is comparable to or less than the pressure scale height, the velocity field of the horizontal two-dimensional turbulence is of the order of 1-10 km/s, and i t s maximum scale ranges from a few times the pressure scale height to one-fiftieth of the stellar radius. If the index of the energy density law is close to 3.5, the turbulent surface layer may be dominated by the large scale (two-dimensional) barotropic eddies whose energy is fed by the small scale (three-dimensional) baroclinic turbulence in a way similar to the planetary atmospheres. In this case we may expect the tangential macroturbulence of the order of 1-10 km/s, though the interaction between this and the small-scale three-dimensional turbulence still remains to be investigated.
The Application of Mesoturbulence to Stellar Atmospheres
Authors: Sedlmayr E. 
Published: July 2016

AbstractFor realistic stellar atmospheres the equations describing mesoturbulent line formation are solved numerically. The general dependence of theoretical line profiles and equivalent widths on the correlation length 1 and the mean square turbulent velocity a is demonstrated. Also empirical relations between the basic parameters of the micro-macroturbulence description (vmic, vmac) and the fundamental mesoturbulence parameters (1,σ) are derived.
Some Effects of Strong Acoustic Waves on Strong Spectral Lines
Authors: Gouttebroze Pierre , Leibacher John 
Published: July 2016

AbstractWe have studied the formation of optically thick lines in time dependent, nonlinear hydrodynamic model of the solar chromosphere. Models of the 200 second, chromospheric oscillation indicate that the emission peaks of self-reversed profiles such as those of Mg II and Ca II are formed at very different depths depending on the phase of the oscillation, while the central absorption feature is emitted at a very nearly constant mass depth. The figure shows the Mg II k line emitted by the mean atmosphere, including “microturbulence” (triangles) and the mean profile (squares). In addition to the substantial intensity increase interior to the emission peaks, one should note that the peaks are broadened only towards line center; i.e. the intensity fluctuations are symmetric outside of the peaks and strengthen more than they weaken within the peaks. A more detailed version has been submitted to the Astrophysical Journal.J.L. wishes to acknowledge support by NASA contracts NASA-3053 and 5-23758.
The Solar Chromospheric Microturbulence and the Emission Observed at Eclipse
Authors: Cuny Y. 
Published: July 2016

AbstractFor a long time there was not a clear interpretation of the fact that the emission observed at eclipse is systematically too high compared with predictions based on disk observations. This discrepancy has been attributed to the hydrogen density, the atomic abundances, or the multiplet excitation temperatures, the physical cause being assumed to be some inhomogeneities. We have shown (Liege 1978, under press) that it can be attributed to the microturbulence. The radiative energy absorbed in the doppler width of the absorption profiles of lines, produces high excitation temperatures. The relative abundances of Ba II, Sr II, Ti II with these excitation temperatures are in agreement with the photospheric abundances.Assuming photospheric abundances, we get the hydrogen density versus the altitude while the observations give ne ne/Te3/2. The solution of these two equations, taking account of non-LTE for hydrogen but without the assumption of hydrostatic equilibrium, gives a chromospheric model. The model obtained is very near the usual quiet chromospheric models based on disk data.This work confirms the validity of our assumption of the influence of microturbulence on the chromospheric emission observed at eclipse.
On the Structural and Stochastic Motions in the Solar and Stellar Atmospheres
Authors: Mogilevsky E.I. 
Published: July 2016

AbstractExistence of discrete structures of velocity and magnetic field in stellar atmospheres follows from the Vlasov’s integral equationwhere is a statistical function of the distribution of the elements of matter having masses and velocities, v, dT is elementary phase volume, F is functional which arrange the ties between macrovelocity, magnetic field, temperature, etc., providing self containment of cosmic plasma.
On the Establishment of Internally Consistent Abundance-Oscillator Strength Scales
Authors: Gurtovenko E.A. , Kostik R.I. 
Published: July 2016

