bluuebunny/crossref_metadata_2025 huggingface dataset:
The Masses and Pulsation Modes of Classical Cepheids
Authors: Cox Arthur N.
Published: July 2016

Recent general observations pertaining to the masses and pulsation modes of classical Cepheids are reviewed. Certain special ones of these variables that display unusual behavior such as long term amplitude variations, overtone pulsations, and double-mode behavior, including one Cepheid in the first and second overtones as well as those in the fundamental and first overtone, are discussed in some detail. I suggest that the amplitude varying supergiant Cepheid HR 7308, which seems to be pulsating in the second radial overtone, is alternating between states where there is enough helium to barely give kappa and gamma effect driving, and where the helium has settled too deep for this driving. Reestablishment of the helium may be due to rapid levitation of the CNONe elements which cause convection and which then dredge-up the helium again to suppress the convection and also to drive pulsations. The use of Fourier fitting to the light and velocity curves have revealed some features that are interesting and are discussed, such as the pulsation mode discrimination and the derivation of accurate Wesselink radii. The seven methods of determining the masses of these stars are presented with a short critique of each. Evolution and pulsation masses depend on uncertain observed luminosities, and these have been revised recently. Evolution masses also depend on the blue looping yellow giant evolution tracks that may need revision because of convective core overshooting and possible opacity, Z, or Y abundance increases. The most discrepant masses, however, are those that depend on period ratios – the “bump” and “beat” masses. The possibility that the Cepheid internal structure can be modified enough by doubling the material opacity seems unlikely. It is suggested, though, that perhaps the period ratio mass anomaly solution can be found in CNONe element enhancement by radiation absorption levitation that would give higher opacities by abundance effects instead of any revisions in the opacity calculations. Some details for this mass anomaly “solution” are presented.
The Baade-Wesselink Technique
Authors: Moffett Thomas J.
Published: July 2016

The historical foundations of the Baade-Wesselink (BW) method were established through the work of Baade (1926), Becker (1940), and Wesselink (1947). All modern versions of the BW-method are constructed upon these foundations. Since the radii of pulsating stars are fundamental to many areas of astronomical research (theories of stellar structure and evolution, galactic structure, and the cosmic distance scale), it is paramount that we strive to improve their observational determination. At the present time, the BW-technique is the only “direct” method for determining the radii of pulsating stars. Until a new technique is devised, better determinations of this important physical parameter will only occur through improvements in the BW-method.
The Pulsation of some δ Scuti Stars with Unusual Light Curves
Authors: Antonello E. , Poretti E.
Published: July 2016

Among 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

It 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

Linear 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

The 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

We 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

We 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

RR 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

We 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

The 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.
The Differential Emission Measure of λ And
Authors: Landini M. , Fossi B.C. Monsignori , Pallavicini R.
Published: July 2016

Simultaneous X-ray and UV observations of λ And from EXOSAT and IUE are analyzed and the differential emission measure as a function of temperature is derived.
Thermal Instability in a Hot Plasma
Authors: Balbus Steve A. , Soker Noam
Published: July 2016

The 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

I 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

We 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

We 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

A 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

Cygnus 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.
Cosmic X-ray Background from Early Active Galactic Nuclei
Authors: Zdziarski Andrzej A.
Published: July 2016

A model for the origin of the cosmic X-ray background (hereafter XRB) is presented. The component of the background left after subtraction of the known classes of sources is explained by emission from a population of black hole sources at the redshift of z ~ 4-5. The model is presented in more detail elsewhere (Zdziarski 1988). Here, we summarize its most important results.
Comments on the Future Observatories and Their X-ray Spectroscopy Capability
Authors: Culhane J L
Published: July 2016

Following 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

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

The 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

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

Cometary 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

The 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.
Cometary Plasma Waves and Instabilities
Authors: Tsurutani Bruce T.
Published: July 2016

This review will discuss various plasma waves and instabilities that have been observed near comets. Comments on nonlinear wave evolution and wave cascading, as well as the role of nonlinear waves in wave-particle interactions, will be made.
High accuracy (≪ 1%) polarisation measurements with the WSRT
Authors: Noordam J.E.
Published: July 2016

The 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

A 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

We 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

The 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

We 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.
Astrometry of SiO Masers: First detections of circumstellar SiO maser emission at λ = 7 mm using three European VLBI stations
Authors: Colomer F. , Graham D. , Krichbaum T. , Rönnäng B.O. , de Vicente P. , Barcia A. , Booth R.S. , Witzel A. , Gómez-González J. , Baudry A.
Published: July 2016

The positions of circumstellar molecular masers relative to each other, to the central star, and to the compact extragalactic sources are of astrophysical and astrometrical interest. The SiO masers at λ = 7 mm are especially interesting as the maser spots are located very close to the photosphere of the star. For astrometry we would like to see a proper distribution of the maser spots and several epochs of observations in order to estimate the position of the central star and eventual motions of the spots.

