bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Pulsation of some δ Scuti Stars with Unusual Light Curves
Authors: Antonello E. , Poretti E. 
Published: July 2016

AbstractAmong the δ Scuti stars with large amplitude there are a few stars with unusual light curve shape and multiperiodicity. These stars are V1719 Cyg, V798 Cyg, V974 Oph and δ Scuti. The Fourier phase difference for the main pulsation mode is very different from that typical of the other large amplitude δ Scuti, SX Phe and RR Lyrae stars. The period ratio is generally high, ≧0.80. Two of the stars (δ Scuti and V1719 Cyg) have known spectral type or Stromgren indices, and these indicate high metallicity which should be related to the δ Delphini type (evolved Am/Fm stars). Some tests with the one-zone model show that the unusual light curve shape could be related to a poor He content in the envelope, while the application of model atmosphere grids shows that the anomalous behavior of the metallicity photometric index of V1719 Cyg is simply due to the high metal abundance of its atmosphere. The conclusion is that diffusion phenomena such as the He settling and the metal enhancement should be able to explain all the observed characteristics of these pulsating stars.
Problems with the Baade Wesselink Method
Authors: Böhm-Vitense E. , Garnavich P. , Lawler M. , Mena-Werth J. , Morgan S. , Peterson E. , Temple S. 
Published: July 2016

AbstractIt is well known that the Baade-Wesselink method leads to different radii for Cepheids depending on which colors are used to determine the effective temperatures. We try to find the reasons for this discrepancy. We employ yet another version of this method using only maximum and minimum radii, thereby circumventing uncertainties in the phase relations between radial velocities and colors. This has essentially no influence on the derived radii. One major uncertainty is the relation between the photospheric expansion velocity and the measured radial velocity. The main reason for the discrepant results obtained by using different colors appears to be an inconsistency in the difference in the applied temperature-color calibrations. Small changes in the d(log Teff)/d(color) can cause major changes in the derived radii.
Irregularity Interpreted as Low Dimension Chaos for Convective Models of W VIR Variables
Authors: Glasner S. Ami , Buchler J. Robert 
Published: July 2016

AbstractLinear and nonlinear pulsational properties of convective stellar envelopes relevant for W Vir and RV Tau stars are surveyed. All models show the same trend to pass from regular to irregular behavior when a control parameter is changed (the effective temperature). The transition to irregular pulsation follows well known systematic routes to chaos (as in the radiative case). Some rich structures were found in special cases; they deserve further research. We show that the chaotic behavior is sustained even when convection is taken into account. The effect of the inclusion of time dependent convection shows up mostly as a shift of Kovacs and Buchler (Ap.J 1988) results in the parameters plane (L,Teff) towards more realistic models.
Is Delta Scuti Star Seismology Possible?
Authors: Kovacs G. 
Published: July 2016

AbstractThe direct fit of theoretical pulsation frequencies to the observations (i.e. stellar seismology) proved to be a very efficient tool in the study of solar oscillations. In the case of other multiperiodic variables, like δ Scuti stars, Ap stars and white dwarfs the method suffers from the disturbing abundance of possible nonradial modes. Colour and/or radial velocity (or line profile) measurements can narrow down the number of possibilities, but these kinds of data are not often available with the desired accuracy and sampling rate. Since pulsational frequencies are the most readily and accurately computed and measured quantities of pulsation, we address the question of the accurate fit of the nonradial pulsation frequencies to the observations in the case of δ Scuti stars.
Observations of Variability in the Radial Velocity of α Boo
Authors: Merline W.J. 
Published: July 2016

