bluuebunny/crossref_metadata_2025_split huggingface dataset:
Activity, Colour Anomalies and Temperature Determination in Solar-type Stars
Authors: Fabregat J. , Reglero V. , Suso J. , Armentia J.E. 
Published: July 2016

The presence of photometric anomalies in broad band colours produced by chromospheric activity was first suggested by Campbell (1984). He defined the colour anomaly δ(B–V)V–K as the deviation from the mean relation (B–V)–(V–K) for Hyades dwarfs, and found this anomaly correlated well with several activity indicators.Since the work of Campbell, several authors have discussed the relation between activity and colour anomalies, obtaining diverging results. This discussion is relevant because long baseline photometric colours are the most reliable temperature indicators, and the presence of such anomalies implies that no consistent temperature scales can be obtained for stars with different activity levels. These topics have been recently reviewed by Soderblom (1989).
Radial Velocity Variability of K Giants
Authors: Hatzes Artie P. , Cochran William D. 
Published: July 2016

AbstractAt McDonald Observatory we have been monitoring the relative radial velocities of a sample of K giants. The technique employed uses the telluric O2 lines near 6300 Å as a reference for measuring the stellar line shifts. We demonstrate that precisions of 10 m s−1 are possible with this technique. We present radial velocity data covering a 2 year time span for α Boo, α Tau, and β Gem. All of these stars show both long term variations (~ several hundred days) with a peak-to-peak amplitude of about 400 m s−1 and short term variations (~ few days) with a peak-to-peak amplitude of about 100 m s−1. The long term variations may be due to the rotational modulation of surface active regions whereas the short term variations may be indicative of pulsations.
Stellar Zeeman Analyses: Effects of Multi-component Atmospheres
Authors: Marcy G.W. , Basri G. , Valenti J.A. 
Published: July 2016

AbstractWe simulate the radiative transfer of Zeeman-broadened lines in an active star containing both quiet and flux-tube regions. A traditional one-atmosphere Zeeman analysis of these synthetic lines yields magnetic field measurements systematically inaccurate by up to 40%. However, two atmospheric components alone cannot reproduce the observed line strengths, luminosities or photometric constancy of active stars.
Two Kinds of Activity in Late-type Stars
Authors: Katsova M.M. 
Published: July 2016

Several years ago we proposed a method for the analysis of X-ray observations of late-type stars. It allowed the determination in a uniform manner of coronal base electron densities for more than 40 late-type stars, in terms of a one-temperature consideration of homogeneous spherically symmetric coronae (Katsova et al., 1987). Fig. 1 shows the results as a function of spectral type. Comparison of our results with values for different kinds of solar regions shows that physical characteristics of F and G star coronae correspond to densities less than those in active regions on the Sun. Values for the active K-M0 stars are comparable with those of dense steady condensations found directly above large sunspots.On this basis, activity can be explained as an increase in that part of the stellar surface that is occupied by strong local magnetic fields. This is illustrated in the table where we compare magnetic field measurements by Saar and Linsky (1988) with our calculations.
Ca II H and K and Hα, and Li Abundances in the Pleiades Late K Main-Sequence
Authors: López R. J. García , Rebolo R. , Beckman J. E. , Magazzù A. 
Published: July 2016

AbstractWe have observed seven main sequence stars in the Pleiades, with B – V between 0.98 and 1.41 (5100–3900 K), and with a wide range of rotational velocities, in Ca II H and K, Hα and have derived Li abundances. Our results, combined with literature data, indicate that the most chromospherically active stars are rapid rotators, and that at a given effective temperature the Li-rich stars show the highest chromospheric activity. A different surface coverage of active regions could influence the dichotomy observed in the Li abundance distribution.
Is ξ Boo AB a Young or an Evolved System? A Model Atmosphere Analysis
Authors: Savanov I.S. 
Published: July 2016