The optical position of this star can be determined with ground-based astrometric facilities, and many of the stars showing SiO emission are in theHipparcoscatalogue. A link between the Optical and Radio Reference Frames is achieved in a second step relating the maser spot positions to the Extragalactic Reference Frame (EGRF) established with VLBI (Baudryet al., 1984).
Microwave Background interferometry in Cambridge
Authors: Jones Michael E.
Published: July 2016

Interferometric methods of studying the Cosmic Microwave Background (CMB) have some distinct advantages over the switched-beam techniques which have mostly been used. However, most existing interferometers are not well suited to CMB observations, for a variety of reasons. These include poor temperature sensitivity due to a low filling factor, and systematic effects which limit the maximum possible integration time. A new instrument, the Ryle Telescope, has been developed in Cambridge which has a high temperature sensitivity (120 μK in 12 h) and the ability to integrate for several hundred hours on the same field. It will be used to study the CMB on angular scales of a few arcminutes, with particular emphasis on the Sunyaev-Zel’dovich effect. A second instrument to study the CMB on angular scales of tens of arcminutes, the Cosmic Anisotropy Telescope (CAT), is also being developed.
One Zone Modeling of Irregular Variability of Stellar Convective Envelope
Authors: Unno W. , Xiong D.-R.
Published: July 2016

One zone modeling of the irregular variability of red super-giants is intended with regard to the nonlinear coupling of finite amplitude pulsation with convection. The nonlocal mixing length is employed for the evaluation of the convective flux, the turbulent pressure and the turbulent power of temperature fluctuations. The radial pulsation and the Boussinesq convection are assumed for simplicity. The one zone is defined as the layer having the entropy maximum and the minimum at the bottom and at the top, respectively. The quasi-adiabatic approximation is consistent with this definition in fixing the zone to the same mass range. The spatial derivatives are evaluated under the assumption of homologous changes with the equilibrium homologous parameters. Then, a set of 6 simultaneous first order nonlinear ordinary differential equations are obtained as the one zone representation of the irregular variability of the convective envelope.
Variability of X-Ray Emission from Cen X-3 Observed with Ginga
Authors: Kanetake R. , Takeshima T. , Makishima K. , Takeuti M.
Published: July 2016

Simplified stellar models like as one-zone models can be oscillated in chaotic state (Saitou et al. 1989). It is natural to consider that the irregular variations of Galactic X-ray sources might be caused from nonlinear oscillation of accretion disks or related flow. We studied the variability of Cen X-3 for applying the nonlinear theory to it.

The X-ray of Cen X-3 observed with Ginga from 22nd to 24th of March, 1989 are analyzed. Since Large Area Counters (LAC) of which the area of detector is 4,000 cm2 are used. The data used here is obtained the range of 1,000 sec from 14 h 25 m on March 23rd with the time resolution of 0.25 sec. The energy bands are 1.7~4.6 keV, 4.6~9.3 keV, 9.3~19 keV, and 19~37 keV, respectively.

Takeshima et al. (1991) show that Cen X-3 is at the high state, when the X-ray from the source is not veiled by non-ionized matter (Schreier et al. 1976). So that we ignore the effect of circumstellar matter around the source.
Predicting Variable Star Light Curves
Authors: Serre T.
Published: July 2016

I present a new method to test determinism, in particular the nonlinear behavior, in observed time series of pulsating stars, based on a recent prediction method which exploits the dynamical system theory. A method for filling gaps in data has thereby been constructed. Estimated bounds to the necessary embedding dimension can be obtained and chaotic divergences can be estimated.
The Onset of Quasi-Periodic Variations in Be Stars
Authors: Kogure T. , Mon M. , Suzuki M.
Published: July 2016

We present some evidence of the quasi-periodic long-term variations (QPLV) in the violet-to-red ratio of double-peaked emission lines (V/R variation) and/or in the radial velocities of shell absorption lines for some Be stars. Although the V/R variations are rather prevailing phenomena among Be stars, the QPLV is remarkable by the following characteristics:

(1)

The QPLV appears as a sudden onset of repeated V/R variations after a long (10 years), almost stable period, and it persists for a few or several periods in ten or more years.

