bluuebunny/crossref_metadata_2025_split huggingface dataset:
Hydrogen Emission Lines from Extended Pulsating Atmospheres
Authors: de Laverny P. , Magnan C. 
Published: June 2019

AbstractIn order to study the basic radiative mechanisms in the extended envelopes of evolved stars (e.g. Long Period Variables), we determine the spectrum emerging from a very optically thick Non-LTE hydrogen layer surrounding a core of high temperature (see figure 1). Such a model was first proposed by Sobolev (1960) and Menzel (1946). It is consistent with the fact that LPVs are likely evolving toward planetary nebulae. The main parameters of the model are the temperature of the illuminating star (Tr), the dilution factor (ω) and the Lyman continuum optical depth of the envelope (τ1c). We examine their influence upon the emerging intensities in the lines and in the continua.The model could explain some observations related to evolved stars and especially the presence of emission features in an otherwise cold environment. Furthermore atmospheric pulsation phenomena produce large variations in the emerging spectra: reddening, emission/absorption transition of the hydrogen lines, profile deformations…
Propagation of Radial Pulsation Modes in the Outer Atmosphere of Arcturus: First Results
Authors: Cuntz M. 
Published: June 2019

AbstractI present first results of a study about propagating radial pulsation modes in the outer atmosphere of Arcturus (K1.5 III). Mechanical energy input is explicitly taken into account by treating shock wave dissipation. I investigate the influence of different wave frequencies on the mass loss behavior of the star. I show that significant time-averaged mass loss can only be produced when periods larger than 5 x 105 s (∼ 1 week) are employed. The initial atmosphere I use extends from 1.2 R* up to 11.8 RR*. All wave models are adiabatic. I found that the mass loss rates and the final flow speeds obtained are extremely sensitive to the wave periods. In a certain regime the effect of increasing the period by a factor of 4 is to increase the corresponding mass loss rate by four orders of magnitude. I note that the mass loss rate and final flow speed of the wind for a 5.6 x 105 s period wave are somewhat close to the observed values. A more complete discussion regarding mass loss generation in Arcturus has been presented by Cuntz (1990). Recent observations of low-amplitude radial velocity variations in the photosphere of Arcturus provide evidence that the theoretically predicted mass loss frequencies might exist. Belmonte et al. (1990) presented evidence for a ∼8.3 d period with an amplitude of ∼50 m s-1, which they attributed to the fundamental radial pulsation mode. Further studies are in progress (Larson et al. 1992). I note that if the 8.3 d period is real than it would be sufficiently large to support continuous mass loss. Judge & Stencel (1991) argued that the mechanical energy in the observed disturbances could be ∼15 times greater than the energy required to drive the wind. For other stars than Arcturus the required minimum mass loss periods can be estimated as PML ∼ R*.
Evolutionary Models of RR Lyrae Stars
Authors: Lee Young-Wook 
Published: June 2019

As tracers of old stellar populations and as primary Population II standard candles, RR Lyrae stars have played an important role in the development of modern astronomy. Our knowledge of stellar evolution has identified these variable stars in a core helium burning phase of low-mass star evolution, the hoiizontal-branch (HB) phase. Consequently, not only to understand fully the nature of RR Lyrae stars, but also to apply them correctly as population probes and distance indicators, we must understand the underlying evolutionary effect of HB stars.In this paper, I briefly review the most important properties of RR Lyrae stars predicted from the HB evolutionary models, and present many pieces of supporting evidence for these models. For the implications of these models on the chronology of the Galactic formation and on the cosmological distance scale, the reader is referred to several recent publications by Lee (1992a,b,c,d).
The Cepheid Instability “Wedge“
Authors: Morgan Siobahn M. 
Published: June 2019

For the most part it had been assumed that the red and the blue edges of the Cepheid Instability region were parallel. However, previous work by Pel and Lub (1978) and recent work by Fernie (1990) seems to reveal a rather interesting structure to the shape of the Cepheid Instability region. Figure 1 shows the shape defined using the data from Fernie (1990) and the observational data of Gieren (1989). It is apparent that the edges defined by the distribution of these points are not parallel.I have calculated a series of pulsation models that included varying values of the mixing length to try and produce a distribution of Cepheids as seen in Figure 1. Calculations were done using the methods outlined by Castor (1971) with a Linear Non Adiabatic pulsation code to determine the characteristics for a given model.
Masses of c-type RR Lyrae Variables in Globular Clusters
Authors: Cacciari C. , Bruzzi A. 
Published: June 2019