AbstractThe method of establishing of internally consistent abundance-oscillator strength scales by using solar fraunhofer lines is elaborated and investigated.The error of internal accuracy should not exceed 0.05 - 0.06 dex. The absolute accuracy depends on the accuracy of “reference” gf-values.The oscillator strengths for about 800 Fe I lines are obtained. The comparison of the results for 19 lines common in our and Blackwell et al. (1976) investigations gives the difference log gfblack - log gfauth = Δ = -0.044 ± 0.010. The accidental part of the difference actually determines the internal accuracy of the obtained oscillator strengths.The Kurucz and Peytremann (1975) oscillator strengths for Fe I lines are analysed. Large systematic errors depending on gf and excitation potential are revealed and investigated. For some lines those errors may change the true values of gf by two orders of magnitude.
The Location of the ZZ Ceti Stars on the Hertzsprung-Russell Diagram
Authors: Cox John P. , Hansen Carl J. 
Published: July 2016

AbstractBy means of a simple analysis, it is shown that the combination of long periods and relatively high effective temperatures in ZZ Ceti variables may be made consistent with each other if the driving region for instability occurs at a temperature of (1–2) × 105K. Since this temperature does not coincide with the zone of He+ ionization, the driving mechanism is then not the same as for the Cepheid variables and the linear extension of the Cepheid strip down to the ZZ Ceti white dwarfs may be regarded as a coincidence. A Stelllngwerf “bump” type mechanism may be more appropriate for the latter. However, it is also shown that the short period modes computed by Dziembowski and found to be unstable, are consistent with the Cepheid mechanism. A special plea is made for further opacity calculations at temperatures of ~105 K and densities within a few orders of magnitude of unity — a ρ-T region of considerable interest for the envelopes of ZZ Ceti variables.
Theory and Observations of Accretion Disc Spectra
Authors: Mayo S.K. , Wickramasinghe D.T. , Whelan J.A.J. 
Published: July 2016

Theoretical continua (3000 ≲ λ (Å) ≲ 20000), UBV colours and Balmer absorption line profiles (Hα – Hδ) have been computed for the optically thick parts of steady-state accretion discs around white dwarfs as a function of mass flow rate, mass of white dwarf and inclination angle. The uncertainties due to disc z-structure and outer radius and viscosity are considered.The theoretical results for continua and UBV colours are compared with the theoretical results of Schwarzenberg-Czerny and Rózyczka (1977) and Herter et al. (1979) and with results obtained using black bodies instead of stellar atmospheres (e.g. Lynden-Bell 1969). Where models can be directly compared the quantitative agreement is good.
Extreme and Far Ultraviolet Astronomy from Voyagers 1 and 2
Authors: Holberg J.B. 
Published: July 2016

AbstractThe instrumental characteristics, observational capabilities and scientific results of the Voyager 1 and 2 ultraviolet spectrometers are reviewed. These instruments provide current and ongoing access to low resolution spectra for a wide variety of astronomical sources in the 500 to 1700 Å band. Observations of the brightest OB stars and hot subluminous stars as faint as V = 15 mag. are possible. In the EUV, at wavelengths shortward of 900 Å, several new sources have been detected and a host of potential sources ruled out. In the Far UV, particularly at wavelengths between 900 and 1200 Å, Voyager is capable of observing a wide range of stellar and non-stellar sources. Such observations can often provide a valuable complement to IUE and other data sets at longer wavelengths. The Voyager spectrometers have proved remarkably stable photon counting instruments, capable of extremely long integration times. The long integration times, relatively large field of view, and location in the outer solar system also provide an ideal platform for observations of sources of faint diffuse emission, such as nebulae and the general sky background.
Röntgen Satellite
Authors: Trümper J. 
Published: July 2016

The ROSAT launch on June 1, 1990 happened to be after the IAU Colloquium No. 123 before the deadline for submitting manuscripts. I therefore take the liberty to deviate grossly from the manuscript of my talk and give a short up to date mission summary. A more complete description of the mission can be found in References 1 and 2.The launch of the Delta II rocket was perfect and the orbital parameters reached are very close to nominal: height 580 km, inclination 53°. The predicted satellite’s lifetime in this orbital is at least 10 years. The switch-on of the spacecraft and instrument subsystems could be completed without any losses. All systems are in good health.
The Gamma-Ray Observatory
Authors: Kniffen Donald A. 
Published: July 2016