(2)

The period and amplitude of V/R variations change from cycle to cycle and from star to star. The variations of radial velocities (RV) of shell absorption lines are usually nearly parallel with the V/R variations.

(3)

The QPLV appears usually in early type Be stars with large rotational velocities, regardless whether the stars are normal Be or shell stars, and whether they are close-binaries or single stars.
Some Insights into Stellar Structure from Nonlinear Pulsations
Authors: Goupil M. J.
Published: July 2016

Efficient tools of investigation of stellar pulsation are the integral relations which link oscillation frequencies to the static structure of stellar models, as provided by the linear theory of pulsation (for a review, see Saio, this conference).

Similarly, oscillation amplitudes and phases, which arise from nonlinear processes, can be related to the stellar structure by means of amplitude equation formalisms (for a review, see Buchler, this conference).

For the simple case of a monoperiodic oscillation, involving only one unstable marginal mode, such a formalism shows that the (limit cycle) radius variations, at time t and mass level m, can be approximated, up to second order of approximation, (Buchler and Goupil, 1984; Buchler and Kovàca, 1986) by:

where A, R, Ω, ĸ, £r(m) respectively are the amplitude, stellar radius, linear nonadiabatic frequency, growth rate, radius eigenfunction. Second order nonlinearities generated first harmonic oscillations and change in equilibrium radius about which the star oscillates, as represented by the last two terms in (la) respectively. Analogous expressions are obtained for velocity and light variations, that can be compared with observations.
Period Doubling with Hysteresis in BL Her-Type Models
Authors: Moskalik P. , Buchler J. R.
Published: July 2016

We have performed recently a survey of the nonlinear hydrodynamical models of the BL Her-type variables (Buchler & Moskalik 1992). Within this project we have studied several sequences of models, i.e., families in which onlyTeff has been varied from model to model, while all other stellar parameters have been kept constant. The fundamental mode pulsations of each model have been converged to strict periodicity with the relaxation code (Stellingwerf 1974). Such approach speeds up the calculations and simultaneously yields information about the stability properties of the resulting limit cycles. In all studied sequences except one, we have found a narrow-range of Teff (typically 100-150K), in which regular solution becomes unstable towards a period doubling bifurcation. The instability has its origin in a half-integer resonance, namely the 3:2 coupling between the fundamental mode and the first overtone (cf. Moskalik & Buchler 1990; hereafter MB90). This is the same resonance, which also causes period doubling in the models of classical Cepheids (Moskalik & Buchler 1991). The bifurcation leads to stable period-two oscillations, characterized by an RV Tau-like, albeit strictly periodic behavior of all variables. In other words, the pulsation light curves and velocity curves will exhibit two alternating minima (as well as maxima) of different values.
Hydrodynamics and Multi-Level Non-LTE Radiative Transfer in Pulsating Atmospheres: Cepheids
Authors: Sasselov D. D.
Published: July 2016

The atmospheres of classical Cepheids cannot be represented adequately by a sequence of hydrostatic equilibrium atmospheric models (see Sasselov and Lester 1990 and references therein). Observational evidence for shocks is also available at least since Kraft (1956). Both the disequilibrium and the non-linear phenomena affect the emergent spectra and spectral line profiles of the Cepheids. In the lower amplitude variables the effects may be subtle, yet still lead to significant systematic discrepancies.

We have developed the code HERMES for time-dependent treatment of hydrodynamics and non-LTE radiative transfer in the atmospheres of classical Cepheids with periods shorter than 12 days. Our approach is applicable to stars in which the formed shock waves do not dominate the energy balance of the atmosphere. Consecutive detailed calculations are performed for several multi-level model atoms, including H, Ca II, Mg II, He I and II. We use a 1-D explicit conservative upwind monotonic second-order (in time and space) Godunov-type Lagrangian hydrodynamic scheme. Being a characteristics based scheme it allows natural handling of boundary conditions. The scheme is stable and without artificial dissipation, a crucial necessity for physically meaningful radiative transfer solutions (see Roe 1990, Sasselov and Raga 1991 for more details).
New Opacities and First Overtone Mode Cepheids
Authors: Antonello E.
Published: July 2016

Some results of linear adiabatic and nonlinear pulsation models of first overtone mode Cepheids are discussed. New and augmented opacities and a nonstandard mass-luminosity relation have been taken into account. The models indicate the possible importance of the resonance P1/P4 = 2 near P1 = 3 days. The resonance could explain the observed characteristics of the light curve shape of first overtone mode Cepheids.
Coupled-Oscillators
Authors: Tanaka Y.
Published: July 2016

