bluuebunny/crossref_metadata_2025_split huggingface dataset:
Generation of Strong Toroidal Magnetic Field Near the Bottom of the Solar Convective Zone
Authors: Krivodubskij V.N. 
Published: July 2016

AbstractThe generation mechanism of the toroidal magnetic field by the angular velocity radial gradient acting on the relict poloidal magnetic field on the boundary between the con-vective and radiative zones is proposed. The magnetic induction magnitude of the toroidal field reaches about 2×10σG, the limiting effect of the magnetic buoyancy being taking into account. This value conforms to the estimation of toroidal field obtained from helioseismological data.
Temperature, Abundance and Magnetic Mapping of Stellar Atmospheres
Authors: Donati J.-F. 
Published: July 2016

AbstractIn this paper, I review recent progress in the domain of indirect imaging of stellar atmospheres. Since all stars other than the Sun are too far away for us to disclose any feature on their surface directly, one has to use an indirect tool to image their atmosphere. This method (Doppler Imaging) takes advantage of a star’s rotation to identify and characterise inhomogeneities on its surface. This is done by studyingbothhow inhomogeneities (carried in and out of the observer’s view by rotation) modulate photometric and spectroscopic data withtime, and how surface features distort theshapeof line profiles at a given rotational phase. In the last decade, Doppler Imaging has first been applied to abundance mapping of magnetic Ap stars, then extended to temperature and magnetic mapping of cool active stars. Allowing a solar-like way of studying the surface of stars, this technique provides a wealth of information about stellar atmospheres and, in particular, about the physical processes which dictate their dynamics and energy balance.
Atmospheric Structure of the Pulsating Ap [CP2] Star HR 3831 from Rapid Multicolour Photometry
Authors: Matthews Jaymie M. , Wehlau William H. , Rice John , Walker Gordon A. H. 
Published: July 2016

Rapidly oscillating Ap (roAp) stars are cool magnetic CP2 stars which vary with periods of a few minutes and amplitudes less than 0.01 mag and 1 km/s in light and velocity. Analysis of their p-mode eigenfrequency patterns and splittings gives information about evolutionary state, rotation rate, magnetic field geometry and internal field strength (see Kurtz 1990; Matthews 1991).
We present here an example of how roAp pulsations can be used to obtain an estimate of the temperature structure of an Ap atmosphere.The pulsation amplitudes of roAp stars decline more rapidly with increasing wavelength than other known pulsators. Matthews et al. (1990) explained this by the wavelength dependence of limb darkening and its weighting effect on the integrated amplitude of an (l, m) = (1,0) mode.
Particle Transport Processes
Authors: Michaud Georges , Proffitt C.R. 
Published: July 2016

AbstractThe effect of gravitational settling and radiation driven diffusion on the evolution of stars near the main sequence is reviewed. New simplified formulae for calculating diffusion are proposed that improve on previous such formulae. The reliability of available diffusion coefficients is discussed and areas where further work is needed are identified. Newly available opacity calculations are used to estimate the effects of radiative acceleration on Fe.The size of the modifications to the evolution are shown to be modest: a reduction of order 10% on the evolutionary age of globular clusters. There are indications from the Li abundance in the high Teff halo stars that some turbulence is present below the convection zone. Models calibrated using solar properties reproduce the Teff at which the Li gap is observed in F stars as well as its depth without any arbitrary parameter. Similarly the presence of AmFm stars is explained over the Teff range where they are observed.
Line Broadening Data: Stark Broadening of Ca II Sc III And Ti IV Lines
Authors: Dimitrijević Milan S. , Sahal-Bréchot Sylvie 
Published: July 2016

AbstractElectron– and proton–impact line widths and shifts for important Ca II, Sc III and Ti IV lines, have been calculated using the semiclassical–perturbation formalism. The obtained results were used to investigate the behaviour of Stark broadening parameters within the K I isoelectronic sequence.
Integral Constraints on Convective Overshooting Two-Dimensional Numerical Studies
Authors: Roxburgh Ian W. , Simmons John 
Published: July 2016