AbstractThe Gamma-Ray Observatory (GRO), a Shuttle-launched free-flying Observatory is currently scheduled for launch in November 1990. The mission provides nearly six orders of magnitude in spectral coverage, from about 50 KeV to about 30 GeV, with a sensitivity over the entire range over an order of magnitude greater than that of previous observations. The 16,000 kilogram Observatory contains four instruments on a stabilized platform. The mission duration is expected to be from six to ten years. A Science Support Center has been established at the Goddard Space Flight Center for the purpose of supporting a vigorous Guest Investigator Program.
First – Far Infrared and Submillimetre Space Telescope
Authors: Frisk U. O. 
Published: July 2016

AbstractFirst is an element of the ESA long term science program. Currently in a detailed study phase it is foreseen as a large (4.5–8 m) diameter passively cooled telescope equipped with a combination of photometer/camera and very high resolution spectrometers. The spectrometers will utilize both direct detection and heterodyne techniques to cover the wavelength band 85–600 micron. The photometers will have both bolometer and photoconductor detector arrays. FIRST is foreseen to be launched shortly after the year 2000 and will be operated as a facility open to the wide scientific community.
SOHO – an Observatory to Study the Solar Interior and the Solar Atmosphere
Authors: Poland A.I. , Domingo V. 
Published: July 2016

AbstractThe Solar and Heliospheric Observatory, Soho, is a joint venture of ESA and NASA. The main objectives of Soho are: a) the study and understanding of solar coronal phenomena; and b) the study of the solar structure and interior dynamics from its core to the photosphere. The primary goals of the coronal and solar wind studies are to understand the coronal heating mechanism and its expansion into the solar wind.These goal will be achieved both by remote sensing of the solar atmosphere with high resolution spectrometers and telescopes and by “in situ” measurement of the composition and energy of the resulting solar wind and the energetic particles that propagate through it. The structure and interior dynamics will be studied by helio-seismological methods and the measurement of solar irradiance variations. The Soho spacecraft will be three-axis stabilized and located in a halo orbit around the L1 Lagrangian point (approximately 1% of the distance from the Earth to the Sun). It is currently scheduled for launch in July 1995.
The Broad Band X-Ray Telescope (BBXRT) on Astro-1
Authors: Serlemitsos P.J. 
Published: July 2016

AbstractBBXRT is one of four pointed instruments comprising NASA’s Astro-1 mission, currently scheduled for a 9–10 day flight in May 1990 on board the Space Shuttle Columbia. X-ray Spectra of over 150 sources will be obtained in the energy range 0.3 to 12 keV, with energy resolution of ~100 eV FWHM and timing resolution of 64 μS. BBXRT’S large throughput, broad band mirrors, coupled to segmented cooled Si(Li) detectors, give us the first opportunity to study in detail X-ray spectral features, from oxygen to the iron K band. In developing this instrument, we have made extensive use of detector background reduction techniques in order to be able to observe the many faint extra-galactic sources discovered with the Einstein observatory. Sources in our observing plan have been selected to give us in a 2–4 ks observation < 103 net counts with negligible background. Specific scientific objectives range from refinements of spectra previously obtained at lower resolution, such as a search for relativistic line shifts in compact objects, to observations of faint extragalactic sources for which spectral information is, at best, sketchy, viz. elliptical galaxies and distant quasars. High on our priorities are a search for emission from SN1987A and the first simultaneous observations in the UV and X-ray band of selected sources such as cataclysmic variables and late-type stars.
Future Deep Space Propulsion Systems
Authors: Stuhlinger Ernst 
Published: July 2016

