bluuebunny/crossref_metadata_2025_split huggingface dataset:
Radiation Hydrodynamics in Pulsating Stars
Authors: Feuchtinger Michael U. , Dorfi E. A. 
Published: June 2019

AbstractWe present nonlinear radiation hydrodynamical calculations of Cepheid envelopes carried out using the method described in Dorfi & Feuchtinger (1991, A&A 249, 414). The radiative transport is treated within the grey Eddington approximation and all equations are discretised in conservative form on an adaptive mesh which is solved simultaneously with the physical equations. The resulting system of nonlinear algebraic equations is solved implicitly to avoid the restrictive Courant-Friedrichs-Lewy time step condition. In order to treat schock waves we use the artificial tensor viscosity developed by Tscharnuter & Winkler (1979, Comp.Phys.Comm. 18, 171) which is suitable for spherically symmetric problems. For the opacity we employ the latest OPAL tables (Rogers and Iglesias, 1992, ApJ, submitted); the equation of state corresponds to the standard evolution calculations according to Baker & Kippenhahn (1962, Zeitschr. f. Astrophys. 54,114).We start from an initial hydrostatic model whose parameters correspond to a typical Cepheid (5 M⊙, 3000 L⊙, 5600 K). The subsequent dynamical evolution is initiated by the excitation of oscillations at the ionization zones in the stellar atmosphere where the opacity changes by several orders of magnitude. After an initial relaxation caused by discretisation errors and numerical noise, a periodic pulsation develops. Each pulsation is accompanied by a shock wave which propagates through the stellar atmosphere. A typical pulsation cycle is covered by about 70 time steps and the obtained periods lie in the range of 1 to 40 days.We discuss in detail the excitation of the oscillations and the structure of the model during a pulsation cycle.
The Periods of X Cyg, T Mon, Y Oph, S Vul and SV Vul
Authors: Heiser A. M. 
Published: June 2019

AbstractPhotoelectric BV observations have been obtained with the 16-inch Vanderbilt APT over a number of recent years for five Cepheids. These data are being examined for periods, period changes, and for possible variations in the seasonal light curves.
Spectrophotometry determination of Teff's for six Cepheids with P < 4 d
Authors: Antonello E. , Fossati S. , Mantegazza L. 
Published: June 2019

The population I Cepheids with 1.8 < P < 7 d are a non-homogeneous group in the sense that in this period range one can find fundamental, first overtone and double-mode pulsators. This implies that these three types of stars fall approximately in the same luminosity range. The knowledge of accurate physical parameters of these stars is therefore of paramount relevance in order to detect possible significant differences among different mode pulsators.We have made spectrophotometric observations at La Silla Observatory of six Cepheids with P < 4 d (3 first-overtone and 3 fundamental mode pulsators). From a comparison with Kurucz's models of atmospheres (1979) we have derived their Teff's during the pulsational cycle. Due to the relevance of a correct estimate of the interstellar reddening in order to obtain accurate temperatures, we have independently determined the colour excesses of the studied objects by means of observations in the uvbyβ system using the method proposed by Feltz and McNamara (1980).
Extremely Low Amplitude Cepheids
Authors: Paul Butler R. 
Published: June 2019

AbstractSeveral photometric studies conducted in the 1970's indicate that at least half the stars in the Cepheid instability strip are stable at the level of 0.02 mag (Fernie & Hube 1971; Percy 1975; Fernie 1976; Percy, Baskerville, & Trevorrow 1979). A precision radial velocity survey of these “stable stars” is currently being conducted by the author. Radial velocity errors have been reduced to a few tens of meters per second with the use of an iodine absorption cell (Marcy & Butler 1992).Extremely low amplitude (200 m/s) periodic radial velocity variations have been found for HR 7796, an F8Ib supergiant (Butler 1992). Two and a half complete cycles have been monitored over three observing runs. The period is found to be 11.87 days. Observations of the reference star HR 509 (G8 V) taken on the same nights, show no periodicities and a scatter of just 20 m/s. Both the period and the shape of the velocity curve appear similar to a normal F8Ib Cepheid. If HR 7796 is a Cepheid, it is the smallest amplitude Cepheid by more than an order of magnitude.
The Blazhko Effect in RR Lyrae
Authors: Teays Terry J. 
Published: June 2019

