bluuebunny/crossref_metadata_2025_split huggingface dataset:
Compressible MHD in Spherical Geometry
Authors: Valdettaro Lorenzo , Meneguzzi Maurice 
Published: July 2016

AbstractThe generation of magnetic field by a conducting, compressible fluid inside a spherical shell is studied by direct numerical simulations. A pseudo-spectral method is used in order to resolve accurately all the scales present in the problem. The range of parameters considered is the following: a unit Prandtl number, Rayleigh numbers up to 100 times critical, Taylor number 625, an aspect ratio of 2, a Mach number slightly less than 1, and pressure and temperature scale heights of the order of the thickness of the shell. A dynamo effect is observed for magnetic Prandtl numbers larger than 1. We present the properties of the turbulent flow, the role of the helicity and of the differential rotation in the enhancement of the magnetic field, and the spectral properties of the flow fields.
Rotational Effects on Reynolds Stresses in the Solar Convection Zone
Authors: Pulkkinen P. , Tuominen I. , Brandenburg A. , Nordlund Å. , Stein R.F. 
Published: July 2016

AbstractThree-dimensional hydrodynamic simulations are carried out in a rectangular box. The angle between gravity and rotation axis is kept as an external parameter in order to study the latitude-dependence of convection. Special attention is given to the horizontal Reynolds stress and the∧-effect (Rüdiger, 1989). The results of the simulations are compared with observations and theory and a good agreement is found.
The Sun’s Internal Differential Rotation From Helioseismology
Authors: Goode Philip R. 
Published: July 2016

AbstractWell-confirmed helioseismic data from several groups using various observational techniques at different sites have allowed us to determine the differential rotation in the outer half of the Sun’s interior. The resulting rotation law is simple – the surface differential rotation persists through much of the convection zone with a transition toward solid body rotation beneath. To date there is no appealing evidence for a rapidly rotating core. There is however, weak evidence for a solar cycle dependence of the Sun’s internal rotation.
Sun-as-a-star: Its Convective Signature and the Activity Cycle
Authors: Livingston W. 
Published: July 2016

AbstractFrom May 1980 to April 1990 we have observed several times a year the solar irradiance spectrum from 5000Å to 6300Å using the 1-m Fourier Transform Spectrometer. During the ascending phase of the solar cycle, spectrum line asymmetry diminishes about 15% as the total magnetic field encreases. At some epoch near the time of total magnetic field maximum, line asymmetry ceases to track surface magnetism and abruptly assumes a value appropriate to solar minimum.
Towards Magnetic Images of Rapidly Rotating Late-type Stars
Authors: Donati J.-F. , Semel M. 
Published: July 2016

AbstractWe present the first magnetic detections using Zeeman-Doppler Imaging (ZDI) in the two bright rapidly rotating RS CVn systems HR 1099 and σ2CrB, and discuss their compatibility with various recent results on magnetic activity in cool stars.
Long-term Starspot Behavior of BY Dra and EV Lac
Authors: Pettersen B.R. , Oláh K. , Sandmann W.H. 
Published: July 2016

AbstractNew starspot photometry of BY Dra and EV Lac, extending over a decade is analyzed in combination with data from the literature to study the long-term behaviour of spottedness on these stars. Both stars show brightness suppression as well as brightness modulation so spots exist both near the poles and at lower latitudes. Polar spots have been continuously present on BY Dra for the last 14 years. (O–C) diagrams for lightcurve minima reveal systematic, sinusoidal variations in BY Dra with a period of 13–14 years. In EV Lac the spots have been located on the same hemisphere for the last ten years.
The Spot Activity of FK Comae Berenices
Authors: Jetsu Lauri , Pelt Jaan , Tuominen Ilkka , Nations Harold 
Published: July 2016

AbstractThe active regions of FK Comae Berenices show a flip-flop behaviour, i.e. the concentrated part of spot-activity shifts exactly to the other side of stellar surface, and then remains on the same longitude for a time interval from a few years to a decade. The activity shows excellent phase coherence with respect to these two active longitudes separated 180 degrees from each other. FK Comae may provide a physical example of a non-linear dynamo, which shows surprisingly simple observational changes in the pattern of the magnetically induced spot configurations.
Recent Advances in the Observation and Analysis of Stellar Magnetic Fields
Authors: Saar Steven 
Published: July 2016

