bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractWe 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

AbstractThe 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

AbstractWe 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

AbstractCCD 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

AbstractElement 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

AbstractWe 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

AbstractIn 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.
Cepheids in Local Group Galaxies
Authors: Schmidt Edward G. 
Published: July 2016

AbstractA 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.
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

AbstractMultisite 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

AbstractA 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

AbstractA 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

AbstractSurface 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

AbstractWe 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

AbstractWe 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

AbstractArguments 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

AbstractHigh-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

AbstractA 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

AbstractHigh 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

AbstractWe 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

Abstract.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

Abstract.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

Abstract.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

AbstractMagnetic 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

AbstractThe 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

AbstractIn 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

AbstractA 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

Abstract73.8 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.