bluuebunny/crossref_metadata_2025 huggingface dataset:
Observations of Spectroscopic Binaries at Ultraviolet and Red Wavelengths
Authors: Fekel Francis C.
Published: July 2016

Recent advances in detector technology as well as the launching of the International Ultraviolet Explorer (IUE) satellite have opened new spectral regions for extensive spectroscopic observation. A program to determine the feasibility of radial velocity measurement for binaries whose secondaries are of earlier spectral type has been initiated with the IUE satellite. It is expected that velocities with uncertainties as small as 2-5 km s−1 may be obtained for some stars. This observing program includes tau Per and other composite spectrum binaries as well as several early-type binaries.

For the past five years, a Reticon solid-state photodiode array detector has been used at the coudé focus of the 2.7 m McDonald Observatory reflector to obtain radial velocities of spectroscopic as well as short-period visual binaries. The high quantum efficiency, red sensitivity, and stability of this detector system permit velocities of high accuracy to be determined and previously unseen spectroscopic components to be detected.
Five Years of Double Star Interferometry and its Lessons
Authors: McAlister Harold A.
Published: July 2016

With nearly 1500 diffraction limited measurements of high accuracy to its credit, speckle interferometry must now be considered an established and productive method for performing binary star astrometry. This paper will attempt to give an explanation of the technique from the perspective of a binary star observer along with a general summary of the accomplishments that have given speckle interferometry its present respectability. Some recommendations aimed at increasing the effectiveness of the method will also be made. Speckle interferometry has provided valuable information on stellar diameters, and tantalizing results are beginning to appear for a variety of faint objects, but only the application to the measurement of binary stars will be considered here.
First Results Concerning the Capability of a TV Camera for Double Star Observations
Authors: Dommanget J.
Published: July 2016

The results of some first laboratory analysis of double star images recorded with a TV camera are presented.

The camera – already in use since a few years for occultation-timings has been described earlier. For double star observations, the equipment has been completed by a video disc memory, a video analyser with analogic-digital converter and a small programmable calculator.

Details are given about this equipment and its possibilities. The reduction method in its present state is described as well as some first results. These show a very good internal agreement leading to a high promising new observational technique.
Double Stars and Space Telescope
Authors: Fredrick Laurence W.
Published: July 2016

With the success of the Shuttle it seems now the Space Telescope will become a reality. The author wrote a study for the Langley Research Center in the summer of 1963 on the astronomical uses of a large space telescope which formed the basic document for the National Academy of Sciences Woods Hole study in 1965. This study recommended construction of a large space telescope scaled to use a Saturn V as the launch booster.

The initial study included some astrometric applications. Among these were various double star applications which is the subject of this symposium.
Remarks on the Thiele-Innes Elements
Authors: Dommanget J.
Published: July 2016

The Thiele-Innes elements show a discontinuity for e = 1. Other elements are proposed having similar expressions and same advantages but offering the interest of being valid in all cases of eccentricity.
New theoretical developments in stellar pulsation
Authors: Christensen-Dalsgaard Jørgen
Published: July 2016

The basic properties of stellar oscillations are reasonably well understood, allowing measurements of their frequencies to be used as probes of stellar interiors. The detailed understanding of the processes responsible for the oscillations, in the very broad range of stars observed to pulsate, has improved substantially over the past decade, as have the techniques for asteroseismic investigations on the basis of the observations. Here I provide a brief overview of the theory of stellar oscillations, emphasizing several cases of recent progress, often inspired by new observational developments.
The DIRECT Project
Authors: Macri Lucas M.
Published: July 2016

The DIRECT project aims to determine direct distances to two important galaxies in the cosmological distance ladder – M31 and M33 – using detached eclipsing binaries (DEBs) and Cepheids. I summarize the results of the project and present some results from follow-up observations.
A Variable Star Survey of the Open Cluster NGC 2126
Authors: Derekas A. , Kiss L.L. , Bedding T.R. , Gáspár A. , Sárneczky K. , Szabó Gy.M. , Váradi M. , Kaspi S. , Kiss Cs.
Published: July 2016

We present the first CCD photometric observations of the northern open cluster NGC 2126. Data were taken on eight nights in 2002 February and December with a total time span of ~57 hours. We have discovered six new variable stars and have estimated the main characteristics of the cluster.
Double-radial-mode SX Phoenicis stars in the globular clusters NGC 5466 and M71
Authors: Jeon Young-Beom , Lee Myung Gyoon , Kim Seung-Lee , Lee Ho
Published: July 2016

Through time-series CCD photometry of the globular cluster NGC 5466, we found three double-radial-mode SX Phe stars, all of which are consistent with the theoretical period ratio of the first overtone mode to the fundamental mode (P1H/PF); their period ratios are 0.7825, 0.7826 and 0.7919, respectively. We also detected a double-radial-mode SX Phe star in M71. Its period ratio is 0.781, which is also consistent with the theoretical value.
Spectroscopy of southern δ Scuti stars
Authors: Cropp Malcolm , Pollard Karen R. , Skuljan Jovan
Published: July 2016

Four δ Scuti stars were observed with the HERCULES fibrefed échelle spectrograph at Mount John University Observatory, New Zealand. These observations were analysed by looking at the radial velocity variations as given by a cross-correlation technique as well as spectral line moment variations. These results were compared to published photometric studies of these stars to see if the modes identified in the photometry were also present in the spectroscopic data obtained.
Metallicity effects in Mira variables: a model study
Authors: Scholz M. , Wood P.R.
Published: July 2016

