bluuebunny/crossref_metadata_2025 huggingface dataset:
“Differential Speckle Interferometry”, a New Tool for Double Star Research
Authors: Beckers Jacques M.
Published: July 2016

Speckle interferometry is a powerful tool for close binary star research allowing angular resolutions as small as 20 milliarcsecond. A technique is proposed to resolve spectroscopic binaries with even smaller separations. It uses the fact that speckle images taken in one or the other of the Doppler shifted spectral lines give a different intensity weighting of the two components of the binary. The location of the speckles in the two speckle images is therefore different, the direction of the displacement being related to the position angle of the binary, the amount of the displacement being related to its separation. Since the location of speckles can be determined with a much higher precision than their diameter, this creates the possibility for submilliarcsecond observations. This paper describes an experiment being started now to use this so-called “Differential Speckle Interferometry” technique for the study of binaries, stellar rotation, stellar chromospheres, Ap stars and other objects.
Shift-and-dd-Type Algorithms and their Application to Capella
Authors: Bagnuolo William G.
Published: July 2016

Bates and Cady (1980) proposed a method for astronomical image reconstruction that involves superimposing regions about the brightest pixels of each frame produced by speckle interferometric techniques. Unlike the method of Lynds, Worden, and Harvey (1976), no attempt is made to center apparent speckle images before superposition. This method is hereafter referred to as “shift-and-add” (SAA). The simplicity of this method makes it highly suitable for analysis and simulation.

This paper summarizes the application of extensions of SAA to the problem of determining reliable double star intensity ratios. Theoretical and simulation results presented in a previous paper (hereafter called “paper I”) by Bagnuolo (1982) will be summarized. In addition, the results of the application of SAA-type algorithms to Capella will be presented.
Long Baseline Interferometry and Binary Stars
Authors: Davis John
Published: July 2016

The observations of α Vir with the Narrabri Stellar Intensity Interferometer demonstrated the potential of long baseline interferometry for the determination of fundamental properties of double-lined spectroscopic binary systems. Since the completion of the programme with the Narrabri instrument the Chatterton Astronomy Department has been conducting a study aimed at developing a stellar interferometer with limiting magnitude V ≳ +8 and maximum baseline ≳ 1 km (resolution at 500 nm ≲ 7 × 10−5 seconds of arc). The way in which a long baseline interferometer may be used in the study of binary stars is outlined, the requirements for this work are discussed, and the current status and future plans of the Chatterton Astronomy Department’s programme to develop a new long baseline interferometer are summarised.
The Faint Object Camera
Authors: Rakos Karl D.
Published: July 2016

The Space Telescope (ST) is one of the most exciting projects presently planned in space astronomy. It will make a considerable contribution to astronomical research throughout the rest of this century. The high resolution 2.4 m telescope will be placed in orbit probably in 1985 by the Space Shuttle and will provide an astronomical capability unattainable by the ground-based telescopes.

The European Space Agency decided to participate in this NASA programme in 1976. ESA’s part in the programme includes the production of a major subsystem (the solar arrays and associated mechanisms), the building of the Faint Object Camera, and future participation in the operational activities and in the running of the Space Telescope Science Institute.
The Nature of the Local Group
Authors: Aparicio Antonio
Published: July 2016

The Local Group provides an interesting and representative sample of galaxies in the rest of the Universe. The high accuracy with which many problems can be addressed in Local Group galaxies is of paramount importance for understanding galaxy formation and evolution. This contribution presents a short review of overall Local Group properties followed by short discussions of five topics in which the study of Local Group members provides particularly significant information. These topics are only examples of the usefulness and potential of Local Group research. The five selected topics are the formation of the Milky Way, galaxy destruction and tidal streams, detailed galactic chemical evolution, star formation history determination, and low surface brightness extended structures.
The Araucaria project – improving stellar distance indicators
Authors: Pietrzyński Grzegorz , Gieren Wolfgang
Published: July 2016

We present first results of an optical wide field imaging survey of the galaxies NGC 300, NGC 6822, WLM and Phoenix, performed as part of the Araucaria Project, a long-term observational program recently initiated at the Universidad de Concepción. Based on our data, a large number of new Cepheids in these galaxies has been found. Our photometric databases were also used to select blue supergiant candidates in these galaxies, and to search for the photometric variability of these stars, as well as for a number of side projects, such as the search of other types variable stars and OB associations in these galaxies.
Age dependence of the vertical distribution of Cepheids
Authors: Dambis A.K.
Published: July 2016

The wavelike oscillations of the vertical scale height of the local population of classical Cepheids located at Galactocentric distances R0 – 1 kpc < Rg < R0 + 1 kpc is analyzed using Cepheid ages computed in terms of evolutionary models of Pols et al. (1998) with and without the allowance for convective overshooting. The resulting periods of vertical oscillations of stars about the galactic plane are found to be PZ = 74 ± 2 Myr and PZ = 104 ± 2 Myr for standard models and models with overshooting, respectively. If interpreted as a manifestation of vertical virial oscillations, the pattern found implies local mass density values of ρstd = 0.118 ± 0.007 M⊙ pc−3 and ρovs = 0.060 ± 0.004 M⊙ pc−3, respectively. The latter value is totally incompatible with recent estimates based on Hipparcos data and the former value, combined with recent estimates of the local density of visible matter, sets an upper limit of 0.023 M⊙ pc−3 for the local density of dark matter.
The distribution of novae in the Magellanic Clouds
Authors: Shida Raquel Yumi , Liller William
Published: July 2016