AbstractThe cause of the Blazhko effect, the long-term modulation of the light and radial velocity curves of some RR Lyr stars, is still not understood. The observational characteristics of the Blazhko effect are discussed in §1. Some preliminary results are presented from aomw recent of campaigns to observe RR Lyr, using the International Ultraviolet Explorer along with ground-based spectroscopy and photometry, throughout a pulsation cycle, at a variety of Blazhko phases. A set of ultraviolet light curves have been generated from low dispersion IUE spectra. In addition, the (visual) light curves from IUE's Fine Error Sensor are analyzed using the Fourier decomposition technique. The values of the parameters ϕ21 and R21 at different Blazhko phasas of RR Lyr span the range of values found for non-Blazhko variables of similar period.
Graphite Grains and Graphite-Core-Ice-Mantle Grains
Authors: Wickramasinghe N. C. 
Published: June 2019

Among the First Requirements for a Grain Model is that it should explain the observed interstellar extinction law or laws as they are now understood; a desirable feature of such a model is that as few ad hoc assumptions be made as possible.A criticism of the classical ice grain theory is that a very narrow range of grain sizes (or a size distribution with a size parameter specified to within a few percent) must be postulated in order to obtain a fit with the mean extinction law. (See refs. 1 and 2.) While it is true that regional variations in the extinction law have recently been detected (refs. 3 and 4), the best available evidence indicates that the extinction law is quite uniform when averaged over individual large regions which are widely distributed in the galaxy. (See ref. 5 and paper by Nandy in the present compilation.) The restriction of particle size to a radius within a few percent of an arbitrarily specified value (r ≈ 3 X 10-5 cm) demanded on the basis of pure ice absorption is therefore considered quite unsatis-factory, particularly in view of the fact that no characteristic size parameter emerges from the Oort-van de Hulst theory for the formation and destruction of grains. (See ref. 6.)
Nucleation and Grain Growth in Interstellar Space
Authors: Donn Bertram 
Published: June 2019

The First Detailed Studies to determine the processes by which interstellar grains may form were made by a group of Dutch astronomers in the 1940's. (See refs. 1 to 5.) Since that time very little systematic work on this problem has been done until very recently when Hoyle and Wickramasinghe (ref. 6) investigated graphite formation in cool stellar atmospheres. Van de Hulst's paper in 1949 (ref. 5) represents the culmination of an intensive attack which had considerable influence on astronomical thought about interstellar grains.Somewhat ironically, beginning about 1949 many significant advances in physics and chemistry having a direct bearing on this problem were made. In 1949, Frank in reference 7 presented a theory which explained how real crystals tend to grow, and much work, both theoretical and experimental, has been done since then. (See ref. 8.) Recent extensive research in chemical reactions at low temperatures both in solids and on surfaces is reported in reference 9.
A Review of the Achievements and Potential of Intensity Interferometry
Authors: Hanbury Brown R. 
Published: June 2019

AbstractThe stellar intensity interferometer at Narrabri Observatory was operated for ten years. Measurements were made of 32 single stars of spectral type 0 to F and were used to establish the first completely empirical temperature scale for hot stars. The application of a high-resolution interferometer to stellar astronomy was explored: observations were made of binary, rotating and emission-line stars, limb-darkening, polarized light and of the effects of atmospheric scintillation and of Cerenkov radiation from the night sky. This programme was completed in 1973. An analysis has been made of the astronomical programme and value to stellar astronomy of a much larger and more sensitive instrument. An intensity interferometer capable of reaching stars of magnitude +7.3 with a baseline of 2 km has been designed and its cost has been estimated.
Incoherent Infrared Spatial Interferometry
Authors: Low Frank J 
Published: June 2019

AbstractBy adapting the principles of interferometry first developed by Michelson, we have built a simple incoherent interferometer which operates efficiently from 2 to 20 micronsThe design and performance of this instrument will be discussed More than 12 stars have been at least partially resolved at one or more wavelengths and, in several cases, their sizes and shapes have been determined A number of objects in the solar system have also been studied and useful studies of extragalactic nuclei are now possible. With the development of the new Multiple Mirror Telescope both the resolution and the sensitivity of this instrument will soon be extended.
The L-Shell Spectroscopy of Heavy Elements in Laser Produced Plasmas
Authors: Hamilton P.J. , Key M.H. , Lewis C.L.S. , Lunney J.A. , Thareja R.K. , Hutcheon R.J. 
Published: June 2019