AbstractWe perform a model atmosphere analysis of ξ Boo A, and make estimates for the secondary component. The abundances of 25 elements for ξ Boo A are obtained with the parameters: Teff=(5300 ± 100) K, log g= 4.1 ± 0.2, ξt = 0.5km/s. The atmosphere of ξ Boo A is deficient in Y and Zr, but overabundant in Ba and rare earth elements. We confirm the high Li abundance in ξBoo A (logA(Li) = 2.1), while Li is strongly underabundant in the secondary relative to the Sun. From comparison with evolutionary calculations we find that ξ Boo A has mass 0.95 M⊙, radius 1.4 R⊙ we also determine a rotational velocity v sin i = 8 km/s. We conclude that, in spite of its relatively high rotation rate and high chromospheric activity, it is already evolved from the main-sequence.
He I 5876 Å Line in the Study of Chromospheric Activity in F-type MS Stars
Authors: López R. J. García , Rebolo R. , Beckman J. E. , McKeith C. D. 
Published: July 2016

AbstractWe have measured the He I D3 line, λ5876 Å, in 32 F-type young open cluster stars and in 4 field stars. Combining our results with previous observations of D3 we strengthen the following conclusions: (a) Sub-surface convective zones appear, on the main sequence, around F0 (B – V ~ 0.3). (b) The stars between F0 and F5 (0.3 ≲ B – V ≲ 0.42) are chromospherically “active” irrespective of age, which implies a heating mechanism not affected by evolutionary effects. In this range the activity in D3 and X-rays are uncorrelated. (c) For stars later than F5 the correlation among all the activity indices, chromospheric and coronal, shows a common energy source. Convective zones here should be deep enough to generate magnetic fields capable of supplying this energy, and also for braking the rotation.
Hα Emission in the Giant Components of RS CVn Systems
Authors: Frasca A. , Catalano S. 
Published: July 2016

The Hα line of hydrogen is a very powerful tool for the study of the solar chromosphere (plages, flares, prominences). Although it is difficult to derive in a simple way quantitative information, like chromospheric temperature and electron density, because of the very extensive depth of its formation and the very complex mechanism of excitation, Hα has been successfully used to study the chromospheres of active stars (Herbig, 1985). Moreover this diagnostic has proved to be very useful for the study of surface inhomogeneities and transient phenomena like flares in systems of the RS CVn type and other active stars (Bopp, 1981; Bopp et al., 1988; Fraquelli, 1982).With the aim of investigating the behaviour of chromospheric activity, we have undertaken an extensive monitoring of Hα emission in a selected group of RS CVn binaries (Strassmeier et al., 1988). We report here preliminary results of the observations obtained in 1988–1989.
Coronal Magnetic Fields of Algol Binaries from Microwave Spectra
Authors: Umana G. , Trigilio C. , Hjellming R. M. , Catalano S. , Rodonò M. 
Published: July 2016

Algol-type binaries are basically known to undergo hydrodynamic processes related to mass exchange between components. Recent observations on radio, X-ray emission and flare-like events have raised the question of possible magnetic activity in the secondary component of these systems (Hall, 1989).From a microwave emission survey we have shown that the radio emission from Algol systems cannot be accounted for by thermal emission from an hot corona (T ≥ 107K) and that their radio luminosities compare very well with those of the magnetically active RS CVn systems (Umana et al., 1990).
Ap-Star Models
Authors: Mestel L. 
Published: July 2016

PhenomenologyThe outstanding observational facts that impress the theorist studying the magnetic Ap stars are as follows:1) The very strong, large-scale magnetic fields, inferred from the integrated Zeeman effect. Typical polar fields are 103- 104gauss, the strongest known being =3.5 x 104gauss.2) The variability of the fields, spectra and luminosities, with the fields often reversing in sign. Typical periods are 5 - 9 days, but periods shorter and very much longer are found, with a small group of stars having periods of several years.3) The low rotations of most Ap and apparently all known Am stars, compared with normal A stars. The possibility that we are seeing normal A stars pole-on seems statistically untenable.The "cross-over effect" (Babcock 1956). This describes how the difference in the widths of two circularly polarized Zeeman components of a spectral line becomes particularly large at the phase when the net field becomes small.
Meridional Circulation, Turbulence, and Diffusion Processes in Stars
Authors: Vauclair S. 
Published: July 2016