M-type Mira models with a moderate deviation from solar metallicity (2x and 0.5 x) have been computed and compared to solar-metallicity models. Selected effects of abundance changes on pulsation properties, atmospheric stratifications, spectral features (colours) and surface brightness distributions are discussed.
Time variations of IR spectra and dust properties in carbon-rich Mira variables
Authors: Sung Hyun-il , Lee Sang-gak
Published: July 2016

We have studied variations of physical parameters in the circumstellar envelopes of carbon-rich Mira variables at different pulsation phases. After comparing the ISO/SWS spectra of these stars with spectral energy distributions calculated from models, we could find the best-fitted parameters for the physical state of the dust envelope at each phase. The variations of the parameters effective temperature, fraction of SiC grains, mass-loss rates, and inner radius of shell, are found to have nearly the same trend as that of the light curve. The other parameters grain size, optical depth, radius of central star and dust temperature at the inner boundary of envelope, are inversely varying to the light curve.
Pulsating white dwarfs in cataclysmic variables: The marriage of ZZ Cet and dwarf nova
Authors: Warner Brian , Woudt Patrick A.
Published: July 2016

There are now four dwarf novae known with white dwarf primaries that show large amplitude non-radial oscillations of the kind seen in ZZ Cet stars. We compare the properties of these stars and point out that by the end of the Sloan Digital Sky Survey more than 30 should be known.
Precessing accretion discs in binary systems
Authors: Retter Alon , Liu Alexander , Bos Marc
Published: July 2016

Superhumps – quasi-periodic oscillations at periods a few percent different than the orbital periods – have been observed in several dozen binary systems of various types. It is well-accepted now that they are caused by the precession of the accretion disc. New findings on TV Col, TX Col and V4742 Sgr are presented. These cataclysmic variables (CVs) have exceptional precessing periods that cast strong doubt on the predictions of the Tidal Disc Instability model.
Theory of Magneto-Acoustic Modes in roAp Stars
Authors: Bigot Lionel
Published: July 2016

In this review I present the effects of a strong magnetic field on the pulsations of rapidly oscillating Ap stars. I show how the field affects the observables, such as the frequencies and eigenvectors, and the selection of modes.
Non-adiabatic Asteroseismology of Near-Main Sequence Variable Stars
Authors: Dupret M.-A.
Published: July 2016

We present in this talk a new linear non-adiabatic pulsation code: MAD. We detail all the possible applications of it in the frame of asteroseismology. By using it, photospheric observables such as multi-color photometric amplitude ratios, phase differences and line profile variations can be determined accurately and confronted with observations. Moreover, MAD enables the precisely analysis of the excitation mechanisms of pulsating stars. Applications to different kinds of variable stars (δ Sct, β Cephei, Slowly Pulsating B and γ Dor stars) are presented by Dupret et al. (these proceedings), Handler & Aerts (these proceedings), De Cat et al. (these proceedings) and De Ridder et al. (these proceedings).
Analysis of Long-Term Single-Site Observations of The Pulsating DB White Dwarf GD 358
Authors: Bradley P.A.
Published: July 2016

We briefly describe the analysis of single-site data collected from 1982 to 1992 for the pulsating DB white dwarf GD 358. We show that the amplitudes of almost all of the l = 1 modes change over time, and some of the l = 1 modes disappear for months at a time. We also find a new mode at k = 19, with a frequency of ~1173μHz.
The Evolution of Stars of Medium Mass
Authors: Chiosi Cesare
Published: July 2016

The main properties of the evolution of low, intermediate, and high mass stars are reviewed focusing on a few issues tightly related to the interpretation of Pop I Cepheid stars. After a summary discussion of the physical mechanism responsible for the Cepheid pulsation, the classical results of stellar evolution theory for the main evolutionary phases (main sequence, core He-burning, and later) all over the HR diagram are presented, putting into evidence the various points of disagreement with current observational data. We then review the models incorporating the effect of convective overshoot, and present in some detail a study on the rich, young cluster NGC 1866 in the Large Magellanic Cloud, in which they are compared with the observational data. Arguments are given to favour the adoption of models with convective overshoot instead of the classical ones. The topics of the mass discrepancy and frequency-period distribution of Cepheid stars are then discussed in the light of models with convective overshoot, showing how they can better account for the observational data. Finally, we address the question of the enhancement in the radiative opacity of heavy elements in the middle temperature region (approximately where the ultimate ionization of the CNO group of elements occurs), which has been suggested to explain the mass anomalies and period ratios encountered in double-mode and bump Cepheids. Evolutionary models for intermediate mass stars incorporating the above modification in the radiative opacity are then presented and discussed in some detail.
Progress Toward an Improved Equation of State and Opacity for Stellar Envelopes
Authors: Mihalas Dimitri
Published: July 2016

A brief description is given of a large international project for computing new stellar envelope opacities and equation of state data.
Stellar Mass Loss and Pulsation
Authors: Willson L.A.
Published: July 2016