AbstractThe integral constraint on convective overshooting (Roxburgh 1976, 1978, 1989) is re-derived for for statistically stationary two-dimensional convection between stress-free horizontal boundaries maintained at constant temperatures, the vertical boundaries being periodic. We report on numerical simulations of such two-dimensional compressible convection in a fluid where the central regions are convectively unstable and the surrounding layers are stable. The results support the conclusion by Roxburgh (1989) that the contribution from the radiative flux is well approximated by the flux carried by the mean temperature gradient, but demonstrate that viscous dissipation is important, reducing the extent of overshooting.
Stellar Distances and Hipparcos
Authors: Gómez A.E. 
Published: July 2016

AbstractHipparcos, the astrometric satellite of the European Space Agency, launched in August 1989, will provide absolute trigonometric parallaxes for about 100 000 stars brighter than the limiting V-magnitude 12.5. In spite of the difficulties caused by a faulty orbit, the mission is successfully going on. A lifetime of more than 4 years is expected which will allow to reach the original objectives of the mission: 2 milli-arcsec for stars brighter than V-magnitude 9 and 4 to 5 milli-arcsec for fainter stars. The observing programme is based upon a single, uniquely-defined Input Catalogue. Field and open cluster stars of almost all effective temperatures, chemical compositions and gravities are being observed. Most types of binaries and variable stars are well represented, but Hipparcos is discovering new ones. Significant parallaxes (relatives errors smaller than 20%) will be obtained for stars closer than 100 pc (about 36 000 stars). Reliable stellar structure models should fit all the possible observational constraints: the main contribution of Hipparcos parallax data will be the huge improvement in the individual luminosities. The determination of stellar masses from the study of visual binaries will also be improved. The preliminary results of the first-year data reduction are an encouraging indication of the expected quality of the final results. On the basis of a mission duration of 4 years, the final results are foreseen for 1995.
New Evolutionary Tracks of Pre-Main-Sequence Stars
Authors: Palla Francesco , Stahler Steven W. , Parigi Giacomo 
Published: July 2016

AbstractWe present the results of new evolutionary calculations of the pre—main-sequence phase of stars in the mass range 1 to 5 Mʙ. The novelty of our approach is that, unlike in the standard theory of PMS evolution, the initial conditions of the gravitational contraction phase are not arbitrarily set, but derive from protostellar theory.
Radial Pulsation in Variable Stars with Mass Loss
Authors: Pijpers Frank P. 
Published: July 2016

AbstractIt is possible to show that cool giants with very large photospheric scale heights do not perfectly reflect pulsational waves at the photosphere. This means that for these stars the classical formulation of pulsation in which the outer boundary for the resonance cavity is assumed to be perfectly reflecting is not valid. This can have significant consequences for the eigenfrequencies of the pulsation of Long Period Variables such as Mira type variables as well as for the stability of their pulsation.
Observational Constraints on Dynamos in Cool Stars
Authors: Schrijver C.J. 
Published: July 2016

AbstractNon-radiatively heated outer atmospheres are common among stars on the cool side of the HR diagram. These atmospheres resemble that of the Sun in which the bulk of the heating is associated with magnetic fields. Consequently it is assumed that other cool stars also generate and maintain an internal magnetic field through a dynamo action. This field gives rise to a wide spectrum of phenomena in the stellar atmosphere. The dynamo process that sustains the field thrives on the interaction of rotation and turbulent convection. In this review I formulate a set of propositions outlining our present knowledge of this interaction as distilled from stellar observations. In doing so, I outline the effects of stellar evolution on rotation rate, discuss some of the proposed “dividing lines” in the HR-diagram, and comment on the possible role of the convective turnover time in dynamo efficiency and on dynamo activity for extremely slowly and rapidly rotating stars.
The Musicos Project: Multi-Site Continuous Spectroscopy
Authors: Catala Claude , Baudrand Jacques , Böhm Torsten , Foing Bernard H. 
Published: July 2016

Many scientific programs, most of them linked to stellar physics (such as asteroseismology, stellar rotational modulation, surface structures, Doppler imaging, Zeeman-Doppler imaging, variable stellar winds) require a continuous spectroscopic coverage during several days.MUSICOS (for MUlti-SIte COntinuous Spectroscopy) is an international project for setting up a network of high resolution spectrometers coupled to telescopes of the 2m class, well distributed around the world, and partly dedicated to continuous spectroscopy.The strategy to reach this objective was defined during two workshops organized at Paris-Meudon Observatory in 1988 and 1990, and consists of three steps: 1) organize multi-site spectroscopie campaigns using resident instruments on various telescopes around the world and transportable fiber-fed spectrographs where adequate spectroscopie equipment is not available; 2) design and develop a cross-dispersed echelle spectrograph, well suited for the scientific programs that require multi-site observations; 3) propose this MUSICOS spectrograph for duplication at several collaborating sites.
Wavelet Analysis of Some Pulsating Stars
Authors: Szatmáry Károly , Gál János 
Published: July 2016