This paper gives the first results of a work in progress, in collaboration with G. Michaud and G. Vauclair. It is a first attempt to compute the effects of meridional circulation and turbulence on diffusion processes in stellar envelopes. Computations have been made for a 2 Mʘstar, which lies in the Am - δ Scuti region of the HR diagram.Let us recall that in Am stars diffusion cannot occur between the two outer convection zones, contrary to what was assumed by Watson (1970, 1971) and Smith (1971), since they are linked by overshooting (Latour, 1972; Toomre et al., 1975). But diffusion may occur at the bottom of the second convection zone. According to Vauclair et al. (1974), the second convection zone, due to He II ionization, disappears after a time equal to the helium diffusion time, and then diffusion may happen at the bottom of the first convection zone, so that the arguments by Watson and Smith are preserved.
Magnetic Accretion and Magnetosphere Physics of Peculiar A Stars
Authors: Havnes Ove. 
Published: July 2016

SummaryWe discuss the possibility for applying the magnetic accretion theory to other peculiar A stars than the Sr Cr Eu and Silicon stars. This requires a moderately strong magnetic field in the so-called “non-magnetic” peculiar stars. We find that magnetic fields with polar strengths of up to ˜ 0.1 Tesla (1000 gauss) may exist undetected if the errors of measurement are &gt; ± 0.025 Tesla. If such magnetic fields exist in these stars, overabundances of the order of ∆ log ˜ 2.0 may be produced in the course of 107years. The high overabundances found for many elements can probably not be produced by magnetic accretion. We suggest that the abundances of several elements (especially Ga are considerably overestimated.
Magnetic Fields: Observational Aspects
Authors: Wood H. John 
Published: July 2016

A relatively short, time was available for the preparation of this paper. A number of excellent reviews of Ap observations are already in the Literature, notably that by Preston (1971) on surface fields. However, a recent review of developments in techniques for the measurement of magnetic fields in Ap stars was lacking.Upon arrival at this symposium, we found so many exciting new results and programs underway (and therefore unpublished) that we decided to throw out our original paper and open this session up for reports from people who are making magnetic observations themselves, or who are familiar with active programs in their home countries.
Line Formation with Magnetic Field: Review and Practical Applications
Authors: Hardorp J. , Shore S.N. , Wittmann A. 
Published: July 2016

SummaryStarting with the full set of equations for the four Stokes parameters as developed by Beckers, we propose a simplified method for computing profiles of Zeeman-split absorption lines in LTE. This method is quick enough to be useful in abundance analyses of Ap stars, but more flexible than earlier approaches.
On the Inhomogeneity of Atmospheric Parameters of Magnetic Peculiar Stars
Authors: Kopylov I.M. , Kumajgorodskaya R.N. , Snezhko L.I. , Sokolov V.V. , Chunakova N.M. 
Published: July 2016

SummaryVariations of θeffand log geffover the surfaces of the magnetic stars α2CVn and HD 184905 are studied by comparison of the Hγ and Hδ hydrogen line contours measured on 15 Å/mm spectrograms with those calculated from Mihalas models. θeffvaries slightly during the period of rotation of these stars. At the same time log geffshows considerable variations over the surfaces of the stars with an amplitude of order of 0.7.
The Energy Distribution of Several Ap Stars
Authors: Hardorp Johannes 
Published: July 2016

SummaryEnergy distributions were observed in closely spaced 50 Å intervals from 3250 Å to 6000 Å and for some stars to 7950 Å. The broad absorption feature at 5200 Å appears in most Ap stars and persists through all phases of light-variation. No sharp absorption edges were found. Of special interest are the large-amplitude light variables HD 51418 and HD 125248. The main differences of their light curves are explained by different aspects of the oblique rotators.
Establishment of Terrestrial Reference Frames by New Observational Techniques
Authors: Bender Peter L. 
Published: July 2016

AbstractThe use of space techniques for establishing transcontinental and intercontinental distances is progressing very rapidly. We can think of the set of station locations used in either LAGEOS ranging or VLBI measurements as forming the vertices of a polyhedron. After correcting for tectonic plate motions using an adopted model, we expect the geometry of the polyhedron to be fairly stable over periods of the order of a year. However, after some period of time, a new set of station coordinates will be required because of improved data, unexpected station motions, etc. Methods for maintaining agreement with the previous set of station coordinates in some average sense are discussed in this paper. Some of the contributions expected from other new measurement methods also are described.
Origin and Scale of Coordinate Systems in Satellite Geodesy
Authors: Zieliński J. B. 
Published: July 2016