Modal coupling oscillation models for the stellar radial pulsation and coupled-oscillators are reviewed. Coupled-oscillators with the second-order and third-order terms seemed to behave non-systematically. Using the equation by Schwarzschild and Savedoff (1949) with the dissipation term of van del Pol’s type which is third-order, we demonstrate the effect of each term. The effects can be understood by the terms of the nonlinear dynamics, which is recently developing, that is. phase-locking, quasi-periodicity, period doubling, and chaos. As the problem of stellar pulsation, especially of double-mode cepheids on the period-ratio, we examine the dependence on the stellar structure from which the coupling constants in the second-order terms are derived. Eigen functions for adiabatic pulsations had been used for the calculation of the constants. It is noted that only two set of the constants are available, that is, for the polytrope model with n = 3 and a cepheid model without convection. Some examples of nonlinear dynamical effects will be shown.

It is shown that if the constants were suitable values, the period-ratio of double-mode cepheids is probably realized. The possibility is briefly suggested.
Pulsational Instability of Accretion Disks around Compact Objects
Authors: Okuda T. , Mineshige S.
Published: July 2016

Linear analysis shows that radial oscillations in accretion disks around compact object are overstable to axisymmetric perturbation under a variety of conditions. Furthermore, numerical simulations confirm that the radial oscillations induce quasi-periodic modulations of the disk luminosity. The disk oscillation model may be responsible for quasi-periodic oscillations (QPOs) observed in low mass X-ray binaries (LMXBs), cataclysmic variables (CVs), and other compact objects.
Examples of Comet-Like Spectra among β Pic-Like Stars
Authors: Butner Harold M. , Walker Helen J. , Wooden Diane H. , Witteborn Fred C.
Published: July 2016

To assess the physical conditions in our early solar system, we have to study the disks around other stars.βPic is a main sequence star surrounded by a large dust disk,βPic’s mid-infrared spectrum bears a striking resemblance to the silicate emission seen in some cometary spectra. We have selected a sample of main-sequence stars with similar IRAS properties to those ofβPic and undertaken a systematic survey of their physical properties. We obtained mid-infrared spectra covering the range 8 to 13μm at a resolution of 200 in June 1995. We report on the wide variety of silicate emission features we see and compare to the spectra reported for many different interstellar and interplanetary sources. Even among our small sample of stars, we find examples of silicate emission features that bear a striking similarity to those seen in the spectra of some comets like P/Halley and P/Bradfield 1987. These spectra also resemble the laboratory spectra of Interplanetary Dust Particles (IDPs).
Asteroid and Comet Impact Speeds upon Mars: Significance for Panspermia and the Supply of Organics
Authors: Steel Duncan
Published: July 2016

Whilst lithopanspermia depends upon massive impacts occurring at a speed above some limit, the intact delivery of organic chemicals or other volatiles to a planet requires the impact speed to be below some other limit such that a significant fraction of that material escapes destruction. Thus the two opposite ends of the impact speed distributions are the regions of interest in the bioastronomical context, whereas much modelling work on impacts delivers, or makes use of, only the mean speed. Here the probability distributions of impact speeds upon Mars are calculated for (i) the orbital distribution of known asteroids; and (ii) the expected distribution of near-parabolic cometary orbits. It is found that cometary impacts are far more likely to eject rocks from Mars (over 99 percent of the cometary impacts are at speeds above 20 km/sec, but at most 5 percent of the asteroidal impacts); paradoxically, the objects impacting at speeds low enough to make organic/volatile survival possible (the asteroids) are those which are depleted in such species.
Astronomical Influences on Biomagnetic Activity Some 120 MA ago: the Potential for Estimating the Evolution of Ancient Planetary Orbits within the Solar System
Authors: Tarling Donald H. , D’argenio Bruno , Iorio Marina
Published: July 2016

Studies of the magnetic properties of sedimentary rocks provide a record of biological activity in the geological past of the Earth. There is increasing evidence that the rate of biological activity reflects, in part, the direct and indirect influence of the Earth’s orbit around the Sun. These orbital changes also influence the strength and direction of the geomagnetic field, showing that orbital changes directly affect the processes generating the geomagnetic field. Therefore the presence of these effects means that past changes in the Earth’s orbit and the rate of rotation of the Earth can be investigated from such geological and geophysical records.
On the Ability of Synchronously Rotating Planets to Support Atmospheres
Authors: Joshi Manoj M. , Haberle Robert M.
Published: July 2016