Some 34 novae have been discovered in the LMC and 14 in the SMC. Historically, the novae seemed to avoid the central bar of the LMC and the main body of the SMC, but with the novae discovered in the past two decades a different picture emerges. The more dramatic situation exists in the SMC where all seven of the most recent novae have occurred in or very near the main body. Indications are that the age of the LMC bar is around 5Gyr (Smecker-Hane et al. 2002), certainly old enough to breed novae. Perhaps the puzzle now is why there have been so many novae in the disk component of the LMC where many HII regions are found.
Pulsating white dwarfs as astrophysical tools
Authors: Sullivan Denis J.
Published: July 2016

The study of pulsating white dwarfs is a scientifically productive exercise as their measured pulsation characteristics yield both unique data on their interiors and also shed light on more general astrophysical questions. The modelling process is simplified due to their assumed chemically homogeneous and layered structures. As well as being archaeological remnants of their nuclear burning progenitors, white dwarfs are cosmic laboratories that can provide some observational constraints on theories of matter in regions of phase space that are unattainable in laboratories on Earth. The hotter white dwarfs should feature neutrino production by several unusual weak interaction processes, while their cooler cousins should exhibit core crystallization. White dwarf pulsators are very stable in period (if not always in amplitude) and suitable objects can be used as sensitive orbiting clocks in a search for stellar companions, including those of planetary size. Finally, the ensemble of cooling white dwarfs can be used as an independent cosmic chronometer over very long timescales if the cooling curve is properly calibrated. The pulsators make important contributions in this regard.
HST/STIS observations of sdBV stars: testing diffusion and pulsation theory
Authors: O’Toole S.J. , Heber U. , Chayer P. , Fontaine G. , O’Donoghue D. , Charpinet S.
Published: July 2016

We present the initial results of an abundance analysis of echelle UV spectra of 5 hot subdwarf B (sdB) stars, which form part of a study to test pulsation and diffusion calculations. SdB stars have been identified as core helium-burning objects on the extreme Horizontal Branch. Around 5% of sdB stars show short-period acoustic mode oscillations, which models predict are due to an opacity bump caused by the ionisation of iron group elements. The necessary metal abundance has to be maintained by diffusive equilibrium between gravitational settling and radiative levitation. However, analyses of high resolution optical spectra has revealed that we cannot discriminate between pulsating and non-pulsating sdB stars on the basis of the surface iron abundance. Our initial analysis of HST/STIS observations of 3 pulsators and 2 non-pulsators in the near and far UV suggests this may also be the case for other iron group elements, although further work will be needed to verify this.
The periods of the β Cephei star κ Scorpii as observed by WIRE
Authors: Cuypers J. , Buzasi D. , Uytterhoeven K.
Published: July 2016

The β Cephei star κ Scorpii is known to be multiperiodic from old photometric observations. A main period of 4.8 hr was detected, and from a beat period of about 7 d a secondary period of 4.9 hr was calculated. These periods were also found in recently analyzed line profile data. In two short data sets (separated by 152 d) obtained by the star camera of the satellite WIRE (Wide-field Infrared Explorer) the known main period of 4.8 hr is clearly present. The secondary period could also be confirmed, although some other periodicities are present as well. While the amplitude of the main variation is about 0.0055 mag, the amplitude of the secondary is only 0.0014 mag. Two longer periods (3.6 and 0.45 d) and a period of 4.2 hr were also found significant in the WIRE data.
Re-classification of ROTSE-I δ Scuti stars
Authors: Jin H. , Kim S.-L. , Lee C.-U. , Lee D.-J. , Kim K.-S.
Published: July 2016

We present new multiband photometric results for ROTSE-I δ Sct stars and the Fourier decomposition analysis. Our result shows that most of the stars classified as ROTSE-I δ Sct stars seem to be W UMa type eclipsing variable stars.
Photometry of eclipsing binary stars in the Large and Small Magellanic Clouds
Authors: Bayne Glenn P. , Tobin W. , Pritchard J.D. , Pollard K.R. , Albrow M.D.
Published: July 2016

CCD uVJIC photometry was obtained for three eclipsing binaries in the Magellanic Clouds and the preliminary analyses of their light curves has been made using a modified Wilson code. The LMC system, MACHO*05:36:48.7-69:17:00, is detached and eccentric, most likely comprising of two similar stars. The system has apsidal motion with a period of 100 ± 5 years. Initial results for two other systems in the SMC, MOA J005018.4–723855 and MOA J005623.5–722123, indicate circular orbits with the former semi-detached and the latter detached with two stars of very similar temperature.
Element Stratification in Main Sequence Stars and its Effect on Stellar Oscillations
Authors: Vauclair Sylvie
Published: July 2016