AbstractThe center of mass of the Earth is commonly taken as origin for the coordinate systems used in satellite geodesy. In this paper the notion of the “geocenter” is discussed from the point of view of mechanics and geophysics. It is shown that processes in and above the crust have practically no impact on the position of the geocenter. It is possible however that motions of the inner core may cause variations of the geocenter of the order of 1 m. Nevertheless the geocenter is the best point for the origin of a coordinate system. Mather’s method of monitoring geocenter motion is discussed, and some other possibilities are mentioned. Concerning the scale problem, the role of the constant GM and time measurements in satellite net determinations are briefly discussed.
Dynamics of an Artificial Satellite in an Earth-Fixed Reference Frame: Effects of Polar Motions
Authors: Farinella P. , Milani A. , Nobili A.M. , Sacerdote F. 
Published: July 2016

AbstractIn an Earth-fixed reference frame, polar motions (precession, lunisolar nutation, free nutation) introduce small apparent forces in the equations of motion of an Earth satellite. We discuss the possibilities (a) of integrating the orbit in an Earth-fixed frame when tracking data are used for geophysical applications, and (b) of determining from orbital data a set of unknown parameters describing the long-period wandering of the pole.
Relativistic Reduction of Astronomical Measurements and Reference Frames
Authors: Brumberg V. A. 
Published: July 2016

AbstractWith the accuracy of modern observations the relativistic treatment of the basic astronomical reference frames only requires the consideration of comparatively simple types of metrics such as heliocentric Schwarzschild metric, geocentric Schwarzschild metric and metric of the Earth-Sun system. Dynamical (related to the motion of the bodies) and kinematical (related to the light propagation) characteristics of these metrics enable one to perform the accurate relativistic reduction of astronomical measurements. In this reduction, the choice of specific quasi-Galilean coordinates may remain arbitrary. This paper presents expressions for the main relativistic terms in coordinates of the principal planets and Moon using the PPN formalism parameters β, γ and coordinate parameter α. General formulae for the reduction of radar, radio-interferometric and astrometric observations of planets and for the interpretation of lunar laser ranging are given. For estimating the actual magnitude of relativistic effects, the ephemeris data should be expressed in terms of physically measurable quantities.
Fast Time-Resolution Spectroscopy of BW Vulpeculae
Authors: Young Arthur , Furenlid Ingemar 
Published: July 2016

AbstractAn experimental Fairchild CCD-211 was placed in the 2.1-m coudé spectrograph at the Kitt Peak National Observatory and used to record an 87 Å spectral band centered on Hα. On the night of 1979 July 11 UT date, this system was used to observe continuously the extreme β Cephei variable BW Vulpeculae throughout one of its 4h 49m cycles with an average exposure of 13 minutes per observation. A total of 18 observations was secured. The results show dramatic profile variations of Hα, including features not previously reported, and extreme variations of the C II λλ6578, 6582 lines, including variations of equivalent width. The fast time-resolution capability and the photometric linearity of the CCD have permitted the detection of subtle effects that have been missed by photographic observations and has led directly to important new interpretations of the complex atmospheric pulsations in this star, including effects of altered opacity on the formation of spectral lines and the suggestion of a helium-ionization heat engine as a mechanism for driving atmospheric pulsations.
The Critical Frequency in the Stellar Pulsation Theory
Authors: Takeuti Mine 
Published: July 2016

AbstractThe standing oscillation in an isothermal semi-infinite plane-parallel atmosphere has a critical frequency. The significance of critical frequency in the stellar pulsation is discussed briefly. The critical period is calculated for the cepheid instability strip. A star having relatively large radius compared with the mass should be examined by the criterion. The pulsational instability is reduced by the linear running wave.
The Evidence for Planetary Progenitor Rotation and for the Long Term Acceleration of the Ejected Nebular Shell
Authors: Kirkpatrick Ronald C. 
Published: July 2016