The detection of a variable period is very important as it gives information on the evolutionary state or on the binary nature of a pulsating star. If a pulsator is moving in a binary system, its light curve is frequency-modulated by the orbital period (light-time effect). Mode switching or chaos also may be the cause of the changes in the amplitude and period of the light variations.The determination of period variability is difficult. Usually the conventional Fourier-spectrum cannot give any information about a possible period variation. The 0-C diagram is useful to detect a variable period, but it does not give the amplitude and phase values.
Spectroscopic Observations of The Be Star PHI-PER
Authors: Singh Mahendra , Mon Masahiro , Kogure Tomokazu , Suzuki Masakazu 
Published: July 2016

AbstractBased on a series of high dispersion Coude spectra taken with 188-cm telescope of Okayama Astrophysical Observatory over an extensive period of time (nearly 20 years), spectroscopic behaviours were measured and discussed. Synchronous variations of V/R (intensity ratio of violet to red emission peaksjvariability with the radial velocity curves (RV) for Balmer and Hel 3888 shell absorption lines were confirmed. Using the peak separations of the Balmer emission lines, envelope extension was also estimated. Main difficulty was found in explaining the envelope extension of the primary star of the system which overflows the Roche-lobe limit when we consider the binary nature of the system.
The VLA and VLBA at Millimeter Wavelengths
Authors: Fomalont Edward B. 
Published: July 2016

AbstractAlthough the Very Large Array (VLA) and the Very Large Baseline Array (VLBA) were originally intended as centimeter wavelength instruments, the exciting results from high resolution millimeter radio astronomy over the last ten years have generated interest in pushing these arrays into the millimeter region. This report will describe two aspects of recent development at NRAO: the new capability of the VLA at 7mm wavelength which will be operational in 1994, and the completion of the VLBA with its anticipated use at 7mm and 3.6mm.
Lunar Occultations of the SiO Masers in RLeo
Authors: Cernicharo J. , Brunswig W. , Paubert G. , Liechti S. 
Published: July 2016

VLBI observations show that the SiO maser emitting regions in oxygen-rich stars are very clumpy and that these clumps extend over a few stellar radii (Mclntosh et al. 1987; Colomer et al. 1992). These observations indicate that the ideal instrument for the study of the SiO masers is an interferometer with baselines covering between a few and several hundreds/thousands km. Such an instrument is so far unavailable.A classical way to get high angular resolution and a full beam synthesis with a single telescope of moderate size is through lunar occultations. This observing technique provides the angular resolution of a single linear antenna several kilometers long. However, at millimeter wavelengths the Fresnel fringes produced by the Moon limb as the source under study is occulted have never been observed. We present here the observation with the 30-m IRAM radio telescope of the v=1 J=2-1 line of SiO during an occultation (and reappearance) of R Leo by the Moon.
CO (J=2-1) Observations of the Molecular Cloud Complex in the Galactic Center
Authors: Oka Tomoharu , Hasegawa Tetsuo , Hayashi Masahiko , Handa Toshihiro , Sakamoto Sei'ichi 
Published: July 2016

AbstractWe report a large scale mapping observation of the Galactic center region in the CO (J=2-1) line using the Tokyo-NRO 60cm survey telescope. Distribution of the CO (J=2-1) emission in the I-V plane suggests that molecular clouds forms a huge complex (Nuclear Molecular cloud Complex, NMC). Tracers of star formation activities in the last 106-108 years show that star formation has occured in a ring ~ 100 pc in radius. Relative to this Star Forming Ring, the molecular gas is distributed mainly on the positive longitude side. This may indicate that much of the gas in NMC is in transient orbit to fall into the star forming ring or to the nucleus in the near future.
Evidence for Protostellar Wind-Cloud Interactions
Authors: Rudolph Alexander L. 
Published: July 2016