AbstractWe have made precise radial velocity observations of α Boo (K1 III) which raise the possibility that giant stars redward of the Cepheid instability strip can pulsate. Our first measurements, made in early 1986 (Smith, McMillan and Merline 1987, Ap.J.,317, L79) revealed a systematic, apparently periodic, variation in radial velocity having a “sawtooth” waveform with a full amplitude of 160 m·sec−1 and period of 1.84 or 2.18 days. This discovery was qualitatively confirmed by Cochran (1987, B.A.A.S.,19, 1052). Additional data show that large night-to-night velocity variations of magnitude about 80 m·sec−1 persist over a 192 day baseline. A sharp velocity transition has been monitored on at least one night, which covers a range of 100 m·sec−1 with an acceleration of 19m·sec−1·hr−1. The positive slope during this transition argues for a 2.18 day rather than the 1.84 day period. Furthermore, the amplitude and shape of the variation may be changing, perhaps suggesting multiple periods. To test the likelihood that radial pulsation could be responsible for the observed variations, stellar pulsation models are being conducted with Dr. Arthur Cox at Los Alamos National Laboratory. Using a structure model based on our best estimates for the stellar parameters, we have found that the 2 day period would require the third or fourth radial overtone, which are driven (unstable) with relatively high growth rates.
The Effects of Metal Enhancement on the Period Ratio of Double Mode Cepheids
Authors: Morgan Siobahn , Cox Arthur N. 
Published: July 2016

AbstractWe increase the metal abundance in the envelope of Double Mode Cepheids (DMCs) in order to remove the period ratio mass discrepancy. By increasing the value of Z, we are able to increase the opacity by a factor of 2 to 2.5. We find that for a star of 5 solar masses, with a temperature of 5957 K, and a luminosity of 1050 times that of the sun, an increase in the metal abundance starting at the Log (T) value of 4.79 and reaching a maximum Z value of 0.3 in the Log (T) range of 4.8 to 5.3 produces a period ratio P1/P0 = 0.7089, and a value of P0 = 3.001 days. The value of Z returns to 0.02 at Log (T) = 5.62. The enhanced zone (part of which convects) is located well below the H and He convective regions. The enhanced region is also stable against downward mixing since the radiative gradient is so sub-adiabatic.
A Search for RR Lyraes in Wide Binary Systems
Authors: Prosser Charles F. 
Published: July 2016

AbstractRR Lyraes that have nearby visual companions may be examined to decide if they do indeed belong to a real binary system. Identification of RR Lyraes in binary systems could lead to the determination of physical characteristics of the variable and to an understanding of the history and evolution of such systems in the Galaxy. Where possible, proper motion analysis, using astrograph plates from the Lick Proper Motion Program, for some visual binaries has been done and has enabled the rejection of some variables from consideration. The variables which have been shown not to be physically associated with their visual companions are YZ Cap, V445 Oph, BH Peg, and TX Vir. EZ Lyr, RW Ari, and V494 Sco still remain possible cases requiring relative radial velocities between the variable and companion. Limited plate material has meant that one must resort to radial velocity measurements for the RR Lyrae and close companion in order to determine the nature of the pair.
The Chromosphere of β Cassiopeiae
Authors: Teays Terry J. , Schmidt Edward G. , Fracassini Massimo , Pasinetti Fracassini Laura E. 
Published: July 2016

AbstractWe have carried out high dispersion, long wavelength IUE observations and ground based photometry of the δ Scuti star, β Cas. Our ground based observations were used, together with the previous results of Antonello et al. (1986, I.B.V.S. 2958, 1986), to ensure that the IUE observations were correctly phased relative to the photometric variation. Fluxes for the emission core of the Mg II k line (2796 Å) were obtained from 23 ultraviolet spectra taken over several cycles in 1986 and 1987. The emission flux, if present, was measured with respect to the mean line profile of all of the spectra. Emission was present at phases between 0.4 and 0.5. This is in contrast to what has been observed for another δ Scuti star, ρ Pup, and for the classical Cepheids, where the emission appeared at maximum light.
Spectroscopic Diagnostics for Ions Observed in Solar and Cosmic Plasmas
Authors: Mason Helen E 
Published: July 2016

AbstractThe X-ray wavelength region (1-200Å) is rich in spectral lines from highly ionised systems. Spectra from the solar atmosphere have been studied extensively with various instruments covering different wavelength regions. In this paper, we discuss the solar spectral line emission with particular reference to iron ions and helium like ions observed during solar flares. The atomic processes involved in the calculation of theoretical intensities for low density plasmas are outlined together with the diagnostic properties of the emission lines. Comparisons are made with available cosmic X-ray spectra and predicted spectra for future projects, such as AXAF.
Thermal Instability in a Hot Plasma
Authors: Balbus Steve A. , Soker Noam 
Published: July 2016