AbstractThere has been considerable progress recently in the study of magnetic fields on late-type stars. Advances in the theory include investigation of systematic effects, new and improved methods of analysis for unpolarized and circularly polarized lines, new models of stellar dynamos and of broadband linear polarization, and the first studies of the thermodynamic nature of stellar magnetic regions and their vertical structure. Observationally, there have been new measurements, particularly of young and active stars (including the first detection of a field on a pre-main sequence object), a new monitoring campaign, the first indications of the relative temperatures in stellar plages, and the first measurements of fields in a single stellar active region. I discuss the results in the framework of stellar activity and surface structure.
Spectroscopy of Southern Active Stars
Authors: Vilhu O. , Gustafsson B. , Walter F.M. 
Published: July 2016

AbstractHα and HeI D3 lines were observed in five active cool stars over their relatively short rotation periods. Three of the stars (AB Dor, Rstl37B, HD82558) are in the pre main sequence phase (as deduced from their lithium abundances), while two are post main sequence FK Comae type stars (HD32918 and BD-223467) (Vilhu et al., 1991). We present the results of these observations and point out the physical similarity of the Hα and He D3 transitions (Fig.1).
A Dynamo Model for Magnetic Stars with Long Periods
Authors: Schüssler M. 
Published: July 2016

SummaryA α - effect dynamo model is presented which can be relevant for the group of magnetic stars.with observed periods between 1 y and 72 ys. The model is based on an axisymmetric α2- dynamo including non-linear effects due to the “cut off α- effect”; no differential rotation is taken into account. There are oscilliations of the magnetic field with periods in the right order of magnitude under the assumption of an outer convection zone between R ≥ r ≥.5 R ….7R. In the sense of this model therefore these stars should be young objects passing from their Hayashi track down to the main sequence.
Magnetic Field And Chemical Anomalies of Ap Stars
Authors: Dolginov A.Z. 
Published: July 2016

SummaryIt is shown that owing to the “battery” effect the spatial nonuniform helium distribution in Ap stars causes a toroidal magnetic field of about 103- 104gauss. Possible circulation of stellar matter in this field produce a poloidal magnetic field of the same order as the toroidal one even if the velocities of circulation are about 10-4-10-5cm/s. The chemical anomaly in turn is supported and amplified by the magnetic field. It is pointed out that the “battery” effect is likely to take place in the liquid core of the earth and in components of close binaries if they are nonuniformly heated by their companions.
Abundance Anomalies, Diffusion and Constraints on F, A and B Stellar Envelope Models
Authors: Michaud G. 
Published: July 2016

SummaryA review of diffusion in F, A and B stars is presented. The basic equations are introduced in a simple and approximate way. Meridional circulation, magnetic fields, surface gravity and age are all factors that contribute to variations in anomalies for stars of a given effective temperature. Detailed calculations for each of these effects, as well as of radiation forces in the atmosphere, are needed before the required detailed comparison between theory and observations can be made for individual stars. When it is made this comparison should put strong constraints on stellar atmosphere or envelope models. At the present level of uncertainty no difficulty between theory and observations appears definitely established.
Diagnostics of Strontium Abundance In Ap Stellar Atmospheres
Authors: Praderie Francoise 
Published: July 2016

SummaryAnalysis of strontium abundance has been performed from Sr II resonance lines in radiative equilibrium model atmospheres characterized by Teff= 9650 K and Teff= 12500°K. One first compares LTE and nLTE determination of the abundance in the case where the abundance is uniform with depth. Second, one derives the effect of concentrating all strontium in a “cloud”, which is the situation produced if diffusion acts to sort the various elements.
The Peculiar A Stars: Atmospheric Parameters and Abundance Peculiarities
Authors: Hack M. 
Published: July 2016

We have known for more than 40 years (Morgan 1933) that a group of stars having abnormal spectra exists among the main sequence stars included between B5 and F2. They represent at least 15% of the normal stars with the same color and luminosity.They can be divided in two large groups and their properties are summarized in Tables 1 and 2. They are the Ap stars (including the various subgroups, Mn stars, Si stars, Sr-Cr-Euor cool Ap stars) and the Am, Fm stars.
Variation of Heavy Element Lines in the Ap Stars
Authors: Aslanov I.A. , Rustamov Yu. S. , Kowalski M. 
Published: July 2016