Element settling due to the combined effects of gravity, thermal gradient, radiative acceleration and concentration gradient may lead to important abundance variations inside the stars that cannot be neglected in the computation of stellar structure. These processes were first introduced to account for abundance anomalies in “peculiar stars”, but their importance in the so-called “normal” stars is now fully acknowledged, specially after the evidence of helium settling in the Sun from helioseismology. These microscopic processes work in competition with macroscopic motions, such as rotation-induced mixing or mass loss, which increase the settling timescales. We have recently obtained clear evidence that asteroseismology of main sequence solar-type stars can give signatures of the chemical variations inside the stars and provide a better understanding of these processes.
Decoupling of Helium in The Winds of Chemically Peculiar Stars
Authors: Kubát Jiří , Krtička Jiří
Published: July 2016

We calculate hydrodynamic multicomponent models of radiatively driven winds in B type stars. Our wind models consist of four components, namely hydrogen, helium, accelerated ions, and electrons. We solve equations of continuity, motion, and energy for all components and we take into account mutual collisions. The resulting models show large heating caused by friction. Results of our calculations show that the explanation of helium chemical peculiarity by the helium decoupling from the mean wind is unlikely.
R Coronae Borealis stars
Authors: Skuljan Ljiljana , Cottrell Peter L.
Published: July 2016

In the last ten years a significant step forward has been made toward a better understanding of the evolutionary status and unusual nature of the R Coronae Borealis (RCB) stars, a rare class of hydrogen-poor and carbon-rich variable stars. More detailed abundance analyses of the majority of RCB stars, and objects related to them, have become available in the last couple of years. In addition to this, recent theoretical studies of the most popular evolutionary models (“Double Degenerate” and “Final Flash”) provide a new insight into the origin of these stars. Regarding the nature of the RCB declines, more observations from the light maxima and the decline phase are now available, including more data from space. However, the characteristics of the various emission lines appearing during the RCB declines, and the nature of their emitting regions, are still not entirely understood.
Variable Stars and the Cosmic Distance Scale
Authors: Mould Jeremy
Published: July 2016

For three quarters of a century pulsating variable stars have lain at the foundation of the extragalactic distance scale. The construction of larger telescopes, advances in detector technology, hard work by observers, and our understanding of stellar structure have all contributed to the expansion of the realm of the Cepheids to the distance of M101. Now, with the advent of Hubble Space Telescope (HST), we can look forward to the detection of Cepheids in the Virgo cluster and the removal of much of the remaining uncertainty in the Hubble constant.
Cepheids in Local Group Galaxies
Authors: Schmidt Edward G.
Published: July 2016

A program is underway to obtain accurate, well sampled light curves for Cepheids in several Local Group galaxies. We have succeeded in attaining accuracies adequate for light curve analysis for stars with periods as short as about 12 days. Photometry for six known Cepheids in NGC 6822 is discussed. Although better sampling in phase is needed to allow full light curve analysis, a Fourier decomposition was performed for one star and it appears to show some differences from galactic Cepheids. A preliminary comparison of some of the gross properties of the light curves with galactic Cepheids shows both similarities and interesting differences.
Mira Variables, Stellar Evolution and Galactic Structure
Authors: Feast M.W.
Published: July 2016

The Mira variables make important contributions to four of the main problems under discussion at this meeting, (1) stellar pulsation, (2) stellar evolution, (3) the morphology and history of the Galaxy, (4) the comparative study of different galaxies. The Miras also show how these rather different fields of study overlap, so that it is no longer possible to deal with any one field in isolation.
Multiple Close Frequences of the δ Scuti Star θ2Tau: The Second Multisite Campaign
Authors: Breger M. , Garrido R. , Lin Huang , Shi-yang Jiang , Zi-he Guo , Frueh M. , Paparo M.
Published: July 2016

Multisite photoelectric b and V photometry of the δ Scuti variable θ2(78) Tau has been obtained on three continents. Five close pulsation frequencies (13.22965, 13.48073, 13.69360, 14.31764 and 14.61454 cycles per day) with visual amplitudes between 0.001 and 0.007 mag were found. The four frequencies found in the previous campaign were confirmed. All three main data sets (1982-1986) are in excellent agreement with each other and fit the solution to ±0.0028 mag per single measurement. Over the four years the amplitudes and frequencies of pulsation were constant.
Hydrodynamic Models of Low-Mass Pulsating Supergiants with Radiative Transfer
Authors: Foken A.B.
Published: July 2016

A method of calculating nonlinear stellar pulsations including nonstationary radiative transfer in a grey spherical atmosphere is described. With the help of this method eleven type II supergiant radiative models were constructed with masses of 0.6M⊙, luminosities ranging from 128L⊙ to 3123L⊙ and periods in the range from 1.123 to 46 days. A stable limit cycle was found to be accessible only by models with an effective temperature between 5700K and 6165K. The model with Te = 6165K is stable, whereas the models cooler than 5700K show nonregular behavior. Transition from strictly periodic to nonregular pulsation arises when M/R ≲ 0.018, due to high amplitudes, δr/r ≈ 1, and strong shocks in the atmosphere. The radiative transfer effects lead to some decay in the radial amplitude, as well as to a more significant decrease, about 0.6 magnitudes, in the light variation. A photometric comparison between the light curves of the models calculated with and without transfer and the observed light curve of the variable star No. 154 in M3 shows that the results predicted by the transfer model are in much better agreement with obervational data.
A Study of Period Doubling in a One-Zone Pulsating Stellar Model
Authors: Foken A.B.
Published: July 2016