Abstract:Several theories seek to explain the peculiar shapes of planetary nebulae. Those of Louise, Kirkpatrick, and Phillips and Reay rely on progenitor rotation. The velocity-radius relation for the shells of well observed planetaries do not extrapolate back through the origin, but rather fall short, suggesting that the shell acquires its velocity over a significant period of time. Kirkpatrick’s theory relies heavily on long term acceleration of the nebular shell, and other theoretical studies support the idea of acceleration of the nebular shell up to the time it becomes optically thin to the ionizing radiation from the central star.
Spherical Oscillation Patterns
Authors: Dean Pesnell W. , Coggins James M. 
Published: July 2016

AbstractSpherical harmonics are an integral part of the study of stellar pulsations. To gain an understanding of how the star is affected by non-radial pulsations, the radial part of the oscillation is plotted, with an assumed sinusoidal time variation. The amplitude of the motion is arbitrarily set at 20% of the radius.
The Core Helium Flash
Authors: Cole Peter W. , Deupree Robert G. 
Published: July 2016

AbstractThe role of convection in the core helium flash is simulated by two-dimensional eddies interacting with the thermonuclear runaway. These eddies are followed by the explicit solution of the 2D conservation laws with a 2D finite difference hydrodynamics code. Thus, no phenomenological theory of convection such as the local mixing length theory is required.
R Coronae Borealis Pulsations
Authors: King David S. 
Published: July 2016

AbstractThe Cepheid-like pulsations of some of the R CrB stars should in principle make it possible to determine their masses and hence to place constraints on possible evolution scenarios. We briefly review the evidence for these pulsations and discuss the problem of how these low-mass, hydrogen-deficient carbon stars could have evolved to their present position in the H-R diagram. Linear and nonlinear pulsation calculations are reviewed. It is found that for these large luminosity to mass ratio (L/M) stars a region of pulsation instability extends considerably hotter than for normal high luminosity Cepheids. The envelopes of these models are so nonadiabatic that the identification of modes becomes very difficult since there is frequently no clearly defined nodal structure. For the most extreme L/M cases it is found that the models are unstable in the sense that they appear on the verge of ejecting the outer layers.
The Magnetosphere of Uranus
Authors: Axford W.I. 
Published: July 2016

The magnetosphere of a planet, as the name implies, is the region surrounding the planet in which the planetary magnetic field plays a dominant role in determining the behaviour of the medium. The inner boundary of a magnetosphere is the surface of the planet if it has no significant atmosphere (as in the case of Mercury), or the lower ionosphere in the case of planets with atmospheres. The outer boundary, usually termed the “magnetopause”, is shaped by stresses exerted by the solar wind, being blunt on the upstream side, with normal stresses playing a dominant role, and extending in a comet-like “magnetotail” on the downstream side away from the Sun largely as a result of the action of shear stresses. The characteristic size of a magnetosphere, namely the distance Lm (in planetary radii) to the subsolar point is determined approximately by balancing solar wind ram pressure and the magnetic pressure:where n is the ion number density and Vs the speed of the solar wind, m is the average ion mass, Bo the (dipole) field strength at the surface of the planet and C is a constant of order unity.
The Rotation of Uranus
Authors: Goody Richard M. 
Published: July 2016

From the start of this century until the mid-1970’s the rotation rate of Uranus was reported to be 10.8 h in a retrograde sense (see, for example, Allen, 1955), but a cursory examination of the origin of this datum reveals that little confidence should be placed in it.Three independent techniques for measuring the rotation rate are available, each very difficult and not including the most direct method of observing the motion of features across the disc. Visual observers have reported features from time to time (see Alexander, 1965, for a full historical account), but the mean diameter of Uranus is only 3.6 arcsec and large high contrast features are rare in the visible spectrum, if they exist at all.
ASTRONET. The network for analysis and retrieval of astronomical data in Italy
Authors: Sedmak G. 
Published: July 2016

AbstractASTRONET, the network for analysis and retrieval of astronomical data planned in Italy, is described with regard to the organization of the network, the hardware, and the standard software environment. ASTRONET starts with four VAX centers operating within 1981, to be expanded to six within 1983.The network includes colour graphics and PDS plates digitization facilities. The software and graphics standards will be set as compatible as possible to major existing astronomical standards. ASTRONET is planned to be operative within 1983 as an open international astronomical
facility.