SummaryLines of U II were found in the spectrograms of the Ap stars HR 465, 17 Com A and HD 224801. Pm II lines were found in HR 465. These lines vary in intensity in HR 465 with a period of 6h41m, in 17 Com A of 71m, and in HD 224801 of 6h.
Some Peculiar Early-Type Stars in the Southern Milky Way (1 = 237° - 332°)
Authors: Rickman Hans 
Published: July 2016

SummaryIn the course of the Stockholm Observatory Spectral Survey of the Southern Milky Way apparent peculiarities have been found in a number of stars of early spectral types. Some of these stars have been subject to further observations at the European Southern Observatory in Chile. A list of the stars at present considered as peculiar is presented together with finding charts in view of forthcoming observations. The list contains all pieces of information collected up to the present time for each one of the stars.
On the Inhomogeneity of Chemical Composition over the Surface of 21 Per
Authors: Glagolevsky Yu.V. , Kozlova K.I. , Lebedev V.S. , Polosukhina N.S. 
Published: July 2016

SummaryThe magnetic variable star 21 Per has been studied from 4 and 8 Å/mm spectra obtained with the 2.6 - meter reflector of the Crimean Astrophysical Observatory. Spectral line intensities (Wλ) and radial velocities (Vr) have been measured.
The Spectroscopic Binary Characteristics of the Mercury-Manganese Stars
Authors: Aikman G.C.L. 
Published: July 2016

SummaryThe 80 confirmed and suspected mercury manganese stars identified prior to 1974 and north of declination -20° have been studied from 654 high dispersion spectrograms. (2.4 - 15 Å mm-1) for velocity and degree of peculiarity. The binary frequency based on velocity variability criteria alone appears to be a normal main sequence value of about 50%, and is probably independent of the degree of enhancement of the mercury and manganese lines. New spectroscopic orbital elements are given for six systems: α And, 2 Per, κ Cnc, φ Her, HR 6620 and 46 Dra. The Hg Mn spectrosopic binaries appear to be fairly normal in their orbital characteristics, except that periods less than 3 days are conspicuously absent; there is also weak evidence for more nearly circular orbits amongst the short period Hg Mn binaries than in comparable normal binaries.
Can Osawa'S Star HD 221568 be an Oblique Rotator?
Authors: Kodaira Keiichi 
Published: July 2016

SummaryRadial velocity data of HD 221568 are critically reexamined. The revised data lead to a velocity curve essentially the same as obtained by Kodaira (1967). It is shown that the velocity variation of HD 221568 could be attributed to orbital motion, if an oblique rotator model is to be assumed for the cause of line-intensity variation.
On the Adoption of a Terrestrial Reference Frame
Authors: McCarthy Dennis D. 
Published: July 2016

AbstractThe report of the IAU Working Group on Nutation endorsed by Commissions 4, 8, 19 and 31 at the 1979 General Assembly points out that “… the complete theory of the general nutational motion of the Earth about its center of mass may be described by the sum of two components, astronomical nutation, commonly referred to as nutation, which is nutation with respect to a space-fixed coordinate system, and polar motion, which is nutation with respect to a body-fixed system …”. Unlike the situation for the space-fixed frame, there is not an adequate, formally accepted, body-fixed system for this purpose. The Conventional International Origin (CIO) as it is presently defined is no longer acceptable because of recent improvements in observational techniques. The effective lack of this type of terrestrial reference frame limits the complete description of the general nutational motion of the Earth. In the absence of a terrestrial reference frame suitable for specifying the orientation of the Earth, it is suggested that a body-fixed system could be represented formally in a manner analogous to that used to represent the space-fixed frame. This procedure would be quite similar to methods employed currently by the International Polar Motion Service and the Bureau International de l’Heure, and would allow for the use of observations from new techniques in the definition of a terrestrial reference frame to be used to specify the complete nutational motion of the Earth.
Critical Remarks on the Possibility of Determining Variations of the Geocenter Position Using Geostationary Satellite Observations
Authors: Klokoĉník J. , Kostelecký J. 
Published: July 2016

AbstractThe possibility of determining the variations of the Earth’s cejiter (expressed in terms of the geopotential harmonic coefficients , , or of some functions of them), using geostationary satellites, was discussed by Burŝa and Ŝíma (1978) after discerning the difference between observed and theoretical positions of libration points. Although their effect on geostationary orbits cannot be expected to be of high magnitude, the question of determination of these terms arose. It was suggested that the “resonant phenomenon” observed here might be analogous to the resonant effects in close satellite orbits, because the satellite is at the 1/1-resonance (1 nodal revolution per 1 sidereal day).
Concepts of Reference Frames for a Deformable Earth
Authors: Richter Burghard 
Published: July 2016