A simple one-zone model for nonlinear stellar pulsations is outlined and applied to the study of period doubling observed in some W Virginis and RV Tauri stars. The model reveals a number of period doubling bifurcations as the parameters are varied, similar to those found by Buchler & Kovacs in their series of hydrodynamic models. In the vicinity of a stable limit cycle, despite its large number of degrees of freedom, the model develops an essentially one-dimensional Poincaré’s return map, determining the modulation of the amplitude. The analysis of these maps confirmed that period doubling has its origin in a strong nonlinear increase of total energy losses per period as radial amplitude, δr/r, increases. An additional study of nearly periodic hydrodynamic models with P > 15 days, calculated including radiative transfer effects, shows that the rate of energy dissipation per period by shocks in the atmospheres increases rapidly with δr/r, whereas the excitation rate, δo, remains rather stable. This permitted us to construct an analytic return map for maxima of the total kinetic energy which clearly demonstrates the mechanism of successive period doubling as δo, the sole bifurcation parameter, grows monotonically.
The Period-Radius Relation from 101 Cepheid Radii
Authors: Gieren W.P. , Barnes T.G. , Moffett T.J.
Published: July 2016

Surface brightness solutions using the (V-R) color index have been carried out for 52 southern Cepheids and 63 northern Cepheids, with 14 stars in common. For the southern stars the data came from the study by Gieren (1986, M.N.R.A.S, 222, 251), with new observations, and for the northern stars, from Moffett and Barnes (1987, Ap.J., 323, 280). All stars were reduced using the same surface brightness relation and the same transformation from radial velocity to pulsational velocity. The 14 stars in common show that there is only a small systematic difference between the southern and northern data sets and reductions. The combined solution for the 101 Cepheids in the period range 3 to 45 days is log R = 1.108 + 0.743 log P with uncertainties of ±0.023 in the zero point and ±0.023 in the slope. This result is consistent with the individual results of the above references but has much smaller uncertainty.
Low Energy Lines in Spectra of Gamma Bursts
Authors: Bisnovatyi-Kogan G.S. , Illarionov A.F.
Published: July 2016

We connect the phenomenon of gamma ray bursts with nuclear explosions on the old neutron stars. The matter of the neutron star in the non-equilibrium layer at depths of 30 m ≤ h ≥ 100 m consists of superheavy (A ≥ 300) nuclei with a surplus of neutrons (A/Z = 3 ÷ 4). These nuclei are metastable and exist only at high pressure. After the starquake some of the matter from non-equilibrium layer may move upwards and its nuclei become unstable. The β-decay is followed by a chain reaction of fission and nuclear explosion. The gamma ray burst is observed as radiation of the star surface heated to high temperature. Some mass may be ejected, forming expanding cloud. It consists mainly of the iron Fe56 with small (≤ 1%) additions of heavy elements (Ba, I, …) arising from the fission. The passage of stellar gamma radiation through the expanding plasma clouds leads to the formation of short-lived spectral features. Strong absorption of the soft gamma rays on K-electrons of Fe56 must be observed in the early stages. The gamma quanta with energies ε = 40–70 keV beyond the K-edge of the heavy elements (Ba, I, …) are absorbed in the later stages. A wide Kα line (εα = 30 keV) appears simultaneously. The free-free emission of expanding hot plasma cloud may be observed as a short flash in optical band.
Spectroscopy of Stellar Coronal Sources with the Medium Energy Experiment on EXOSAT
Authors: Pallavicini R. , Pasquini L. , Schmitt J.H.M.M. , Tagliaferri G.
Published: July 2016

We summarize some of the results obtained by spectral analysis of EXOSAT ME observations of stellar coronal sources. We focus on time-resolved spectroscopy of stellar flares and on the determination of the temperature structure of quiescent RS CVn binaries
Alternatives to the Existence of Large Cooling Flows
Authors: Tucker Wallace
Published: July 2016

Arguments against the existence of large scale cooling flows in clusters of galaxies are presented. The evidence for cooling flows is all circumstantial, consisting of observations of cool gas or hot gas with a radiative cooling time less than the Hubble time, or a central peak in the x-ray surface brightness profile. There is no evidence for large quantities (several tens to several hundreds of solar masses per year) of matter actually flowing anywhere. On the contrary, several lines of evidence — stellar dynamics, observations of the amount of star formation, x-ray surface brightness observations, theoretical calculations of the growth of thermal instabilities, the amount of cold gas — suggest that cooling flows, if they exist, must be suppressed by one to two orders of magnitude from the values implied by simple estimates based on the radiative cooling time of the x-ray emitting gas. Two heat sources which might accomplish this — thermal conduction and relativistic particles, are considered and an alternative to the standard model for cooling flows is presented: an accretion flow with feedback wherein the accretion of gas into a massive black hole in the central galaxy generates high energy particles that heat the gas and act to limit the accretion.
High Resolution X-ray Spectroscopy of Active Galactic Nuclei
Authors: Krolik Julian H.
Published: July 2016