The L-shell spectra of heavy elements from germanium to molybdenum have been observed in plasmas generated by high power neodymium lasers at Queens University, Belfast and the Central Laser Facility (Rutherford Laboratory).The spectra were recorded with convex crystal spectrometers with either a mica crystal (radius of curvature 3.81 cm) or a PET crystal (radius of curvature 15 cm). The spectral resolution is 1 in 1000. The spectra show lines from neon-like ions and associated satellites, fluorine-like ions, hydrogen and helium-like ion wavelength calibration lines and satellites from the latter. These will be discussed and some identifications given.
Introductory Remarks on Spectroscopic Parameters
Authors: Abt Helmut A. 
Published: June 2019

I hope that you will excuse a relative newcomer to this field and a spectroscopist for presenting his views, which may be somewhat different than those of others. I would like to enumerate the aims of research on visual systems and the needs of the field, particularly for spectroscopic data, as I see them.I. Aims of Multiple-Star Astronomy. (1). Statistics of multiple systems. We would like to know the fraction of stars that are visually double, triple, etc. and in each case as a function of spectral type. For instance, is the frequency of visual doubles the same for Population II stars as for Population I stars? Or are the frequencies of visual doubles the same for stars off the main sequence as for their antecedents on the main sequence? For such information a negative result on duplicity is as important as a positive result, i.e. we wish to know which stars are not seen to be visually double as well as which ones are seen as doubles.
A Self-Consistent Model Atmosphere Program with Applications to Solar 0I Resonance Lines
Authors: Athay R. Grant , Canfield Richard C. 
Published: June 2019

AbstractProfiles and total intensities are computed for solar 01 resonance lines at λ1302 and λ1305 using a model atmosphere program that includes non-LTE effects in both hydrogen and oxygen and that includes microturbulence both as a line broadening mechanism and as a contribution to the gas pressure. Good agreement is obtained between computed and observed intensities. The computed profiles appear to have too much 'self-reversal.
What Should We Do to Know More About Chromospheres and Coronae of Stars?
Authors: Thomas Richard N. 
Published: June 2019

AbstractChromospheres-coronas satisfy the last two of the proposed classification schemes: inadequacy of the classical atmosphere (CA) model to represent observations and a priori rejection of the CA model So we survey the question of what is required for more knowledge from the standpoint of asking what conceptual modifications will increase knowledge and what new observations are required. We stress that continued progress requires a continual interchange of ideas between the solar situation and the range of stellar situations.
Observations of ζ Aurigae Stars and Their Interpretation
Authors: Groth H. G. 
Published: June 2019

AbstractA summary of information on the extended atmosphere of the K component coming from three eclipsing binaries consisting of a K supergiant and a main sequence B star. Various anomalies exist: (1) Discrepancy between electron density and metallic density inferred from observations, noting that the electrons should come from the metals. This suggests a cloud structure in the atmosphere. (2) Anomalous excitation temperatures and populations of the second quantum level of hydrogen suggests a solar-chromosphere type behavior for these K atmospheres. (3) Significant changes in the profiles of the Ca+ lines from one eclipse to another. The necessity to distinguish between effects coming from radiation of the B star on the K atmosphere and chromosphericlike effects is emphasized.
Microquasars and ULXS: Fossils of GRB Sources
Authors: Mirabell I. F. 
Published: June 2019