AbstractAmong several potential future deep space propulsion systems, the two which are closest to realization are selected for closer consideration: solar-electric, and nuclear-electric propulsion. In particular, the paper describes a manned Mars mission using a particle bed reactor and Brayton cycle converter as power source. Technical details of the design and the mission profile of a 4-astronaut expedition to Mars, and a proposed course of action for project implementation are presented.
Evolution of the Unsolved Problems
Authors: Oda Minoru 
Published: July 2016

I would like to trace the evolution of the major problems or questions since the 1950’s by showing several diagrams (Fig. 1a-e) based on my own recollections. As for the coordinates of the diagrams, the latitude towards North indicates the magnitude of surprise and that towards the South indicated is the deepening of our knowledge. Of course such a visual approach is incomplete and is full of personal prejudices, and many important subjects and considerations may be missing.I will not go into detail of the interpretation of the diagram. But from the series of the diagram we may see that unification and diversification of the questions alternate as one of our co-panelists, Freeman Dyson, pointed out in one of his inspiring books.In the 1970’s to the 1980’s we had good hopes that GUT would develop and in its evidence mono-poles and Proton-decay’s would be discovered and that proposed candidates of the dark matter may also be discovered from fairly reasonable particles to much more exotic particles.
The Stellar X-Ray Polarimeter for the Spectrum-X-Gamma Mission
Authors: Kaaret P. , Novick R. , Martin C. , Shaw P. , Fleischman J.R. , Hamilton T. , Sunyaev R. , Lapshov I. , Silver E. , Ziock K. , Weisskopf M. , Elsner R. , Ramsey B. , Chanan G. , Manzo G. , Giarrusso S. , Santangelo A. , Costa E. , Piro L. , Fraser G. , Pearson J.F. , Lees J.E. , Perola G.C. , Massaro E. , Matt G. 
Published: July 2016

AbstractWe describe an X-ray polarimeter which will be flown on the SPECTRUM-X-Gamma mission. The instrument exploits three distinct physical processes to measure polarization: Bragg reflection from a graphite crystal, Thomson scattering from a metallic lithium target, and photoemission from a Cesium Iodide photocathode. These three processes allow polarization measurements over an energy band of 0.3 keV to 12 keV. The polarimeter will make possible sensitive measurements of several hundred known X-ray sources. X-ray polarization measurements will allow us to constrain the geometry of gas flow in X-ray binaries, identify nonthermal emission in supernova remnants, test current models for X-ray emission in radio pulsars, determine the radiation mechanisms in active galactic nuclei, and search for inertial frame dragging (Lense-Thirring effect) around the putative black hole in Cygnus X-1.
Project of a Three Reflection Telescope for Wide Field Ultraviolet Observations
Authors: Amoretti A. , Badiali M. , Preite-Martinez A. , Viotti R. 
Published: July 2016

AbstractWe present a project of a three-reflection 1.5 m f/3 aplanatic wide-field telescope, in view of a small-satellite mission for the all-sky survey in the Ultraviolet.
Low Frequency Radio Astronomy from Earth Orbit
Authors: Weiler Kurt W. , Kassim Namir E. 
Published: July 2016

AbstractLow frequency radio astronomy for the purpose of this discussion is defined as frequencies ≲100 MHz. Since the technology is fairly simple at these frequencies and even Jansky’s original observations were made at 20.5 MHz, there have been many years of research at these wavelengths. However, though radio astronomers have been working at low frequencies since the first days of science, the observing limitations and the move of much of the effort to ever shorter wavelengths has meant that most areas still remain to be fully exploited with modern techniques and instruments. In particular, the possibilities for pursuing the very lowest frequencies by interferometry of ground to space, in Earth orbit, or from the Moon promises a rebirth of work in this wavelength range.We present concepts for space-ground VLBI and a fully space-based array in high Earth orbit to pursue the astrophysics which can only be probed at these frequencies. An Orbiting Low Frequency Radio Astronomy Satellite (OLFRAS) and a Low Frequency Space Array (LFSA) are two concepts which will open this last, poorly explored area of astronomy at relatively low cost and well within the limits of current technology.
High-Resolution Imaging Spectroscopy at Terahertz Frequencies
Authors: Brown Robert L. , Kerr Anthony R. , Thompson A. Richard , Schwab Frederic R. 
Published: July 2016