High-resolution X-ray spectroscopy has the potential to reveal a number of interesting features of active galactic nuclei, primarily, though not exclusively, through the measurement of absorption lines. After a brief review of the principal problems of AGN research, selected potential high-resolution observations are discussed with a view toward assessing their scientific value and the degree of resolution they will require. Two classes of observations pertaining directly to AGNs are discussed: Fe Kα spectroscopy relevant to the dynamical and thermal character of the emission line zones; and measurement of resonance line absorption by highly-ionized species in BL Lac objects, which should tell us about entrainment of interstellar material by relativistic jets. A third class of potentially important observations uses AGNs as background light sources in order to directly measure the distance to clusters of galaxies.
Cosmic X-ray Background from Self-absorbed Low Luminosity AGNs
Authors: Grindlay Jonathan E. , Luke Michael
Published: July 2016

A model is proposed for the origin of the diffuse cosmic x-ray background whereby it is primarily due to the contribution of low luminosity active galactic nuclei which are increasingly self-absorbed at low luminosities. Strong self-absorption for low luminosity objects allows the observed background spectrum to be flatter in the ~2-20 keV band than the asymptotic spectrum, assumed to have mean index γ ~ -0.7 up to a high energy cutoff at ~125 keV. The model can account for the spectral shape and intensity of the background spectrum, as well as its possible fluctuations, if the AGN undergo modest density evolution and the bulk of the CXB arises from AGNs at redshifts z ~ 1-3. The model can be tested with AXAF observations of low luminosity AGNs at 5-10 keV and sensitive new hard x-ray observations.
The SPEKTROSAT Mission
Authors: Predehl Peter
Published: July 2016

High resolution spectroscopy will be an important diagnostic tool in future X-ray astronomy. SPEKTROSAT, the follow-up mission to ROSAT will be equipped with a transmission grating spectrometer with a spectral resolution of 0.2 Å over the wavelength range between 6 Å and 300 Å. Third order aberations are minimized by a mounting of the grating elements according to the Rowland-torus geometry. The ROSAT mirror system will be slightly changed by a redesign of the optical stops in order to reduce the field of view. The Focal Instrumentation has to be modified for a better match with the spectroscopic requirements. Since SPEKTROSAT is almost identical to ROSAT, this concept offers a large amount of science at relatively low costs. Its limiting line sensitivity will be about 20 times better than that of the EINSTEIN-OGS.
A High Resolution Echelle Spectrometer for Soft X-ray and EUV Astronomy
Authors: Green James , Bowyer Stuart
Published: July 2016

We present a new design for high resolution spectroscopy from 80 to 400 Å. This design employs grazing incidence optics and variable line-spaced gratings to achieve high resolution. Unlike some previously proposed EUV echelles, this design employs straight groove planar gratings, which are a well-proven, easily manufactured design. The instrument delivers a peak resolution of λ/Δλ = 7500 and a peak effective area of 3 cm2.
Characterization of the Rotation of Cometary Nuclei
Authors: Belton Michael J. S.
Published: July 2016

I review the primary methods used to determine the spin state of cometary nuclei and the pitfalls and successes experienced in their use. There are in excess of 60 reported determinations of rotational periodicities, but only a few (~4) appear to be reliable, and even these do not necessarily fully describe the true rotational state. An adequate rotational ephemeris is not, at present, available for a single cometary nucleus.

Because recent studies indicate that active cometary nuclei could be in excited spin states, I also review the theory of rigid body rotation from the point of view of remote (astronomical) observers, covering what is known of the effects of nutation on lightcurves, the influence of torques induced by jet activity, and the effects of internal energy and mass dissipation, and nuclear splitting.

The available knowledge on rotation for 8 comets, including P/Halley, is reviewed. Outstanding questions that need early resolution are: (1) Can a consensus be achieved on the rotational state of P/Halley? (2) Is it possible to accurately determine the amplitude of the transverse non-gravitational force associated with rotation? (3) Are the orientations of fan-like comas a valid indicator of the orientation of the spin vector – can definitive observational checks be made in a few cases?

Improved observational and interpretational techniques are needed to advance this field. Improved time-series and zero-date analyses are needed to connect existing and future data sets and to search for multiple periodicities in cometary lightcurves; improved sampling and extension of time-series observations with moderate- and large-aperture telescopes at good sites is needed; near-simultaneous photometric and radiometric observations made when cometary activity is low are particularly significant; well-sampled time-series imaging of near-nuclear phenomena (together with adequate software to analyze them) is essential to diagnose rotational states if nutation is present.
Possible Mechanisms for Cometary Outbursts
Authors: Hughes David W.
Published: July 2016

A cometary outburst is an unexpected flare-up in the brightness of a comet, with the brightness normally increasing by 2 to 3 magnitudes, but on occasion by up to 5. This outburst is superimposed on the general change in brightness that is related to the comet’s heliocentric and geocentric distance, spin phase, spin axis orientation and activity geography. The brightness variation during outbursts usually has an abrupt, eruptive onset followed by a quasi-exponential decrease and lasts for a few tens of days. Several cometary outbursts are accompanied by the expansion of a spherical halo from the brightest point at the centre of the visible cometary coma.
Multidimensional Dusty Gasdynamical Models of Inner Cometary Atmospheres
Authors: Gombosi Tamas I.
Published: July 2016