AbstractIn order to describe geometrical facts in geodesy, geodynamics, or astronomy, one must have suitable reference frames. This paper deals with some basic concepts for the definition of both terrestrial and celestial systems which make it possible to describe position and motion of mass elements of the earth or stars, respectively. It is suggested to use such frames with respect to which coordinate changes of points on the earth’s surface or of stars, respectively, become minimal. The rotation vector should be defined by applying the rotation operator on the velocity field.
Apport des Observations d’Occultations Stellaires en vue du Rattachement du Systeme Dynamique au Systeme Geometrique
Authors: Froeschlé M. , Meyer C. 
Published: July 2016

AbstractThe possibilities of tying the geometrical and dynamical reference frames by use of lunar occultations are reviewed. Actually, the best accuracy available is about 0”.4, essentially limited by stellar positions and limb corrections. One can expect an improvement of one order of magnitude with the accomplishment of POLO and HIPPARCOS Projects.
Development of a Radio-Astrometric Catalog by Means of Very Long Baseline Interferometry Observations
Authors: Fanselow J. L. , Sovers O. J. , Thomas J. B. , Bletzacker F. R. , Kearns T. J. , Cohen E. J. , Purcell G. H. , Rogstad D. H. , Skjerve L. J. , Young L. E. 
Published: July 2016

AbstractThe Jet Propulsion Laboratory of the California Institute of Technology has been developing a radio-astrometric catalogue for use in the application of radio interferometry to interplanetary navigation and geodesy. The catalogue consists of approximately 100 compact extragalactic radio sources whose relative positions have formal uncertainties of the order of 0”.01. The sources cover nearly all of the celestial sphere above -40° declination. By using the optical counterparts of many of these radio sources, we have tied this radio reference frame to the FK4 optical system with a global accuracy of approximately 0”.01. This paper describes the status of this work.
Non-Radial Pulsation Models of the H-Alpha Profile in BW Vulpeculae
Authors: Odell Andrew P. 
Published: July 2016

AbstractThe profile of the Hydrogen alpha spectral line in the large-amplitude Beta Cephei star BW Vulpeculae was modeled in terms of a linear, non-radially moving, and rotating stellar atmosphere. The pulsation and rotation parameters were derived from fitting the radial velocity amplitude, and the best fit was obtained for l=2, m=-2, i=90°, rotation velocity=120 km/sec, and pulsation amplitude=27 km/sec. When the line profiles are compared to this model, the fit is quite encouraging for most phases, but several discrepancies remain. At the time of writing, it is not clear whether tliese can be made to disappear by increasing the sophistication of the model, or whether a completely different physical model (e.g. radial) is necessary.
Mode Discrimination in Nonradially Oscillating Stars From Light, Colour and Velocity Observations
Authors: Balona L.A. , Stobie R.S. 
Published: July 2016

AbstractExpressions for the amplitudes and phases of the light, colour and radial velocity variations are derived for a star in nonradial oscillation. For stars in the cepheid instability strip the spherical harmonic mode of the oscillation can be obtained from the phase difference between the light and colour variations. For 3 Cep stars the mode can be estimated from the amplitude ratio of the light and colour variations.
HR 7308: A Unique Cepheid+
Authors: Percy John R. , Evans Nancy Remage 
Published: July 2016

AbstractWe have obtained photometric observations and radial velocities of the F6I-IIb star HR 7308. The spectral type, the period (1d.49), the relative range and phase of light, colour and velocity variations all suggest that the star is a small-amplitude Cepheid. The metal abundance appears to be normal. However, the light amplitude decreased slowly during 1978 from 0m.3 to 0m.1, and increased during 1979 and 1980 from 0m.05 to 0m.15. Such behaviour is unprecedented in a population I Cepheid.
Bump Masses and Radii of BL Herculis Variables
Authors: Petersen Jørgen Otzen 
Published: July 2016