AbstractRecent molecular line observations of HH flows (HH objects and optical jets) have revealed abundant evidence of interactions between protostellar winds and the surrounding molecular clouds. HCO+ observations taken with the BIMA Array have traced out the distribution of both stationary and high-velocity gas near the protostellar objects HH 7-11, L1551, and HH 34.
Warm and Cold Gas in Galactic Nuclei: The Near-IR/Millimeter Connection in NGC 253
Authors: Van Der Werf Paul P. , Cameron M. , Genzel R. , Blietz M. , Krabbe A. , Forbes D. A. , Ward M. J. 
Published: July 2016

AbstractWe discuss the use of seeing-limited near-IR spectroscopic imaging combined with high resolution millimeter and submillimeter wave observations, as a diagnostic in the study of the nuclear interstellar medium in starburst galaxies and active galactic nuclei. As an example, recent near-IR spectroscopic imaging of the starburst galaxy NGC 253 is analyzed. It is shown that the central ~ 100 pc of NGC 253 contains a number of giant star forming complexes, the stellar content of which is at least as large as that of the 30 Dor region in the LMC. We suggest the use of the [FeII]/Brγ ratio as an approximate age indicator for such complexes. The warm component of the nuclear molecular medium in NGC 253 detected in submillimeter CO spectra and in near-IR rovibrational lines of H2 is probably heated by stellar UV radiation or slow shocks in star forming regions, rather than by supernova remnant shocks. There are indications that molecular material is being removed from the nuclear region by the “superwind” observed in optical emission lines.
Self-Gravitating Gas Dynamics in a Nuclear Region of a Weak Barred Galaxy
Authors: Wada K. , Habe A. 
Published: July 2016

Starburst regions are frequently located in galactic central regions and CO observations indicate that these regions contain a large amount of molecular gas (e.g. Lo et al. 1987). However, the triggering mechanism for starbursts and the mechanism of the high mass supply rate of gas into a galactic center are still unclear.It is suggested that tidal encounters of galaxies remove angular momentum of gas and trigger rapid gas accretion and starburst. Noguchi(1988) has shown by computer simulations that galaxy-galaxy interactions induce a stellar bar, and gas loses its angular momentum and accumulates to a galactic center. In his numerical simulation, non-axisymmetric potential of a stellar bar plays an important role in the accretion of gas. However, it is not obvious whether or not gas accretes into a nuclear region within a few hundred pc only by the effects of stellar bar.Wada and Habe (1992) investigated the dynamics of self-gravitating gas in a barred potential by 2-D numerical simulation, and show that even if the bar is weak, for the initial gas mass ratio to stellar mass grater than 10%, a central elongated gas ring formed at near ILRs becomes unstable and collapses. As a result, a large amount of gas can be supplied to galactic center. They conclude that both the effect of the self-gravity of the gas and the existence of ILRs are necessary to the rapid gas fueling in a weak barred potential.
1.7” Resolution CO(1-0) Observations of ARP220: Nuclear Gas Ring of Merger Remnant
Authors: Okumura S. K. , Kawabe R. , Ishiguro M. , Ishizuki S. 
Published: July 2016

AbstractWe made aperture synthesis CO(l-O) observations of the central region of Arp220 with the Nobeyama Millimeter Array. Central CO emission was resolved with a size of 975 kpc. It shows a ring-like structure (ɪ ~ 500 pc) with a large velocity gradient, 393 km · s−1 · kpc−1, from southwest to northeast direction. The ring-like emission is located around double radio compact sources. No emission peak was found in the center of the double sources within the velocity range 5100 km s−1to 5800 km s−1. These results suggest that an inclined massive gas ring has been or is being formed in the central 1 kpc of Arp220. Most of the molecular gas in Arp220 is concentrated on this nuclear ring. The radio compact sources are probably located at the inner egde of the ring.
An Abundance Analysis of the Silicon CP Star HD 43819
Authors: Lopez-Garcia Zulema , Adelman Saul J. 
Published: July 2016

AbstractA fine analysis performed using four 4.3 Å/mm IIaO Mt. Wilson Observatory spectrograms shows that HD 43819 has abundance anomalies similar to those of other Silicon stars, namely it is helium, carbon, magnesium, and sulfur poor while being silicon, titanium, chromium, manganese, iron, strontium, zirconium, and rare earth rich.
MK Process and the Am Stars
Authors: Sreedhar Rao S. , Abhyankar K. D. 
Published: July 2016