The presently available multidimensional dusty gasdynamic cometary atmosphere models are reviewed. This paper examines the simplifying assumptions and their potential consequences. Special attention is paid to the question of potential nightside dust accumulation presently predicted by Korosmezey and Gombosi (1989).
Tape recorders for VLBI
Authors: Rogers Alan E. E.
Published: July 2016

Magnetic tape recorders developed for VLBI have taken advantage of the best available technology to achieve high data rates and large storage capacity. The Mark I, II and III VLBI recording systems have data rates of 0.72, 4 and 224 Mbits/sec respectively. The recorder developed for the VLBA has a data rate of 256 Mbits/sec and can store 5.5 Terabits of data on one tape.
Imaging with non-coplanar Data
Authors: Perley R. A. , Cornwell T. J.
Published: July 2016

The general relation between interferometric data and sky brightness requires the use of three-dimensional Fourier transforms. Only in special cases does the standard two-dimensional Fourier relations result in images free of distortions. For the VLA, use of two-dimensional transforms introduces errors which are important at low frequencies and high resolutions. We review options and progress in solving the computational problems associated with the full solution.
Imaging for the Australia Telescope: Present needs and future strategies
Authors: Norris Ray P.
Published: July 2016

In this paper I outline the strategy we have adopted to produce images from the Australia Telescope. Our main production system is based on AIPS, but we have made several departures from conventional strategy. These departures are amongst several paths that we are exploring in the hope that some will lead to richer astronomical pastures. I will describe the start that we have made on these explorations and our impressions of the scenery along the way.
A modified algorithm for Cleaning wide-field maps with extended structures
Authors: Dwarakanath K. S. , Deshpande A. A. , Shankar N. Udaya
Published: July 2016

A simple but effective modification to the conventional CLEAN algorithm is suggested. This modification ensures both stability and speed when CLEAN is applied to maps containing a mixture of point sources and extended structures. The method has been successfully applied to the recently-completed sky survey at 34.5 MHz. This survey was made using the Gauribidanur T array (GEETEE) in 1-D aperture synthesis mode. Since in this case the ‘dirty beam’ (point spread function) cannot be directly computed, a method to obtain this is discussed in detail. The results of this deconvolution procedure have been encouraging in terms of reduced computing time and improved dynamic range in our maps. This algorithm should find wider application in deconvolving maps which have both extended structures and point sources.
The first VLA observations of radio sources at 73.8 MHz
Authors: Kassim N. E. , Perley R. A. , Erickson W. C.
Published: July 2016

73.8 MHz instrumentation for the VLA is being developed and is currently installed on 8 VLA dishes. Test observations of strong radio sources have been made. We describe techniques that we have developed to analyze these test observations and we present examples of our first maps.
Exploring the limits of Phase Referenced VLBI Imaging; a progress report
Authors: Vermeulen R.C. , Conway J.E.
Published: July 2016

The practical limits of phase referenced VLBI imaging are not well known. If it would be feasible to use target-reference separations of a few degrees, most of the sky would be accessible to the technique. Preliminary results at 6 cm using baselines of up to 1000 km are encouraging.
Quantitative assessment of the science return from an Orbiting, Imaging, Optical Interferometer
Authors: Meier David L. , Synnott Stephen P. , Dumont Philip J. , Jones Dayton L.
Published: July 2016

We describe an investigation of the science potential of the filled Mills cross orbiting, imaging, optical interferometer proposed by Synnott et al. and the constraints that science places on the design of the instrument. Imaging simulations were performed with this design in order to determine its capabilities in the area of spatial resolution, dynamic range, and sensitivity. In addition, specific astronomical objects of high interest were chosen as candidate program sources to determine what science was desired for these objects, what spatial resolution, etc. was needed to perform that science, and whether or not this interferometer design could do the science.
Imaging Characteristics of the proposed NRAO MMA, with and without a single large element, and a pointing correction algorithm
Authors: Emerson D. T.
Published: July 2016

This is a continuation of earlier imaging studies for the MMA. Earlier work is confirmed, and the differences in imaging quality resulting from small and larger central antennas used for short-spacing information are investigated. An algorithm is presented which uses interferometer data to correct for an important component of central element pointing errors.
The Canadian Geophysical LBI System: Status and Future Plans
Authors: Petrachenko William T. , Mathieu Pierre , Popelar Joseph , Daniels Micheal
Published: July 2016