The mass of RR Lyrae variables has been a controversial problem for about a decade: while the stellar evolution theory predicts masses ranging between 0.65 and 0.75 M⊙ for Oosterhoff type I and II clusters respectively, the stellar pulsation theory predicts smaller masses (0.55 and 0.65 respectively) using the double-mode pulsators. Simon (1990, M.N.R.A.S. 246, 70), comparing hydrodynamical models with observed stars by means of Fourier parameters, has found relations between the stellar mass and its luminosity, pulsation period, Helium content and Fourier parameter ϕ31. Combining his equations we obtain:(1)from which one can estimate the stellar mass by using the observable quantities P and ϕ31 and a luminosity scale, e.g. the one derived by Cacciari, Clementini and Fernley (1992, Astrophys. J. in press).
Cepheids in Magellanic Cloud Clusters
Authors: Welch D.L. , Mateo M. , Olszewski E.W. 
Published: June 2019

AbstractThe Magellanic Clouds remain an ideal place to study the properties of Cepheid variables. In this paper, we review historical and current work on Cepheids in LMC and SMC clusters, present new results for NGC 1866 and NGC 2164, and describe a new technique for automated selection of Cepheid variables using two-color photometry. We also emphasize the numerous advantages of high-precision radial velocities in the study of Magellanic Cloud variables.
First overtone pulsators among Cepheids
Authors: Mantegazza L. , Poretti E. 
Published: June 2019

The Fourier decomposition has been successfully applied to several classes of pulsating variables. Antonello and Poretti (1986) and Antonello et al. (1990a) applied it to the Cepheids with P < 8 d. The latter authors redefined the s – Cepheids a Population I Cepheids that do not follow the Hertzsprung progression, but have progression of their own. The same authors proposed a new denomination (Antonell et al., 1990b): C-a stars to indicate the Classical Cepheids and C-b stars to indicate the redefined s–Cepheids.The new photometric data obtained at La Silla and Merate Observatories (Mantegazza and Poretti, 1992) increase the evidence of a separation of Cepheids into two well defined subclasses on the basis of the Fourier parameters of their light curves.In the ϕ21 – P plane, the s– and Classical Cepheids are characterized by two sequences well separated for P<5.5 d.
Harmonics and Coupling-terms in the Pulsation of the Double-mode Cepheid TU Cas
Authors: Szabados L. 
Published: June 2019

AbstractPreliminary results of the period analysis of the double-mode Cepheid TU Cassiopeiae are given. Up to now 29 frequencies have been identified: the frequency of the fundamental mode pulsation and the first overtone, and their linear combinations. The amplitudes of the frequency constituents show temporal variations.
Spectroscopic Study of Cepheids in the Globular Cluster Omega Cen
Authors: Gonzalez Guillermo , Wallerstein George 
Published: June 2019

AbstractUntil now there have been few spectroscopic studies of cepheids in globular clusters. In this preliminary report we present sample spectra of the stars Vl and V29 in ω Cen. Eventually, we hope to use the abundance patterns, masses, and period changes of cepheids to better understand post-horizontal branch evolution in globular clusters.
Ultraviolet Studies of Cepheids
Authors: Böhm-Vitense Erika 
Published: June 2019

AbstractWe discuss whether with new evolutionary tracks we still have a problem fitting the Cepheids and their evolved companions on the appropriate evolutionary tracks. We find that with the Bertelli et al. tracks with convective overshoot by one pressure scale hight the problem is essentially removed though somewhat more mixing would give still better fits.Using results of recent nonlinear hydrodynamic calculations by Morgan we find that we also have no problem matching the observed pulsation periods of the Cepheids with those expected from their new evolutionary masses, provided that Cepheids with periods less than 9 days are overtone pulsators.We investigate possible mass loss of Cepheids from UV studies of the companion spectrum of S Mus and from the ultraviolet spectra of the long period Cepheid Carinae. For S Mus with a period of 9.6 days we derive an upper limit for the mass loss of M < 10-9 M⊙, if a standard velocity law is assumed for the wind. For Carinae with a period of 35.5 days we find a probable mass loss of M ∼ 10-5±2 M⊙.
The Stability of Cepheid Lightcurves
Authors: Fernie J.D. 
Published: June 2019