Mass loss at rates sufficient to alter the evolution of stars is known to occur during the pre-main sequence evolution of most stars, on the main sequence for massive stars, and during advanced evolutionary phases when the luminosity is high and the effective temperature is low. While most investigations of the effects of mass loss on stellar evolution have assumed continuous (parametrized) mass loss laws apply, there is increasing evidence that mass loss rates are substantially higher for stars that are pulsating with large amplitude and/or in selected modes. Some new insights into the mass loss that terminates the AGB evolution of intermediate mass stars, and leads to the formation of planetary nebulae, come from recent detailed studies of the mass loss process from the Mira variables.
Anomalous Cepheids and Population II Blue Stragglers
Authors: Nemec James M.
Published: July 2016

Recent studies of anomalous Cepheids (ACs) and Pop II blue stragglers (BSs), including photometrically variable BSs (VBSs), are reviewed. The VBSs represent about 25% of the BSs, the majority of which are SX Phe short-period variables in the Cepheid instability strip. Mass estimates derived using various techniques suggest that both ACs and BSs are relatively massive (about 1.0–1.6 Me). The recent discovery that two BSs in the globular cluster NGC 5466 are contact binaries, and the earlier discovery that one of the BSs in ω Cen is an eclipsing binary, provide direct evidence that at least some BSs are binary systems. If all BSs are binaries, and the time scale for coalescence is a few Gyr, then the majority are likely to be coalesced. Because ACs and BSs are found in the same stellar systems, and are probably related through their evolution, it is highly likely that most ACs are also coalesced binary systems. The fact that ACs and BSs are found only in low density environments, suggests that they were primordial binaries.
Cepheid Evolution with Pulsationally-Driven Mass Loss
Authors: Brunish Wendee M. , Willson Lee Anne
Published: July 2016

We have studied the effects of a pulsationally-driven wind on Cepheid evolution. Mass loss due to the wind, which occurs only when the star is crossing the Cepheid instability strip, is a function of luminosity and radius. We have investigated the evolution of 4, 5, 6, 7 and 8 M⊙ stars using the updated 12C(α,γ) 16O rates.
Discontinuity Modes in Polytropes
Authors: Carroll Bradley W.
Published: July 2016

In the calculation of linear nonradial oscillation modes in composite polytropes with a small density discontinuity, a discontinuity mode may occur. This mode consists of a wave propagating along the discontinuity interface with a large amplitude that declines exponentially away from the interface. The period P of this mode is well-estimated (to within 10%) by

where Δρ/<ρ> is the fractional density discontinuity and k is the horizontal wavenumber (c.f. Gabriel and Scuflaire 1980, in Nonradial and Nonlinear Stellar Pulsation, eds. H. Hill and W. Dziembowski, Springer-Verlag). For a 12 solar-mass polytrope with a radius of 4.27 R⊙, a 3% density discontinuity at fractional radius 0.15 produces a discontinuity mode with a period of 7.329 hours. As the density discontinuity increases the period P decreases, resulting in avoided crossings with the normal g-mode spectrum. Between these avoided crossings, the discontinuity mode has an unusually large amplitude at the location of the discontinuity.
S Sge: A Cepheid Triple System
Authors: Evans Nancy Remage , Slovak Mark H. , Welch Douglas L.
Published: July 2016

IUE spectra have been used to investigate the companion of the classical Cepheid S Sge. At the phase of the IUE observations, 45 Dra (F7Ib) is a good match to S Sge, both in (B–V)o and from 2200 to 3200 Å. However, from 1700 to 1900 Å, the S Sge spectrum has excess flux, as compared to 45 Dra. The flux remaining after subtracting 45 Dra from S Sge is a good match to an A5V to A7V star. The magnitude difference between the Cepheid and the companion is consistent with the absolute magnitude and colors of an A5V star. This corresponds to a mass of 1.9 solar masses (Popper, 1981, Ann. Rev. Astr. Ap., 18, 115). A spectral type range from A3V to A7V corresponds to an uncertainty in the mass of ±0.2 solar masses. However, the mass function from the orbit and an evolutionary mass for the Cepheid require a companion mass of 2.7 solar masses or greater. We infer that the companion is itself a binary.
The Blue Edge of the Helium Star Instability Strip
Authors: Fadeyev Yu. A.
Published: July 2016

Hydrodynamic calculations of nonlinear pulsations were done for models of helium stars with mass 1 M⊙, luminosity from 3220 L⊙ to 12820 L⊙ and effective temperature from 6000K to 8000K. The models with L > 8000L⊙ were found to pulsate in the fundamental mode with large amplitude (ΔR/R ~ 1), whereas less luminous models (L < 8000L⊙) revealed small amplitude oscillations (ΔR/R ~ 0.2) in the first overtone. In the luminosity range considered the blue edge of the instability strip corresponds to an effective temperature of 7500K, that is to the upper limit of the effective temperatures of R CrB stars. Application of the period-luminosity relation to the variables R CrB and RY Sgr gives their luminosities to be of 9000L⊙.
Envelope Distention and Mass Loss IN W VIR Pulsating Variables
Authors: Fadeyev Yu. A. , Muthsam H.
Published: July 2016

The hydrodynamic calculations of nonlinear self-excited radial pulsations were done for the models of W Vir stars with mass 0.6 M⊙ and luminosities from 794L⊙ to 3981L⊙. The periods of the models are longer than 10 days. The pulsations are shown to be accompanied by periodic shocks that change the density distribution in the pulsating stellar atmosphere. At radii less than 5 Rph, where Rph is the radius of the photosphere, the mean dynamic scale height is nearly five times the static scale height. In this region of the atmosphere the mean radii of outer mass zones do not change perceptably. On the other hand, at radii larger than 5 Rph the scale height is of the order of Rph so that the outermost layers ultimately expand with a velocity exceeding the local escape velocity. The mass flux in the atmosphere increases with decreasing mass to radius ratio and mass loss rate in W Vir type variables is in the range from 2 X 10−6M⊙·yr−1 to 10−5M⊙·yr−1.
Forthcoming Cepheid Database
Authors: Fernie J. Donald , Evans Nancy Remage
Published: July 2016