Emphasis in the design of the Canadian Geophysical LBI (CGLBI) system has been placed on economy, operational efficiency, and performance in geophysical applications. Unique features of the system include the use of modified VHS recorders to store digitized astronomical data and the capability to track delay and fringe rotation during observation. A prototype system has been completed and tested at L-band using the ARO-DRAO interferometer baseline. The prototype system converts a single fixed 12 Mhz wide IF channel to baseband. Since delay resolution with this system is not sufficient for modern geophysical applications a frequency switching capability is being designed to synthesize a wide IF bandwidth while still maintaining the economy of a single channel system. Further improvements include the design of a phase cal detector and an input module compatible with NASA geodetic S/X receivers. Finally, efforts are being coordinated with ISTS so that record terminals developed for the Radioastron project can be interfaced to the CGLBI system.
Non-Linear Oscillations and Beats in the Beta Canis Majoris Stars
Authors: Baranov A. S.
Published: July 2016

Notwithstanding a great number of hypotheses, suggested for explaining superpositions of the light- and of the velocity variations of the ß Canis Majoris stars, no one of these does it satisfactorily. Possibly it is due to an inadequate elaboration of the non-linearly oscillation theory. Analysis and critical evaluation of the existing hypotheses are given by Mel’nikov and Popov (1970). Our explanation consists in existence of close frequencies corresponding to various oscillation modes which are non-linearly interacting.

Equations of motion of an ideal incompressible fluid under condition of preserving the equilibrium figure symmetry with respect to the equatorial plane (lateral oscillations) have the form (Baranov 1988):
Theoretical Modes Fitting on Miras Light Curves
Authors: Barthés D. , Tuchman Y. , Mennessier M. O. , Mattei J. A.
Published: July 2016

Visual observations of long period variable stars over 20 years were provided by the American Association of Variable Stars Observers, and were analysed as part of the preparation of the HIPPARCOS mission.

A set of frequencies is extracted from the light curve by using Fourier transform, preliminary Van Cittert deconvolution and comparison of the results obtained through different kinds of spectral windows. The same procedure is applied to the residual obtained after nonlinear fit of the main frequency. After final comparison of both sets, a nonlinear fit of the common frequencies gives the ‘clean’ power spectrum.

Different equilibrium stellar model (i) give theoretical linear nonadiabatic pulsation modes (vj) with their growth rates (ηj)i) (Tuchman 1978). The metallicity is taken between 0.005 and 0.02; the mixing length is λ = 1±0.2; the upper bound is r = 0.7. Assuming two peaks of the power spectrum to be the fundamental (vo) and first overtone (v1) modes, one looks for the corresponding models. The best one is selected by checking the other theoretical overtones they give. So are obtained the mass, the luminosity, the effective temperature and the effective radius of each star.
Period Variation of BW VUL
Authors: Shi-Yang Jiang
Published: July 2016

Due to the large fluctuation of the time of light maxima, in each observational season only very few observations is very dangerous for period variation research. Especially when the observation is not accurate enough such as for the early radial velocity data. Of course parts of the fluctuation are real which are caused by the chaotic behavior of the complicated pulsation. Generally the pulsation is quite periodic but is not like a timer such as the oscillation of the quartz or the rotating of the pulsar, all of these as much simple.

The period increasing rate of BW Vul from our results is 0.34 micron days per year which makes a evolution time scale about 3 million years. For a star with mass of 10 solar mass, the life age on the main sequence is about 10 million years. So BW Vul should not be very far away from the main sequence.
Optical Observations of FY Persei
Authors: Okazaki Akira
Published: July 2016

The results are presented of multi-channel photometry and spectroscopy of the suspected nova-like variable FY Per. These results suggest that FY Per may be a Herbig Ae/Be star rather than a nova-like variable.
Fourier Analysis of the Hydrodynamic Limit-Cycle Models of Pulsating Stars
Authors: Fadeyev YU. A.
Published: July 2016

The pulsation motions of the limit-cycle model can be described as a superposition of the Fourier harmonics, in the adiabatic layers each harmonics being identified with the corresponding standing wave. Near the resonance Π0/Πl = k the harmonics of order k is also identified with the overtone of order l The spectra of the oscillatory moment of inertia obey to the power dependence on the Fourier harmonics order k. In cepheids with periods shorter than 9 days the bump is due to the wave packet generated by the second overtone, whereas at periods longer than 10 days the bump feature is due to the traveling pulse reflected off the stellar core.
A Linear and Nonlinear Study of Mira
Authors: Cox A.N. , Ostlie D. A.
Published: July 2016

Both linear and nonlinear calculations of the 331 day, long period variable star Mira have been undertaken to see what radial pulsation mode is naturally selected. Models are similar to those considered in the linear nonadiabatic stellar pulsation study of Ostlie and Cox (1986). Models are considered with masses near one solar mass, luminosities between 4000 and 5000 solar luminosities, and effective temperatures of approximately 3000 K. These models have fundamental mode periods that closely match the pulsation period of Mira. The equation of state for the stellar material is given by the Stellingwerf (1975ab) procedure, and the opacity is obtained from a fit by Cahn that matches the low temperature molecular absorption data for the Population I Ross-Aller 1 mixture calculated from the Los Alamos Astrophysical Opacity Library. For the linear study, the Cox, Brownlee, and Eilers (1966) approximation is used for the linear theory variation of the convection luminosity. For the nonlinear work, the method described by Ostlie (1990) and Cox (1990) is followed. Results showing internal details of the radial fundamental and first overtone modes behavior in linear theory are presented. Preliminary radial fundamental mode nonlinear calculations are discussed. The very tentative conclusion is that neither the fundamental or first overtone mode is excluded from being the actual observed one.
A Simplified Model for a Nonlinear Tidal Effect on Accretion Disks in CVS
Authors: Zhang Zhong-Yong , Chen Jian-Sheng
Published: July 2016