AbstractThe nature of local thermal instability in static and dynamic radiating plasmas described by an equilibrium cooling function has been reexamined. Several new results have been found. In a plasma in both thermal and hydrostatic equilibrium, if the cooling function is not an explicit function of position, and does not display isentropic thermal instability (i.e. sound waves are thermally stable), then isobaric thermal instability by the Field criterion is present if and only if convective instability is present by the Schwarzschild criterion. In this case, thermal overstability does not occur. For the case of a dynamical plasma we present a very general Lagrangian equation for the development of nonradial thermal instability. In the limit of large cooling time to free-fall time ratio, the equation is solved analytically by WKBJ techniques. Results are directly applicable to cluster X-ray cooling flows. Such flows are surprisingly stable except for perturbation wavenumbers that are very nearly radial. We believe that the origin of cooling flow optical filaments is not to be found in linear thermal instability.
Goals for the Application of High-Resolution X-ray Spectroscopy to the Diagnosis of Stellar Coronal Plasmas
Authors: Linsky Jeffrey L. 
Published: July 2016

AbstractI provide examples of how high-resolution x-ray spectra may be used to determine the temperature and emission measure distributions, electron densities, steady and transient flow velocities, and location of active regions in stellar coronae. For each type of measurement I estimate the minimum spectral resolution required to resolve the most useful spectral features. In general, high sensitivity is required to obtain sufficient signal-to-noise to exploit the high spectral resolution. Although difficult, each measurement should be achievable with the instrumentation proposed for AXAF.
X-ray Spectral Diagnostics for Coronal Loops in the Active K Dwarf AB Doradus
Authors: Vilhu O. , Cameron A. Collier 
Published: July 2016

AbstractWe discuss theoretical X-ray spectra for coronal loop models in the rapidly rotating young K dwarf AB Doradus (HD 36705), as a typical representative of active X-ray bright stars. The loop models are based on EXOSAT and IUE observations, and further motivated by a possible connection between the observed X-ray flares and co-rotating clouds of neutral hydrogen (a few/day). The resulting synthetic spectra between 0.5 – 7 keV can be approximated by a linear combination of three distinct temperature components. Two components are sufficient between 0.5 – 2.5 keV. Below 0.1 keV the loop spectra deviate significantly from the few component fits. To test some basic assumptions (dynamic vs. static, constant vs. variable cross-sectional area), useful constraints on the DEM(T)-distribution could be obtained with the grating-spectrometer of XMM with 103 – 104sec exposure times. The ratio of the He-type (O VII) forbidden and intercombination lines at 0.56 keV will provide sufficient density diagnostics, to distinguish e.g.between compact and large loops. The crystal-spectrometers of XMM and XSPECT could achieve the same but with longer (105 sec) observing time. The strong Ly α line of O VIII at 0.65 keV can be observed with the crystals in 104 sec, and used even for rotational modulation and flare studies, and giving additional information about flows in flaring loops. At the Iron 6.7 keV lines, where the gratings do not work, the crystals should be used together with low resolution devices, to set constraints on the hottest gas at loop summits. Our discussion applies also to several brighter cool stars but with shorter observing time (like Capella and HR 1099, which are over 3 times X-ray brighter than AB Dor).
Fe VIII, IX, and X Line Emission From the Soft X-ray Background: Previous Limits and a Future Measurement
Authors: Bloch J. J. , Priedhorsky W. C. , Smith Barham W. 
Published: July 2016

AbstractWe discuss pulse height analysis of Be band data in relation to the important 72 eV Fe line cluster emission from the soft X-ray background (SXRB). Pulse height fits to the Be band data suggest that the Fe lines must be suppressed by a factor of ~10 with respect to the rest of the X-ray spectrum. The broad band rates and the mean energy of the pulse height data for the Be band can be brought into agreement by using depleted elemental abundance emission models. A planned measurement of the SXRB Fe lines using the Array of Low Energy X-ray Imaging Sensors (ALEXIS) experiment could resolve this issue.
X-ray emission from Reverse-Shocked Ejecta in Supernova Remnants
Authors: Cioffi Denis F. , McKee Christopher F. 
Published: July 2016