AbstractGamma-ray bursts (GRBs) of long duration probably result from the core-collapse of massive stars in binary systems. After the collapse of the primary star the binary system may remain bound leaving a microquasar or ULX source as remnant. In this context, microquasars and ULXs are fossils of GRB sources and should contain physical and astrophysical clues on their GRB-source progenitors. Here I show that the identification of the birth place of microquasars can provide constrains on the progenitor stars of compact objects, and that the runaway velocity can be used to constrain the energy in the explosion of massive stars that leave neutron stars and black holes. The observations show that the neutron star binaries LS 5039, LSI +61°303 and the low-mass black hole GRO J1655-40 formed in energetic supernova explosions, whereas the black holes of larger masses (M ≥ 10 M⊙) in Cygnus X-l and GRS 1915+105 formed promptly, in the dark or in underluminous supornovao. The association with clusters of massive stars of the microquasar LSI +61°303 and the magnetars SGR 1806-20 and SGR 1900+14, suggest that very massive stars (M ≥ 50 M⊙) may -in some cases- leave neutron stars rather than black holes. The models of GRB sources of long duration have the same basic ingredients as microquasars and ULXs: compact objects with accretion disks and relativistic jets in binary systems. Therefore, the analogies between microquasars and AGN may be extended to the sources of GRBs.
Results of a Search for New Microquasars in the Galaxy
Authors: Ribó M. , Paredes J. M. , Martí J. , Casares J. , Bloom J. S. , Falco E. E. , Ros E. , Massi M. 
Published: June 2019

AbstractWe present here the results of a search for new microquasars at low galactic latitudes, based on a cross-identification between the ROSAT all sky Bright Source Catalog (RBSC) and the NRAO VLA Sky Survey (NVSS) and follow-up observations. The results obtained up to now suggest that persistent/silent microquasars such as LS 5039 are rare objects in our Galaxy, and indicate that future deeper surveys, and harder than the RBSC in X-rays, will play a fundamental role in order to discover them.
Revealing the Nature of the Highly Obscured Galactic Source IGR J16318-4848
Authors: Chaty S. , Filliatre P. 
Published: June 2019

AbstractThe X-ray source IGR J16318-4848 was the first source discovered by INTEGRAL on 2003, January 29. We carried out optical and near-infrared (NIR) observations at the European Southern Observatory (ESO La Silla) in the course of a Target of Opportunity (ToO) programme. We discovered the optical counterpart and confirmed an already proposed NIR candidate. NIR spectroscopy revealed a large amount of emission lines, including forbidden iron lines and P-Cygni profiles. The spectral energy distribution of the source points towards a high luminosity and a high temperature, with an absorption greater than the interstellar absorption, but two orders of magnitude lower than the X-ray absorption. We show that the source is an High Mass X-ray binary (HMXB) at a distance between ~ 1 and ~ 6 kpc, the mass donor being an early-type star, probably a sgB[e] star, surrounded by a rich and absorbing circumstellar material. This would make the second High Mass X-ray Binary (HMXB) with a sgB[e] star after CI Cam, indicating that a new class of strongly absorbed X-ray binaries is being unveiled by INTEGRAL.
Accretion Flows in Magnetic Cataclysmic Variables
Authors: Norton A. J. , Somerscales R. V. , Parker T. L. , Wynn G. A. , West R. G. 
Published: June 2019

AbstractWe present the results of simulations that explore the variety of accretion flows possible in magnetic cataclysmic variables (mCVs). Our simulations evolve to equilibrium periods spanning 0.01 < Рspin/Рorb < 0.6 and the resulting flows vary from disc-fed systems at Рspin/Рorb ~ 0.01 - 0.1, to stream fed systems at Рspin/Рorb ~ 0.1 - 0.5 and systems fed from a ring at the outer edge of the white dwarf's Roche lobe at Рspin/Рorb ~ 0.6.
A Strong Variation of Accretion Disc Corona Size with Luminosity in LMXB
Authors: Church M. J. 
Published: June 2019

AbstractThe controversy that has existed for many years over the nature of the continuum X-ray emission components in X-ray binaries is reviewed, in which workers have been polarized between the Eastern model with a small central Comptonizing region around the compact object and alternative models. We present measurements of the radial extent of the Comptonizing ADC in low mass X-ray binaries which rule out the Eastern model and show that, the ADC is extended (the Birmingham model). Dip ingress timing shows conclusively that the ADC radial extent varies from 7% of the accretion disc radius in faint sources, to 65% in bright sources. Remarkably, the size depends strongly on the source luminosity suggesting that the ADC is formed by irradiation of the disc by the neutron star and the hot inner disc. These results have fundamental implications for the correct description of Comptonization in X-ray binaries, and the spectral form is derived for the Comptonized emission of an extended ADC led by soft, seed photons from the underlying disc which differs substantially from that of the Eastern model. Measured ADC electron températures provide values of the Compton radius in broad agreement with measured values of the radial extent of the ADC. Finally, we show that the results are inconsistent with the non-thermal emission being produced in a jet, and so provide evidence against the recent suggestion that all LMXB have jets.
A Simple, Unifying Model of the Extreme Ultraviolet Spectra of Dwarf Novae in Outburst
Authors: Mauche Christopher W. 
Published: June 2019