AbstractHISAT, a multi-element heterodyne interferometer attached to Space Station Freedom, will provide spectroscopic images with unprecedented detail of those submillimeter lines of C,O and C+ which are critical diagnostics of UV excitation in the Galaxy. With the arcsecond angular resolution achievable from the space station, HISAT will reveal:
– The distribution of sources of ultraviolet radiation in the Galaxy;–The effective temperature of the UV radiation as a function of galactocentric radius;– The chemical and isotopic enhancement of atomic carbon and oxygen with galactic radius;– The propagation of UV radiation in molecular clouds and its stimulative, or inhibitive, effect on star formation;– The density structure, dumpiness or fragmentation, of molecular clouds throughout the Galaxy. HISAT has been selected by NASA for a concept-phase study.
Structure of Radiatively Cooled Jets
Authors: Kondo Masa-Aki 
Published: July 2016

AbstractRadiative cooling strongly affects the thermal structure of dense jet, such as in SS433, through free-free emission. From the dynamical aspect, the beam width of a cooled jet does not expand, unlike from an adiabatic jet. From the thermal aspect, cooling efficiency determines the ratio of X-ray region of high temperature to optical one of low temperature. However, this ratio is influenced by the heating due to contained high-energy particles, which produce synchrotron radiation in the tail of the jet.Extragalactic jets can also be considered in a similar way due to other energy loss mechanisms.
Studying the Galactic Central Engine from Space Observatories
Authors: Greyber Howard D. 
Published: July 2016

Three general models have been constructed for the fantastically powerful “central engine” that powers the enormous energy output from quasars and active galactic nuclei (AGN). One model assumes a rapidly rotating accretion disk around a central black hole (however the disks, thick or thin, are subject to violent instabilities). Another assumes that in some postulated circuitry energy is extracted from the rotational portion of the deepest potential hole known, a black hole. Both models appear implausible.The third model is the STRONG MAGNETIC FIELD MODEL (SMF) in which an extremely strong gravitationally bound current loop (GBCL) is formed during the gravitational collapse that forms the galaxy or quasar, producing a very intense dipole magnetic field anchored in the nucleus. SMF, first published in 1962, thus predicted the vertical magnetic field configuration seen today at our own galactic nucleus; to some the radio arcs observed suggest a dipole magnetic field there, just as SMF predicts.
Monte Carlo Simulations of Particle Acceleration at Oblique Shocks
Authors: Baring Matthew G. , Ellison Donald C. , Jones Frank C. 
Published: July 2016

AbstractThe Fermi shock acceleration mechanism may be responsible for the production of high-energy cosmic rays in a wide variety of environments. Modeling of this phenomenon has largely focused on plane-parallel shocks, and one of the most promising techniques for its study is the Monte Carlo simulation of particle transport in shocked fluid flows. One of the principal problems in shock acceleration theory is the mechanism and efficiency of injection of particles from the thermal gas into the accelerated population. The Monte Carlo technique is ideally suited to addressing the injection problem directly, and previous applications of it to the quasi-parallel Earth bow shock led to very successful modeling of proton and heavy ion spectra, as well as other observed quantities. Recently this technique has been extended to oblique shock geometries, in which the upstream magnetic field makes a significant angle ΘB1 to the shock normal. In this paper, spectral results from test particle Monte Carlo simulations of cosmic-ray acceleration at oblique, nonrelativistic shocks are presented. The results show that low Mach number shocks have injection efficiencies that are relatively insensitive to (though not independent of) the shock obliquity, but that there is a dramatic drop in efficiency for shocks of Mach number 30 or more as the obliquity increases above 15°. Cosmic-ray distributions just upstream of the shock reveal prominent bumps at energies below the thermal peak; these disappear far upstream but might be observable features close to astrophysical shocks.Subject headings: acceleration of particles — cosmic rays — shock waves