A summary of basic parameters of classical Cepheid variable stars is being prepared. It will contain stars from the fourth edition of the General Catalogue of Variable Stars in the categories CEP, CEP(B), DCEP, and DCEPS, excluding those which belong to Population II according to the criteria of Harris (1985, A.J.,90, 756). V and B intensity means, as well as velocity means are derived from Fourier fits using the N. Simon program MINFIT. In addition to the data contained in the catalogue by Fernie and Hube (1968, A.J., 73, 492), it will provide a comparison of reddenings from a number of sources, binary status, and period change information. We anticipate a hardcopy version, and also a version on floppy disks. The computer version uses dBaselll, and will include programs, for instance, to calculate phases using changing periods.
CCD Observations of Variable Stars in Globular Clusters
Authors: Smith H.A. , Kuhn J.R. , Curtis J.
Published: July 2016

BVR observations of the relatively metal-rich globular cluster NGC 6388 have been obtained with a CCD on the CTIO 0.9 m telescope. Eighteen possible short period variable stars have been discovered in or near the cluster. At least 10 of these are probable RR Lyrae members of NGC 6388. We confirm the finding of Hazen and Hesser that this cluster is one of the most metal-rich to contain a significant number of RR Lyraes. A program of CCD photometry of field and cluster variable stars has been initiated on the 0.6m telescope of the Michigan State University Observatory.
X-ray Line Formation in Astrophysical Environments: The L-shell Spectra of Ionized Iron
Authors: Liedahl Duane A. , Kahn Steven M. , Osterheld Albert L. , Goldstein William H.
Published: July 2016

We have initiated an extensive atomic modeling effort applicable to X-ray line emission in high-temperature astrophysical plasmas. The emphasis of our program is on the detailed accounting of the mechanisms which populate excited states of highly ionized atoms over a wide range of electron temperatures and densities. As a first demonstration we have calculated spectra for the important L -shell ions Fe XVI-XIX in a complete collisional-radiative model under conditions appropriate to solar coronal plasmas. Using methods presented here, we have synthesized the X-ray spectra of solar flares and active regions over the wavelength interval 13 – 18 Ǡ. The atomic model, which includes 705 atomic energy levels, is the largest and most detailed of its kind. In this introductory paper, we discuss the effects of dielectronic recombination on the spectrum of Fe XVII and present a new technique whereby the 3s lines can be used as a diagnostic of the electron temperature. Also included are new values for the rate of resonance excitation of the n=3 Fe XVII excited states. These rates are lower than those previously obtained and suggest that resonance excitation does not contribute significantly to the population kinetics. Finally, we present a direct comparison of our a calculated model spectrum with data from a solar flare.
Role of Magnetospheric Plasma Physics for Understanding Cosmic Phenomena
Authors: Das Indra Mohan Lal
Published: July 2016

Cosmic phenomena occur in the remote regions of space where in-situ observations are not possible. For proper understanding of these phenomena laboratory experiments are essential, but the in-situ observations of magnetospheric plasma provides even a better background to test various hypothesis of cosmic interest. This is because the ionospheric-magnetospheric plasma and the solar wind are the only cosmic plasmas accessible to extensive in-situ observations and experiments.
X-ray Spectroscopy across the HR Diagram
Authors: Schmitt J.H.M.M.
Published: July 2016

X-ray spectra of stellar X-ray sources taken with high, moderate and low spectral resolution are discussed. Only low resolution spectra are available for a sufficiently large number of late-type stars. It is shown that high temperature plasmas (log T > 7) produce the dominant emission component in the coronae of red dwarfs as well as yellow giants, while the coronae of F stars – usually – have X-ray temperatures similar to those found in the quiet Sun. The need for density diagnostics of stellar coronae is stressed and it is argued that the waveband region between 90 – 140 Å is particularly well suited for this purpose.
Implications of Abundance Measurements of the Intracluster Medium
Authors: Forman W. , Jones C. , David L.
Published: July 2016

The correlation of Mgas/Mstellar with the intracluster gas temperature in clusters of galaxies has implications for the efficiency of galaxy formation and predicts trends for the heavy element abundances in these systems.
The ROSAT Mission
Authors: Trümper J.
Published: July 2016

The scientific payload of ROSAT consists of a 83 cm X-ray telescope (6 – 100 Å) and a 60 cm XUV-telescope (60 – 300 Å) which are looking parallel. An important objective of the mission is to perform the first all-sky survey with imaging X-ray telescopes providing an improvement in sensitivity by several orders of magnitude compared with previous surveys. A large number of new X-ray sources (~ 105) is expected to be discovered and located with an accuracy of 1 arcmin or better, depending on source strength. The sources discovered will represent almost all types of astronomical objects, ranging from nearby normal stars to distant quasars.