AbstractThe MK morphology of the spectra in the region λλ 3850-4400 Å of 28 Am stars has brought to light the tendency of these stars to form into groups in contrast to earlier findings by various authors (S. Sreedhar Rao & K. D. Abhyankar, 1991). The potential of such a study is seen in the revelation of the inherent flexibility of the stars in this sample, apparently leading to a selective binning of the species with distinctly similar signatures, viz., Ca ll K and Sr ll 4077 features, the blue and violet regions and the Am-shell features. These are found to be the essential discriminants on the basis of which a multi-dimensional classification of Am spectra in the domain of the MK process is attempted.
Magnetic Field Diagnosis Through Spectropolarimetry)
Authors: Mathys Gautier 
Published: July 2016

AbstractAn original approach, the moment technique, is applied to analyze the shapes of spectral lines of Ap stars recorded in both circular polarizations. The longitudinal magnetic field, the asymmetry of the longitudinal magnetic field, and the quadratic field of the studied stars are derived. From the consideration of these quantities and of their variations through the stellar rotation cycle, constraints on the spatially unresolved structure of the magnetic fields are obtained.
Do Bp Stars Have “Flares”?
Authors: Vető Balázs 
Published: July 2016

AbstractUBV(RI)c photoelectric observations of four variable magnetic Bp stars (HD 21699, HD 177003, HD 184927 and HD 217833) were carried out between 1989 and 1992. Random fluctuation of 0m.1 range was detected in the U colour band in the case of HD 177003 and HD 184927. The U light curve of HD 21699 shows 0m.02 peaks superposed on the periodic light variation which definitely exceeds the typical error of the observations. It is not likely that atmospheric extinction or observational error would cause the phenomena described. On the other hand there are no physical processes known which lead to such changes of the U band continuum radiation in Bp stars.
Rotational Velocities for the Brighter A-Type Stars
Authors: Abt Helmut A. , Morrell Nidia I. 
Published: July 2016

AbstractIn 1968 van den Heuvel showed that the rotational velocities at many A types have peaks below V sin i = 45 km s−1 and above 90 km s−1. We explored this bimodal behavior by obtaining new rotational velocities of 1761 A-type stars with coudé CCD spectra and, to date, new MK classifications for 956 of those. We find that the slow rotators consist of Ap, Am, and short-period binaries while the rapid rotators are normal and λ Bootis stars. The overlap is only 8–10% and can probably be attributed to undetected abnormal stars and binaries.The λ Bootis stars discovered purely by their having weak λ4481 Mg II constitute about 18% of the early A-type stars but 4% at the late ones. Statistically all such λ Bootis stars have V > 100 km s−1 but they constitute only a small fraction of those stars.More than half of the stars classified as A2 IV (and some at Al IV and A3 IV) seem to be a hitherto unrecognized class of slowly-rotating abnormal-abundance stars of luminosities like those of class V stars.
Chemical Separation vs Rotation in a and F-Stars
Authors: Charbonneau Paul 
Published: July 2016

AbstractThe role played by rotationally-induced mixing in the diffusion-based models for non-magnetic chemically peculiar stars is investigated. This paper focuses on one specific rotationally controlled mixing mechanism, namely thermally-driven meridional circulation. Its effects on the time evolution of chemical abundances are illustrated by means of three specific examples. The first two concern the diffusion model for FmAm stars, where it is shown that while circulation has a determining influence on the settling of Helium, it has no significant effect on the diffusion of heavier metals once the He superficial convection zone has disappeared. The third example is concerned with the diffusion/mass loss model for λBootis stars. It is shown that the inclusion of circulation prevents the appearance of generalized underabundances at any epoch of the evolution, indicating that the diffusion/mass loss model for these objects must be abandoned.
Models for Vega
Authors: Castelli Fiorella , Kurucz Robert L. 
Published: July 2016

AbstractBy using the ATLAS9 code which requires the new opacity data from Kurucz (1992), we derived a new atmospheric model for Vega with parameters Teff=9500 K, log g=3.95, [M/H]=−0.5, ξ=2 kms s−1. The model was fixed by comparing observed and computed energy distributions and Balmer profiles.
The Magnetospheres of the Helium Peculiar Stars of the Upper Main Sequence
Authors: Shore Steven N. 
Published: July 2016

AbstractA brief review is presented of the evidence for magnetospheres surrounding the upper main sequence helium peculiar stars. Some of the underlying physical processes are discussed. The helium strong stars all appear to possess comparatively dense magnetospheres. The phenomenon is also present amongsomehelium weak stars, but is quite rare.