This paper investigates the tidal effect on accretion disk in CVs and sets up a simplified model in which the secondary’s gravitation is substituted by a mean tidal torque. We find that a linear tidal torque will not be able to maintain an equilibrium disk. By using the result of the radius of the equilibrium disk approximately equals to the tidal radius, which was obtained by using the two dimensional numerical simulation invoking nonlinear tidal effect, we give the modified tidal dissipation function for our simplified model which could be used to interpret the outburst of the dwarf nova with tidal effect. The paper also shows that the radius of an equilibrium disk with a torus is slightly small than the Lubow-Shu radius, and the tidal effect may also cause the cycle of quiescence-superoutburst in addition to the cycle of quiescence-outbursts-superoutburst.
Hydrogen Cyanide Polymers from the Impact of Comet P/Shoemaker-Levy 9 on Jupiter
Authors: Matthews Clifford N. , Pesce-Rodriguez Rose A. , Liebman Shirley A.
Published: July 2016

Hydrogen cyanide polymers – heterogeneous solids ranging in color from yellow to orange to brown to black – may be among the organic macromolecules most readily formed within the Solar System. The non-volatile black crust of comet Halley, for example, as well as the extensive orangebrown streaks in the atmosphere of Jupiter, might consist largely of such polymers synthesized from HCN formed by photolysis of methane and ammonia, the color observed depending on the concentration of HCN involved. Laboratory studies of these ubiquitous compounds point to the presence of polyamidine structures synthesized directly from hydrogen cyanide. These would be converted by water to polypeptides which can be further hydrolyzed to α-amino acids. Black polymers and multimers with conjugated ladder structures derived from HCN could also be formed and might well be the source of the many nitrogen heterocycles, adenine included, observed after pyrolysis. The dark brown color arising from the impacts of comet P/Shoemaker-Levy 9 on Jupiter might therefore be mainly caused by the presence of HCN polymers, whether originally present, deposited by the impactor or synthesized directly from HCN. Spectroscopic detection of these predicted macromolecules and their hydrolytic and pyrolytic by-products would strengthen significantly the hypothesis that cyanide polymerization is a preferred pathway for prebiotic and extraterrestrial chemistry.
Prebiotics on Titan: from Available to Expected Measurements
Authors: Gautier Daniel
Published: July 2016

An intense organic chemistry occurs in the atmosphere of Titan. The detected species are far to be uniformly spatially distributed. Their mixing ratios vary with altitude and latitude. Tentatives to interpret these data are examined. Available photochemical models are discussed and shown to fail to reproduce simultaneously hydrocarbons and nitriles observed abundances., whatever the assumed eddy diffusion coefficient profile. Future models will have to explicitly take into account polymerisation, distributions of aerosols, seasonal effects and atmospheric circulation. The Cassini-Huygens mission, to be launched towards Saturn and Titan in 1997, will provide a tremendous information on the atmospheric composition of organics and on the mapping of the most abundant species. The nature of the aerosols will be investigated. Current theories on the chemical composition of the subnebulae of Saturn in which Titan was formed are discussed. We conclude that billions of like- Titan objects may exist in the Universe.
Direct Detection of Extra-Solar Planetary Systems from Balloon Borne Telescopes
Authors: Terrile Richard J. , Ftaclas Christ
Published: July 2016

Analysis of a 1.5 meter diameter High Altitude Balloon Circumstellar Imaging Telescope (HABCIT) indicates that it offers a fast and low cost path to direct detection of extra-solar planets. Above an altitude of about 30 kilometers, scatter in the visible due to the atmosphere is more than 1000 times smaller than the diffraction sidelobes for a 1.5 meter telescope at 1 arcsecond in the visible. This permits a thousandfold reduction in the total background against which the planet must be detected. By flying a scatter compensated 1.5 meter telescope and high efficiency coronagraph on a balloon platform, a very near term and low cost opportunity exists to achieve exciting scientific results and technology demonstrations. For a thousandfold reduction in the background, a Jupiter like planet could be detected around more than 70 stars with an average integration time of 16 hours. For the very nearest stars complete planetary systems can be characterized down to nearly Earth sized planets, over a range of orbital distances (1-5 AU).
Application of the Strömvil Photometric System for a Search of Solar-Type Stars
Authors: Straižys Vytautas
Published: July 2016

The seven-color photometric system, introduced by Vilnius astronomers in 1963-1967, makes possible a completely photometric determining of spectral classes (and temperatures), absolute magnitudes (and surface gravities), interstellar reddenings and distances of stars of all normal spectral types. Recently it was suggested to join this system with the Stromgren four-color system. The new seven-color system, called the Stromvil system, has the mean wavelengths of the passbands at 350, 375, 411, 467, 516, 547 and 656 nm and half-widths of the order of 20 nm. The system can be effectively used for detection and classification of solar-type stars which are expected to be the best sites for existence of planetary systems and life.