AbstractBump masses and radii are derived for 18 BL Her stars from the observed bump phase and the accurately known fundamental period. The mean mass M/M⊙ = 0.60 ± 0.09 agrees precisely with predictions from standard stellar evolution theory and gives a new test of the theoretical models. The derived radius of V553 Centauri is in good agreement with the radius recently determined by an independent modified Baade-Wesselink method by Balona. Finally, a preliminary discussion of possible continuations of the BL Her bump progression is given.
Core Collapse, Bounce and Shock Propagation
Authors: Bowers Richard L. , Wilson James R. 
Published: July 2016

Abstract.Recent developments in the physics input for iron core collapse models of type II supernovae are reviewed. The effect of these developments on collapse calculations is also discussed. The inner core collapses homologously, with little change in specific entropy, bounces in the neighborhood of nuclear density, and sets up; an outward moving shock. In adiabatic models an explosion may result. The inclusion of neutrino effects may produce substantial shock damping. Current results indicate that core collapse, bounce and shock propagation does not produce an explosion when neutrino effects are included.
White Dwarf Models for Type I Supernovae and Quiet Supernovae, and Presupernova Evolution
Authors: Nomoto Ken’ichi 
Published: July 2016

AbstractSupernova mechanisms in accreting white dwarfs (WDs) are presented, i.e., the carbon deflagration as a plausible mechanism for producing Type I supernovae and electron captures to form quiet supernovae leaving neutron stars. These outcomes depend on accretion rate of helium, initial mass and composition of the WD. The various types of hydrogen shell-burning in the présupernova stage are also discussed.
The Effect of a Magnetic Field on Stellar Pulsations as a Singular Perturbation Problem
Authors: Goossens M. , Biront D. 
Published: July 2016

AbstractThe perturbation problem that describes the effect of a weak magnetic field on stellar adiabatic oscillation is considered. This perturbation problem is singular when the magnetic field does not vanish at the stellar surface, and a regular perturbation scheme fails where the magnetic pressure is comparable to the thermodynamic pressure. The application of the Method of Matched Asymptotic Expansion is used to obtain expressions for the eigenfunctions and the eigenfrequencies.
IUE Data Distribution
Authors: Heck André 
Published: July 2016

AbstractThe IUE project is described, with special emphasis on the IUE data management and archive retrieval.This paper briefly describes a more specific data center providing also a service to the astronomical community, or more precisely a group of data centers dedicated to a given instrument.This instrument is the IUE ( International Ultraviolet Explorer) launched on Jan. 26, 1978 and collecting since then spectral images of all kinds of celestial objects in the ultraviolet wavelength range.
A Coronographic Mode for Large Telescopes
Authors: Courtes G. , Lamy Ph. , Mauron N. , Saisse H. 
Published: July 2016

AbstractThe study and efficiency test of the optics of the Space Telescope Faint Object Camera carried out at the Laboratoire d'Astronomie Spatiale, led the authors to design a similar coronographic mode for large ground based telescopes. Adaptation of this design to the Mount Chiran 1 meter and E.S.O. 3.6 meter Ritchey-Chrétien telescopes has given new levels of performance in the detection of faint features in stellar and planetary observations.
Recent Progress with Multi-Anode Microchannel Array Detector Systems
Authors: Timothy J. Gethyn 
Published: July 2016

AbstractThe Multi-Anode MicroChannel Arrays (MAMAs) are a family of photoelectric, photon-counting array detectors that are being developed specifically for use in instruments on space-borne and ground-based telescopes. In this paper we review briefly the construction and modes of-operation of the MAMA detectors and describe the current development status of the different detector systems.
Application of the Electronographic Image Converter for Spectroscopy at the 2.6m Reflector of the Crimean Astrophysical Observatory
Authors: Petrov P.P. 
Published: July 2016

AbstractAn Electronographic Image Converter (EIC) of the “Spectracon” type is being used for routine spectroscopic observations at the 2.6m reflector of the Crimean Astrophysical Observatory. Some details of the EIC and film-loop applicator are given and the spectrophotometric accuracy and detective quantum efficiency are discussed.
First Results with an Electronographic Camera Used at the Prime Focus of the C.F.H. Telescope in Hawaii
Authors: Picat J.P. ,   Groupe Caméra
Published: July 2016

AbstractFirst results have been obtained with an electronographic camera used at the prime focus of the C.F.H. Telescope in Hawaii. The camera is briefly described. It is an electronic camera with a new technological design and a tight valve allowing the recovery of the nuclear plates without destroying the photocathode. Some preliminary results are given.