After completion of the sky survey which will take half a year, the instruments will be used for detailed investigations of selected sources with respect to spatial structures, spectra and time variability. In this pointing mode, which will be open for guest observers, ROSAT is expected to provide substantial improvements over the imaging instruments of the Einstein observatory.
XSPECT: A Telescope/Spectrometer System on SPECTRUM RÖNTGEN GAMMA
Authors: Schnopper Herbert W.
Published: July 2016

The SPECTRUM RÖNTGEN-GAMMA mission is being developed by the Babakin Center (BC) together with the Space Research Institute (IKI) of the Academy of Sciences, USSR and is scheduled for launch in 1993. Mission objectives include broad and narrow band imaging spectroscopy over a wide range of energies from the EUV through gamma rays with particular emphasis on the study of extragalactic objects. The Danish Space Research Institute (DSRI) BC and IKI share the responsibility for the preparation of the XSPECT system. Two thin foil telescopes which are conical shell approximations to Wolter 1 geometry, each with an aperture of 60 cm and a focal length of 8 m, are designed to have a half-power width of less than 2 arcmin and will have collecting areas of 1700 and 1200 cm at 2 and 8 keV, respectively. Images and spectra will be recorded with position sensitive proportional counters with good spectral resolution. An objective Bragg crystal panel, placed in front of one of the telescopes, will make high resolution spectroscopic studies (E/ΔE ~103) of point- and extended sources. Other instruments are under consideration.
Expected Scientific Performance of the Three Spectrometers on the Extreme Ultraviolet Explorer
Authors: Vallerga J. V. , Jelinsky P. , Vedder P. W. , Malina R. F.
Published: July 2016

The expected in-orbit performance of the three spectrometers included on the Extreme Ultraviolet Explorer (EUVE) astronomical satellite is presented. Recent calibrations of the gratings, mirrors and detectors using monochromatic and continuum EUV light sources allow the calculation of the spectral resolution and throughput of the instrument. An effective area range of 0.2 to 2.8 cm2 is achieved over the wavelength range 70-600Å with a peak spectral resolution λ/Δλ (FWHM) of ~ 360 assuming a spacecraft pointing knowledge of 10 arc seconds (FWHM). For a 40,000 sec observation, the average 3σ sensitivity to a monochromatic line source is 3 × 103 photons cm-2 sec-1. Simulated observations of known classes of EUV sources such as hot white dwarfs and cataclysmic variables are also presented.
Dispersive Spectroscopy on AXAF
Authors: Markert T.H.
Published: July 2016

There are two transmission grating spectrometers and one Bragg crystal spectrometer being developed for the Advanced X-ray Astrophysics Facility (MIT is building the crystal spectrometer and one of the grating spectrometers; the Laboratory for Space Research in Utrecht is responsible for the other grating spectrometer). The gratings divide the AXAF energy band (80 eV – 10 keV) into three regions (the MIT instrument contains gratings with two different periods) and attain resolving powers for point sources between 100 and 1800. The gratings are composed of arrays of small facets mounted on plates which can be inserted immediately behind the AXAF telescope. The dispersed spectra from the grating arrays are read out by one of the AXAF imaging instruments.

The Bragg Crystal Spectrometer (BCS) is a focal plane instrument. One of eight selectable curved diffractors intercepts the AXAF X-ray beam as it diverges beyond the focal point X-rays that satisfy Bragg’s law are reflected from the crystal which, because of its curvature, re-focuses the beam onto an imaging detector. Narrow spectral regions are scanned by rocking the crystal over a range ~0.1 to 1°. Nearly the entire AXAF energy range can be studied by selecting the appropriate crystal and rotating it to the proper Bragg angle. The BCS achieves the highest spectral resolutions of the AXAF spectrometers: for 500 eV < E < 1600 eV, the FWHM of a narrow line (ΔE) is ≲ 1 eV.
Structure of the Coma: Chemistry and Solar Wind Interaction
Authors: Huebner W. F. , Boice D. C. , Schmidt H. U. , Wegmann R.
Published: July 2016

Much has been learned about the physics and chemistry of comets from the successful spacecraft encounters and intensive remote observing programs of Comets Halley and Giacobini-Zinner. Instead of being the panacea for our comet curiosity, these tantalizing “snapshots” have raised new questions, and many fundamental problems remain unsolved. To reap fuller benefits, extensive modeling is necessary to characterize the physical structure and chemical properties of the coma and to infer the composition and structure of the nucleus.
Self-noise in interferometers
Authors: Vivekanand M. , Kulkarni S.R.
Published: July 2016

We study the distribution of noise in images synthesized by radio interferometers, focusing particularly on the noise due to source power fluctuations. For a total power interferometer, the r.m.s. fluctuation in the image consists of a term that is constant all over the map, which is the uncorrelated system noise divided by the number of telescopes in the interferometer, and a term that is the image itself. For a correlation interferometer the expression is not so simple but is qualitatively similar. Our results are consistent with intuitive expectations in various observing situations. We find similarities in self-noise at optical and radio wavelengths.
The effect of spatial coherence of sources on synthetic aperture mapping
Authors: James Daniel F.V.
Published: July 2016

The interferometric mapping of astronomical objects relies on the van-Cittert Zernike theorem, one of the major results of the theory of partially coherent light [see, Bom and Wolf (1980), chapter 10]. This theorem states that the degree of spatial coherence of the field from a distant spatially incoherent source is proportional to the Fourier transform of the intensity distribution across the source. Measurement of the degree of spatial coherence, by, for example, measuring the visibility of interference fringes, allows the object to be mapped by making an inverse Fourier transform. (For a full description of this technique see Thompson, Moran and Swenson, 1986.)