AbstractPublished lightcurves of 25 to 40 day classical Cepheids and of BL Herculis stars have been examined closely for evidence of alternating deep and shallow minima, as predicted by Moskalik and Buchler (1991) and Buchler and Moskalik (1992). No such effect has been detected. Two stars, SZ Mon and EN Tra, which are sometimes taken to be classical Cepheids, do show this RV Tauri-like effect, but neither star is, in fact, likely to be a classical Cepheid.
Strömgren Photoelectric Photometry of the Dwarf Cepheid Stars DY Peg and BP Peg
Authors: Peña J.H. , Peniche R. , Garrido R. 
Published: June 2019

AbstractA study of short period high amplitude Dwarf Cepheid stars has been undertaken in order to discriminate between the two possible models, namely Bessel's (1969) proposal of low mass Pop. II or old Pop. I. The telecsope utilized was the 1.5 m at the SPM Observatory. The Danish spectrophotometer that allows the simultaneous acquisition of data in the uvby filters and almost simultaneously in the Hβ narrow and wide niters was attached. With this advantage of simultaneous observations, no phasing adjustment was needed, eliminating the risks of losing information due to amplitude varition explained either by multiple periodicity or by the Blazhko effect. We have restricted our discussion to the descending branch and light minimum phase interval between 0.175 to 0.725. For this interval the unreddened indices (b – y)0 and (cl)0 were calculated as in Nissen (1988), which will serve to determine the effective gravity and temperature variation of both stars, through the model atmsphere calibrations by Breger (1974), based on the ATLAS and Kurucz model atmospheres. Mean log g values of 4.0 and 4.33, and mean temperatures of 7500 and 7700 K were determined for DY Her and BP Peg, respectively. It can be concluded that both pulsating stars DY Her and BP Peg are Pop.I, normal Dwarf Cepheid stars.
The Color Dependence of Interstellar Polarization
Authors: Behr Alfred 
Published: June 2019

Measurements of the Interstellar Polarization of Starlight at different wavelengths have been reported by several observers in references 1 to 8. With a few exceptions the wavelength range of the measurements is still relatively small, and up to now only a few dozens of stars have been measured with high precision. There is still a remarkable discrepancy between the results of different observers, as can be seen in figure 1. The scattering of the individual values is significantly larger than the probable error given by the respective authors. One of the most serious error sources seems to be the instrumental polarization which itself is highly color dependent. (See ref. 9.)Nevertheless, even if a much greater individual error is assumed, a remarkable conclusion can be drawn: the wavelength dependence of polarization is not the same for all stars.
Dispersion of Interstellar Polarization
Authors: Gehrels Thomas 
Published: June 2019

The Wavelength Dependence of Polarization as observed in 32 stars, for which the Henry Draper numbers are given, is shown in figure 1. Details of some of these observations are presented in reference 1.The equipment is now being used with the new 154-cm Catalina reflector of the Lunar and Planetary Laboratory at the University of Arizona. The instrumental polarizations are nearly zero. The data processing and observing techniques have been further improved; the precision is mainly determined by statistics such that the internal probable error in the percentage polarization is ±0.03 percent (±0.0006 magnitude) for a half-hour observation per filter on objects brighter than about 7 magnitudes. The wavelength λ ranges from 0.33 to 0.95 μ covered by seven filters of bandwidth of about 0.05 μ. The wavelength range is being extended to 1.2, 1.6, and 2.2 μ and, with high-altitude ballooning, to 0.28 and 0.22 μ.
Accreted Molecules
Authors: Gehrels Thomas 
Published: June 2019