AbstractA simple physical model of the dynamics of a young supernova remnant is used to derive a straightforward kinematical description of the reverse shock. With suitable approximations, formulae can then be developed to give the X-ray emission of the reverse-shocked ejecta. The results are found to agree favorably with observations of SN1006.
Line Structures in the X-ray Spectra of Cygnus X-2 Observed With EXOSAT
Authors: Freeman P. E. , Kahn S. M. , Chiappetti L. , Tanzi E. G. , Ciapi A. , Maraschi L. , Treves A. , Branduardi-Raymont E. G. , Ercan E. N. 
Published: July 2016

AbstractCygnus X-2 was observed with EXOSAT at five phases of a single orbital cycle in September of 1983. We will summarize the results of spectral fits of the LE + ME (Argon) data in terms of a superposition of thermal bremstrahlung and blackbody components. During the first observation a grating spectrum was obtained and this is described in some detail. The GSPC data are used to investigate the presence of iron features, and their behavior during dips.
Comments on the Future Observatories and Their X-ray Spectroscopy Capability
Authors: Culhane J L 
Published: July 2016

AbstractFollowing a brief discussion of the possibilities offered by X-ray spectroscopy at various values of resolving power (E/ΔE), the spectroscopic capabilities of a number of future missions are presented. The possible roles of Charge Coupled Devices (CCD’s), Microcalorimeters, Diffraction Grating and Bragg Crystal Spectrometers are then evaluated.
High Throughput Soft X-ray Spectroscopy With Reflection Gratings
Authors: Kahn Steven M. 
Published: July 2016

ABSTRACT:As dispersing elements, grazing incidence reflection gratings offer the unique combination of high dispersion and wide spectral coverage at high efficiency. They can therefore be coupled with large area, low-resolution mirrors and high quantum efficiency detectors to yield moderate resolution spectroscopy of faint X-ray sources. Various design options are presented and compared, including both objective and convergent-beam configurations and both in-plane and off-plane grating mountings. A specific reflection grating payload design for ESA’s X-Ray Multi-Mirror Mission (XMM) is reviewed in more detail. Predicted performance curves derived from ray trace studies are presented along with preliminary X-ray reflectivity measurements of prototype grating samples.
The SHEAL Diffuse X-ray Spectrometer Experiment
Authors: Sanders W. T. , Snowden S. L. , Edgar R. J. 
Published: July 2016

AbstractThe Diffuse X-ray Spectrometer (DXS) experiment is part of NASA’s SHEAL 2 mission, scheduled to be flown as an attached Shuttle payload in 1992. The DXS is designed to measure the spectrum of the low energy (0.15 to 0.28 keV) diffuse x-ray background with energy resolution better than 0.01 keV. This paper describes the DXS experiment and presents the results of calculations of the anticipated data.
Surface Morphology of Cometary Nuclei
Authors: Möhlmann D. , Kührt E. 
Published: July 2016

Abstract.The first in situ observations of a cometary nucleus by imaging techniques on board the VEGA and Giotto spacecraft in 1986 opened the possibility of directly studying the properties and structures of a cometary surface. This new information, combined with results from other experiments made by these spacecraft and with results from laboratory experiments and Earth-bound radar observations, gave a better basis for coming from speculation to more precise knowledge in describing properties of cometary surfaces and related activity. It is the aim of this paper to discuss the different views developed to understand properties of cometary surfaces and to propose a consistent model for understanding topological surface properties and related cometary activity. We accomplish this by extending former hypotheses and approaches and making them more precise in the light of the new and more complex data.
Optical Properties of Cometary Dust
Authors: Lien David J. 
Published: July 2016

AbstractCometary dust is observed in a variety of ways: scattered light, thermal emission, stellar occultations, coma and tail morphology, radar echoes, and meteors associated with comets. To interpret these observations with respect to the physical parameters of the dust (size, shape, and composition) and the properties of the dust-emitting region, the physics of how electromagnetic radiation interacts with small particles must be understood.The interaction between electromagnetic radiation and a small particle depends on the size, shape, and composition of the particle. The composition determines the optical constants of the material. Classically, a solid can be approximated by a collection of bound charges with a number of resonant frequencies. The optical constants (the index of refraction or dielectric constant) are a measure of the ability of the material to vibrate in response to an incoming electromagnetic wave. The laboratory measurement of optical constants is very difficult, and many published optical constants may be in error. Care must also be taken in choosing the optical constants that best represent the assumed composition of the cometary dust.Scattering theory is usually synonymous with Mie theory, although Mie theory pertains only to spherical particles. In many cases, homogeneous spheres may be good approximations of the dust in the coma, but an understanding of the effects of non-sphericity and heterogeneity is essential to determine the limitations of the spherical approximation. A variety of methods exist that, although computationally intensive, do provide insight into shape effects.
The Magnetic Field Structure of the Cometary Plasma Environment
Authors: Neubauer F. M. 
Published: July 2016