In this paper I present a summary of the results an investigation into what happens when the distant source is not spatially coherent (James, 1990). Using a heuristic model of a spherically symmetric partially coherent source, an analytic expression for the error in the measurement of the effective radius, expressed as a function of coherence area, can be obtained.
The multi frequency front end: a new type of front end for the Westerbork Synthesis Radio Telescope
Authors: Tan G.H.
Published: July 2016

The Westerbork Synthesis Radio Telescope will be equipped with new front ends. These front ends will cover 8 frequency bands in the range from 250 MHz to 8.6 GHz. For the frequency bands above 1.2 GHz the sensitivity of the instrument will be drastically improved. Two independent local oscillator systems make it possible to observe in two frequency bands simultaneously.
Using sub-aperture illumination to solve the shortspacing problem
Authors: Woody David P. , Scoville Nick Z. , Mundy Lee G.
Published: July 2016

A common problem in producing interferometer maps of objects with structure on the scale-size of the primary beam is the acquisition of UV data corresponding to telescope spacings of less than one telescope diameter. One technique for solving this “short-spacing” problem is to divide the aperture of one of the telescopes into smaller sub-apertures and measuring the visibilities between these sub-apertures. This technique is being tested using the Owens Valley Millimeter Interferometer.
Space Ground Interferometer
Authors: Savin A.I. , Zaxon M.B. , Matveyenko L.I.
Published: July 2016

A space project for studying ecological Earth problems is being carried out by means of radio techniques. A 30 m prototype antenna has already been deployed and tested. The radio telescope will be launched in 1994 into a circular orbit haying an altitude 600 km and an inclination of 65°. The planned mission time is ≥ 1.5 year; 25% of this will be available for VLBI observations at wavelengths 6 and 18 cm.
The requirement to the baseline Knowing of a Space VLBI
Authors: Kogan L.R. , Eysmont N.A.
Published: July 2016

It is shown that the influence of a baseline knowing error on the fringe rate change during coherent averaging time interval is extremely increasing for space VLBI due to more angle velocity of a space baseline rotation comparatively with ground VLBI. It is shown on the example of Radioastron orbit that the reguirement to its orbit for such a short wavelength as 1.35 cm is determined by unknowing of the fringe rate change during coherent averaging time interval rather than by delay ambiguity and this reguirement is equal several meters.
Software Systems for the Ryle Telescope
Authors: Titterington DJ
Published: July 2016

An overview of the software systems developed for the broad-banded Ryle Telescope.
Adding Short spacings to synthesis maps in the ‘sky’-domain
Authors: Schwarz U.J. , Wakker B.P.
Published: July 2016

We discuss critically the methods used to complement synthesis maps with short-spacing information derived from single-dish observations. The conclusion is that from a single synthesis field the correct result cannot be derived, a large area has to be synthesized.
The use of redundancy in interferometry – a comparison of redundancy and selfcal
Authors: Wieringa Mark
Published: July 2016

Many synthesis arrays currently in use have at least a few redundant baselines which can be used to derive model-independent telescope corrections. We discuss the relative merits of redundancy-based versus standard selfcalibration methods with respect to sensitivity to noise and interference. We then derive an approach to optimally combine the two methods for partially redundant arrays
Self-calibration of M.O.S.T. data
Authors: Gray A. D.
Published: July 2016

The Molonglo Observatory Synthesis Telescope (MOST), located near Canberra, Australia, is an east-west earth-rotation synthesis array which, unlike conventional synthesis arrays, generates multiple real-time fan-beams and forms images by the back-projection of the recorded intensities rather than by Fourier inversion of visibilities. The images produced in this fashion are often marred by the presence of radial artefacts emanating from strong sources due to residual calibration errors. An algorithm devised to self-calibrate these errors is discussed.
GMRT Mapping Strategies
Authors: Rao A. Pramesh
Published: July 2016

The Giant Metrewave Radio Telescope (GMRT) that is being built to operate at metre wavelengths, will encounter a number of problems that are specific to synthesis imaging at low frequencies. Some of these problems are highlighted and possible solutions that are being considered, are discussed.
Low Frequency imaging and the non-isoplanatic atmosphere
Authors: Subrahmanya C. R.
Published: July 2016

At low frequencies, the typical fields of view over which imaging is required is several degrees across, for which (a) the visibility and brightness distribution cannot be assumed to be related by a two-dimensional Fourier Transform, and (b) the propagation delays introduced by the atmosphere cannot be assumed constant within the field of view. These are adequately represented in a method discussed in this paper. Although compute-intensive, the method is suitable for implementation on parallel processors.
VLA observations at 333 MHz: A Search for pancakes
Authors: Uson Juan M. , Bagri Durgadas S. , Cornwell T. J.
Published: July 2016

The P-band is the most recently completed observing band at the VLA. The aperture efficiency of the antennas is 0.4 and the system temperature is ∼ 130 K. The primary beam is slightly off-axis and its half-power width is steeply frequency dependent, which induces abnormal spectral indices on off-axis (≥ 1°) sources. We have searched for Zel’dovich “pancakes” and reached a level of ΔSrms ∼ 3.5 mJy for spectral channels of width Δν ∼ 100 kHz. We find no pancake with MHI ≳5 × 1014 M⊙.
Speckles in Interstellar Radio-Wave Scattering
Authors: Desai K. M. , Gwinn C. R. , Reynolds J. , King E. A. , Jauncey D. , Nicholson G. , Flanagan C. , Preston R.A. , Jones D.L.
Published: July 2016