AbstractA simple model of the boundary layer and accretion disk wind of dwarf novae in outburst is briefly described and applied to the Chandra Low Energy Transmission Grating spectra of WZ Sge and SS Cyg in outburst.
Measuring the Mass Ratios of X-Ray Transients from the Rotational Broadening of the Secondary Star's Spectrum
Authors: Robinson Edward L. , Bitner Martin 
Published: June 2019

We describe a new computer program for calculating the spectra of stars that fill their Roche lobes in close binary stars.Soft X-ray Transients (SXTs) are interacting binary stars in which a compact star is stripping mass from a relatively-normal companion star (the secondary star). One step in the usual method for measuring the mass of the compact stars in these systems is to determine the mass ratio from the rotational broadening of the absorption lines in the spectrum of the secondary. With a few outstanding exceptions (e.g., Shahbaz 2003), the observations have typically been analyzed assuming that the observed spectrum can be modeled by convolving the spectrum of a non-rotating star with a line-broadening function appropriate for a rotating, spherical star. The results are often inadequate because 1) the profiles of the absorption lines vary strongly with orbital phase, and 2) the limb darkening differs from line to line so that no single broadening function can represent the behavior of all lines.
Using High Resolution Data to Investigate the Variability of GG Carinae System
Authors: Machado Maria Auxiliadora , Araújo Francisco Xavier de , Lopes Dalton de Faria , Pereira Claudio Bastos 
Published: June 2019

GG Carinae is a peculiar emission line object classified as a B[e] star. Using high resolution data we confirm that GG Carinae is at same time a binary system and a variable star.GG Carinae was classified by Lamers et al. (1998) as a B[e] star and also is pointed out as a binary system. So, any investigation to determine its parameters requires an detailed analysis in order to know and to separate the effects of each component. Gosset et al. (1984) used Fourrier Analysis in the data obtained from photoeletric photometry of both the standard UBV and Strongren system and calculated a period of 31D.02.
Clementine Photographs of the Inner Zodiacal Light
Authors: Cooper B. L. , Zook H. A. , Potter A. E. 
Published: June 2019

AbstractOver 700 photographs of the inner zodiacal light were taken from the Clementine spacecraft while it was in orbit around the Moon. These exposures were taken with the 28° x 43° field-of-view Star Tracker camera and the 4.2° x 5.6° field-of-view UV/Vis camera. The images were made while the Clementine spacecraft was on the dark side of the Moon such that the Sun was occulted. Most of the photos were taken at the highest possible sensitivity and longest exposure time (0.7 sec) in order to detect an expected weak lunar horizon glow. Consequently, many of the photos are over exposed where the zodiacal light is the brightest. However, a subset of photos were purposefully taken with a range of exposure times to reveal the entire inner zodiacal light structure, both in latitude and longitude, to within 1° of the Sun. These Star Tracker images show the lenticular shape of the inner zodiacal light. When work to correct the images to absolute photometry is concluded, the detailed structure of the entire inner zodiacal light will be derived.
Understanding vocabulary acquisition, instruction, and assessment: A research agenda
Authors: Schmitt Norbert 
Published: June 2019

AbstractThis paper suggests six areas of vocabulary research which the author believes would be fruitful for future research. They include (1) developing a practical model of vocabulary acquisition, (2) understanding how vocabulary knowledge develops from receptive to productive mastery, (3) getting lexical teaching/learning principles into vocabulary and language textbooks, (4) exploring extramural language exposure and how it can best facilitate vocabulary acquisition, (5) developing more informative measures of vocabulary knowledge, and (6) measuring fluency as part of vocabulary competence. Nine tasks are suggested for how to research these six research directions, with advice on research design and how to set about carrying out the tasks.