AbstractThe plasma surrounding a comet has the interplanetary magnetic field frozen in. The geometric interpretation of this property is considered. The frozen-in character of the magnetic field leads to the draping of magnetic field lines around the inner coma, where, by exclusion of the inner purely cometary ionosphere, a magnetic cavity is formed inside a region of magnetic field pile-up. The consequences of these physical processes can nicely be diagnosed and tested by interplanetary tangential discontinuities serving as tracers of the magnetoplasma flow. The topology of the magnetic field around the cavity and the shape of the ionopause, as well as the formation of the magnetic tail, are discussed. Particularly in the outer regions, the magnetic field is disturbed by strong magnetic turbulence. This turbulence plays a role in accelerating cometary and also solar wind ions to high energies.
High accuracy (≪ 1%) polarisation measurements with the WSRT
Authors: Noordam J.E. 
Published: July 2016

AbstractThe procedure for polarisation measurements with radio synthesis telescopes is fraught with uncertaincies and shaky assumptions. Therefore, their accuracy will be limited to a few percent at best. It is investigated here whether it will be possible to improve this accuracy by a factor 10-100 with the WSRT, which has excellent polarisation characteristics, very small closure errors, and redundant spacings. The latter can be used to calibrate the instrument in a way which is less dependent upon assumptions about the polarisation of the calibrator source. They also cause more reliable convergence of the SELFCAL model, by reducing the number of free parameters. Although the investigation is not yet finished, the tentative conclusion is that it may be possible, with crossed dipoles, to derive completely model-independent values for Q/I, U/I and V/I.
Application of an estimator-free information criterion (WIC) to aperture synthesis imaging
Authors: Ishiguro M. , Morita K.I. , Ishiguro M. 
Published: July 2016

AbstractA statistical criterion for stoppingCLEANprocedure is proposed. The criterion is calledWICand an estimator of Kullback-Leibler information quantity which is a measure of the goodness of statistical models. A numerical example is given.
Observational test of the homogeneous array concept: An X-band image of the Crab Nebula
Authors: Uson Juan M. , Cornwell T. J. 
Published: July 2016

AbstractWe have performed an observational test of the “Homogeneous Array” concept utilized in the MMA design, whereby the array elements are used in total power mode to provide the shortest spacings. Using the VLA in interferometric mode and a VLBA element measuring total power, we have reconstructed a mosaic image of the Crab Nebula, which fits the data adequately and is of good visual appearance.
Are the bandwidth synthesis delay and delay rate observables useful for VLBI imaging?
Authors: Charlot P. 
Published: July 2016

AbstractThe Band Width Synthesis (BWS) delay and delay rate observables measured by VLBI are used in astrometry to determine positions of extragalactic radio sources with a precision of 0.001”. At this level of precision, extragalactic radio sources exhibit “extended structures that induce significant effects in the measured BWS delays and delay rates. The question whether closure BWS delays and delay rates could supplement the amplitudes and closure phases for VLBI imaging is studied. Preliminary results for the radio source NRAO140 observed in July 1983 are presented.
Dual-Frequency Synthesis VLBI Observations of 3C84 at 18 cm wavelength
Authors: Biretta J. A. , Bartel N. , Deng J. 
Published: July 2016

AbstractWe present results of a multi-frequency synthesis VLBI experiment which employed observations at 1622 and 1707 MHz. Combining data from two frequencies gave a factor ∼2 improvement in dynamic range over the single frequency images. We estimate map fidelity by comparing the images made separately from each frequency; while the dynamic range is ∼1800, the fidelity index is only ∼5.