Planets within the habitable zones of M stars are likely to be synchronous rotators, i.e., one side is permanently illuminated while the other side is in perpetual darkness. For such planets to be habitable, horizontal atmospheric motions must be capable of transporting enough heat from the day side to the night side to prevent the atmosphere from freezing out. Simple calculations have shown that the equivalent of approximately 150 mb of CO2is required to prevent atmospheric condensation on the dark side. We now use a 3-dimensional global circulation model in order to better assess the ability of these planets to sustain an atmosphere. The results of this work show that for an earth-sized planet, 100 mb of CO2is required to support an atmosphere, while 1-2 bars are sufficient to allow liquid water everywhere. Planetary systems associated with M stars should therefore not be ruled out in the search for life in the universe.
The Chemical Origin of Life
Authors: de Duve Christian
Published: July 2016

Fifty years ago, the problem of the origin of life was largely inaccessible to scientific research. Not only was almost nothing known of the conditions that surrounded the appearance of life on Earth, but there was so little understanding of life itself that the problem could not even be defined in concrete terms. The situation is very different today. Astronomical observations, geochemical findings, and laboratory investigations have illuminated the problem with much relevant information. Especially, the basic mechanisms of life are now understood in considerable detail. We know what to look for. Some important notions that have emerged in this way will be briefly surveyed in this essay, which summarizes views presented in two recent books (de Duve, 1991, 1995).
Did A Precellular RNA World ever Exist?
Authors: Rizzotti Martino
Published: July 2016

The so-called «RNA world» is the most fashionable hypothesis on the origin of the biosphere on the Earth. It states that cells derived from aggregates made of RNA in contact with the superficial waters. Its presumed pieces of evidence are drawn either from simulation experiments or from RNA molecules of the present-day organisms. However, both kinds of evidence are better explained by alternative hypotheses.
Surmising Cognitive Universals for Extraterrestrial Intelligences
Authors: Narens Louis
Published: July 2016

Cognitive universals are concepts that our civilization and technologically advanced extraterrestrial civilizations can easily interpret. The universality of certain mathematically and perceptually based concepts are discussed. It is argued that continuously based concepts are more fertile ground for surmising cognitive universals than discretely based ones, and in particular, one should be suspicious of the use of inductively based numerical concepts, including the totality of natural numbers. Ideas about intuitive evolutionary theory, physical and perceptual invariance, and the efficient processing of information are linked to provide a framework for searching for cognitive universals.
Millions and Billions: The Meta and Beta Searches at Harvard
Authors: Leigh Darren , Horowitz Paul
Published: July 2016

The META and BETA searches for microwave carriers have now completed a decade of observations of the northern sky, using the 26-meter radiotelescope at Agassiz Station. META's results set limits on the prevalence of advanced civilizations transmitting carrier beacons at the HI wavelength. BETA, the next-generation search system, combines greater bandwidth with quick lookback of candidate signals and robust rejection of signals of terrestrial origin. It covers the «waterhole» of 1.4-1.7 GHz, using a 250-million-channel spectrometer, in conjunction with a 3-beam antenna system, producing a 250 Mbytes/s output stream. The philosophy, implementation, and results of these systems will be described.
Southern Hemisphere Seti Survey: Five Years of Project Meta II
Authors: Lemarchand Guillermo A. , Colomb Fernando R. , Hurrell E. Eduardo , Olalde Juan Carlos
Published: July 2016

Project META II, a full sky survey for artificial narrow-band signals, has been conducted from one of the two 30-m radiotelescopes of the Instituto Argentino de Radioastronomía (IAR). The search was performed near the 1420 Mhz line of neutral hydrogen, using a 8.4 million channels Fourier spectrometer of 0.05 Hz resolution and 400 kHz instantaneous bandwidth. The observing frequency was corrected both for motions with respect to three astronomical inertial frames, and for the effect of Earths rotation, which provides a characteristic changing signature for narrow-band signals of extraterrestrial origin. Among the 2 × 1013spectral channels analyzed, 29 extra-statistical narrow-band events were found, exceeding the average threshold of 1.7 × 10−23Wm−2. The strongest signals that survive culling for terrestrial interference lie in or near the galactic plane. A description of the project META II observing scheme and results is made as well as the possible interpretation of the results using the Cordes-Lazio-Sagan model based in interstellar scattering theory.