Observations of speckles in the scattering disk of the Vela pulsar are presented and speckle techniques for studying and circumventing scattering of radio waves by the turbulent interstellar plasma are discussed. The speckle pattern contains, in a hologrammatic fashion, complete information on the structure of the radio source as well as the distribution of the scattering material. Speckle observations of interstellar scattering of radio waves are difficult because of their characteristically short timescales (≈seconds) and narrow bandwidths (≈kHz). Here, we present first observations, taken at 13 cm wavelength with elements of the SHEVE VLBI network, of speckles in interstellar scattering.
Hot spots in Cygnus A; A mosaiced map at 86 GHz
Authors: Wright M.C.H. , Sault R.J.
Published: July 2016

We describe a new software package, MIRIAD, for making multichannel images of multiple-field aperture synthesis observations. As an example of the use of the new software we present a map of the radio galaxy Cygnus A. We used the BIMA array to map Cygnus A at 86 GHz with 2.28 × 3.06 arcsec resolution in 3 fields centered on the central radio source and the radio hot spots in each radio lobe. The 3 fields were combined and deconvolved to find the positions and flux densities of the radio source components.
Errors in high precision source position determination
Authors: Elosegui Pedro , Marcaide Juan-Maria , Shapiro Irwin I.
Published: July 2016

We have made a detailed analysis of the systematic errors in the determination, from two sets of VLBI observations, of the position of the quasar 1038+528 A relative to the quasar 1038+528 B. This analysis confirms an apparent proper motion at λ=3.6cm of 26±8 μas/yr of the core of the quasar 1038+528 A relative to the quasar 1038+528 B.
MERLIN observations of superluminal motion in the jet of 3C120
Authors: Muxlow T.W.B. , Wilkinson P.N.
Published: July 2016

We present evidence for changes in the radio structure of the jet in 3C120 between December 1980 and December 1988 derived from MERLIN observations at 1.67 GHz. The changes are consistent with outward motion of 19±2 mas (2.4 ± 0.3 mas yr−1, i.e. 3.5 ± 0.4h−1 c, for H0 = 100h km s−1 Mpc−1) in the inner jet, about 0.15 arcsec (∼ 75h−1 pc) from the core. This confirms the suggestion by Benson etal. (1988) that the jet does not slow down quickly just outside the core region usually imaged by VLBI. However, we find no evidence for motion at the position of the knot 4 arcsec (∼ 2h−1 kpc) from the nucleus as suggested by Walker et al. (1988). Our limit is 1.26 mas yr−1, i.e. <1.8h−1 c, compared with Walker et al. ’s measurement of 2.4 ± 0.8 mas yr−1 i.e. 3.7 ± 1.2h−1 c.
The link to extragalactic sources: observations of radio stars
Authors: Baudry Alain , Requieme Yves
Published: July 2016

Optical positions obtained with the Bordeaux automatic meridian circle (accuracy – 0.05”) are compared with VLA positions taken from the literature. Alignment of the FK5 and VLA frames are discussed, and other projects (HIPPARCOS, VLBI) are briefly presented.
High-accuracy equation of equinoxes and VLBI astrometric Modelling
Authors: Gontier A.M. , Capitaine N.
Published: July 2016

In the course of implementing the use of nonrotating origin (Guinot 1979) in astrometric VLBI, for the transformation between celestial and terrestrial frames, we have compared the new approach with the classical one. We have shown that a difference exists between the two procedures at a few milliarcsecond level; this difference is due to the terms generally neglected when considering the equation of equinoxes in the classical representation of the Earth angle of rotation.
Mark-III VLBI astrometry of pulsars
Authors: Jones Dayton L. , Dewey Rachel J. , Gwinn Carl R. , Davis Michael M.
Published: July 2016

Differential 18-cm VLBI measurements of the position of PSR 1937+214 with respect to the quasar 1923+210 have been made to improve our knowledge of the offset between the planetary ephemeris reference frame and the extragalactic radio reference frame. Our preliminary frame-tie result (VLBI – Timing position) is +21 ± 2 milli-arcseconds in right ascension and +14 ± 9 milli-arcseconds in declination. We describe the data analysis methods used and compare our results with other radio interferometric position measurements of PSR 1937+214. Our results illustrate the value of combining the large collecting area of the Arecibo telescope with the Mk-III VLBI recording system.
Imaging Optical interferometry
Authors: Simon R.S. , Johnston K.J. , Mozurkewich D. , Weiler K.W. , Hutter D.J. , Armstrong J.T. , Brackett T.S.
Published: July 2016

Interferometry at optical wavelengths is very similar to radio interferometry, once the fundamental differences in detectors are accounted for. The Mount Wilson Mark III optical interferometer has been used for optical interferometry of stars and stellar systems. Success with the Mark III has lead to the current program at the Naval Research Laboratory to build the Big Optical Array (BOA), which will be an imaging interferometer. Imaging simulations show that BOA will be able to produce images of complex stellar systems, with a resolution as fine as 0.2 milliarcseconds.
The Owens Valley Millimeter Array
Authors: Padin Stephen , Scott Stephen L. , Woody David P.
Published: July 2016

In this paper the telescopes and signal processing systems of the Owens Valley Millimeter Array are briefly described.
Monitoring source variability with the Fast All Sky Telescope
Authors: Johnston Kenneth J. , Fiedler Ralph L. , Simon Richard S.
Published: July 2016

The proposed Fast All Sky Telescope (FAST) is an interferometer which is intended to monitor the northern four-fifths of the celestial sphere every two days at 8.1 GHz and daily at 2.7 GHz. The design goal is to have a rms sensitivity of 10 mJy/beam at both frequencies. The array is planned to comprise 20 3-meter diameter antennas with a maximum baseline of 0.7 km. FAST will provide a valuable database that may be used to study time variability in a sensitivity limited sample of radio sources. This will significantly impact on the understanding of active Galactic and extragalactic radio sources, as well as on the understanding of radio wave scattering in the interstellar medium.
Optimal antenna configurations for the BIMA Array
Authors: Thatte Niranjan , Welch Wm. J.
Published: July 2016

We attempt to determine optimal configurations of six antennas for the proposed Berkeley Illinois Maryland Array. The instrumental response is used as a performance measure. A two dimensional sine function is chosen as the ideal beam, supported by theoretical considerations. A mean square comparison with this ideal is used to perform optimizations for arrays of various resolutions, using the current station locations at Hat Creek. Configurations for larger fields of view are also discussed, employing two days of coverage with six antennas.
Motion of a Charged Particle Around a Black Hole Permeated by Magnetic Field and its Chaotic Characters
Authors: Nakamura Y. , Ishizuka T.
Published: July 2016

Motion of a charged particle around a black hole immersed in magnetic field is calculated. It is shown that this motion has a chaotic property depending on initial parameters.
Chaotic Behavior and Statistical Analysis of Some Mira and Sr Stars
Authors: Yanagita T. , Satoh H. , Saijo K.
Published: July 2016

Visual light curves of some Mira and semiregular variable stars are analyzed to search for their chaotic and statistical behaviour. Discussion are also given.
Understanding Changes in Period Ratios
Authors: Petersen J. O.
Published: July 2016

The discrepancies between observed and theoretically calculated period ratios of the double mode, classical Cepheids and other groups of Cepheid type variables have played a prominent role in investigations of the important physical characteristics of these stars during the last twenty years. Today, there is growing consensus that the drastic increases in astrophysical opacities proposed by Simon (1982) provide the correct solution of all the period ratio problems (e.g. Petersen, 1989; Iglesias and Rogers, 1992; Moskalik et al., 1991).

It seems at first strange that the optimal opacity change (see Andreasen, 1988) can produce both the decrease in period ratio for double mode classical Cepheids and the increased period ratio in models of δ Scuti stars. Here we use ĸ-effect-functions (Refsdal and Stabell, 1972) to provide both a detailed description of the effects of opacity changes and an improved and more direct understanding of these effects.
Long-Term Amplitude and Period Variations of δ Scuti Stars: A Sign of Chaos?
Authors: Breger M.
Published: July 2016

On short time-scales of under a year, the vast majority of δ Scuti stars studied in detail show completely regular multiperiodic pulsation. Nonradial pulsation is characterized by the excitation of a large number of modes with small amplitudes. Reports of short-term irregularity or nonperiodicity in the literature need to be examined carefully, since insufficient observational data can lead to an incorrect impression of irregularity. Some interesting cases of reported irregularities are examined.

A few δ Scuti stars, such as 21 Mon, have shown stable variations with sudden mode switching to a new frequency spectrum. This situation might be an indication of deterministic chaos. However, the observational evidence for mode switching is still weak.

One the other hand, the case for the existence of long-term amplitude and period changes is becoming quite convincing. Recently found examples of nonradial pulsators with long-term changes are 4 CVn, 44 Tau, τ Peg and HD 2724. (We note that other δ Scuti pulsators such as X Cae and θ2 Tau, have shown no evidence for amplitude variations over the years.) Neither the amplitude nor the period changes are periodic, although irregular cycles with time scales between a few and twenty years can be seen. While the amplitude changes can be very large, the period changes are quite small. This property is common in nonlinear systems which lead to chaotic behavior. There exists observational evidence for relatively sudden period jumps changing the period by about 10−5 and/or slow period changes near dP/dt ≤ 10−9. These period changes are an order of magnitude larger than those expected from stellar evolution.

The nonperiodic long-term changes are interpreted in terms of resonances between different nonradial modes. It is shown that a large number of the nonradial acoustic modes can be in resonance with other modes once the mode interaction terms, different radial orders and rotational m-mode splitting are considered. These resonances are illustrated numerically by the use of pulsation model. Observational evidence is presented that these interaction modes exist in the low-frequency domain.
The Delta Scuti Star GX Pegasi: A Theoretical Investigation of its Power Spectrum
Authors: Michel E. , Goupil M. J. , Lebreton Y. , Baglin A.
Published: July 2016

Target of a STEPHI multisite campaign, the Delta Scuti star GX Pegasi has been found to oscillate with at least five simultaneous, close frequencies (table I).

Mode identification together with informations about the star that such an identification can provide are outlined below (see also Michel et al, 1992b).

The mode identification is carried out by means of a comparison between the observed frequencies and the adiabatic frequencies of models appropriate to this star. Models that match GX Peg’s position in a Hertzsprung-Russell diagram have masses in the range 1.9 – M⊙. When included, convective core overshoot is handled as in Maeder and Meynet (1989). According to these models, GX Peg is a rather evolved, main sequence star.