In Some of the Plots of the Interstellar Polarization (for example, fig. 1 of first paper by Gehrels in the present compilation) a striking feature is noted in a discontinuity that frequently occurs near λ-1 = 1.3. The following explanation of the shape of the curve of interstellar polarization as a function of wavelength is proposed in terms of physical optics.Far in the infrared, light passes the particle with relatively little interaction. At shorter wavelengths (red light), scattering by the molecules at the particle skin increases and consequently the amount of polarization rises. The rise with increasing values of λ-1 is steep (although not necessarily with λ-4 for polarization).Prediction of the wavelength at which the rise becomes evident would necessitate a knowledge of the optical depth or scattering cross sections. These factors have not yet been evaluated. Qualitatively, the larger the particles, the larger the wavelength at which the polarization rise becomes manifest because of greater molecular optical depth near the skin.
Effects of Absorption Spectra of Ices on the Ultraviolet Extinction by Interstellar Grains
Authors: Field G. B. , Partridge R. B. , Sobel H. 
Published: June 2019

In Spite of Many Years of Research on Interstellar Grains, their composition is still unknown. Evidently it can be found only through the interaction of the grains with radiation, and one is led to the spectroscopic study of the solids composing the grains. If discrete lines, bands, or other spectral features can be discovered, they may be of great importance in the chemical analysis of the grains. So far, data of this type include the diffuse interstellar lines, the optical lines of CH, CH +, and CN (almost certainly once part of the grains), the radio lines of OH, and the distinct “kink” or curvature in the optical extinction curve (see paper by Nandy in present compilation), which may reflect spectral variations in refractive index and hence indicate a specific material.	The advent of ultraviolet and infrared techniques suggests that consideration be given to new features to be expected in those regions.
Ultraviolet Photochemistry of Some Frozen Gases
Authors: Thompson Warren E. 
Published: June 2019

Here Seems to be Agreement that laboratory studies are necessary in the pursuit of a better understanding of the interstellar medium. Herein is described some experiments performed on the effects of far ultraviolet radiation on molecules containing the astronomically abundant atoms H, O, C, and N. In particular, the attempts that have been made to prepare, isolate, and identify various free radicals and other reactive species, some of which may play a role in the processes occurring in the interstellar medium, are discussed.In one of the studies, a frozen film of pure carbon dioxide at 77° K was irradiated with the light from a low pressure xenon discharge lamp equipped with a LiF window. This lamp is activated by radio-frequency radiation, and the emitted light is rich in the 1470-Å resonance line of xenon.
The Astronomical Potential of Spatial Interferometry in the Infrared
Authors: Low Frank J 
Published: June 2019

AbstractThe angular diameters ad shapes of bright infrared stars are now being measured from 2 micron to 20 microns using several newly developed techniques of high angular resolution interferometry. These objects possess circumstellar shells emitting in the temperature range 300-600K, thus the brightest of their number have sizes ranging from 0 1 to 10 arcseconds, and are readily resolved by instruments a few meters across.The size, shape, and structure of several typesof stars will be discussed in detail as the Colloquium proceeds. It will be seen that this information can be used to help characterize the optical properties of the particulate material contained in the circumstellar envelopes. Conclusions concerning the formation and evolution of these cold outer regions of stars may also be possible It appears that the conditions in some of these circumstellar envelopes may be suitable for the formation of planets, a process which may be studied directly as these new techniques are further developed.Although sensitivities of present instruments are still somewhat marginal, it is now possible to resolve the brightest of the extragalactic nuclei at 10 microns Future instrumental developments should bring the size and structure of many extragalactic nuclei within reach at several wavelengths, thus adding important information on the nature of these extraordinary sources of infrared emission
Amplitude Interferometry at C.E.R.G.A.
Authors: Koechlin Laurent 
Published: June 2019

The 12 to 20 metre baseline interferometer at C.E.R.G.A. built by A. Labeyrie has now been operational for two years. The purpose of this instrument was initially to be a prototype showing the feasibility of long baseline coherent arrays and for testing the techniques needed for them. Yet, pending completion of large interferometers, this small 2 x 26 cm instrument has proven able to yield astrophysical data: a dozen stars up to third magnitude have been observed with a 3 x 10-3 arc second resolution on diameters and a 5 x 10-4 arc second accuracy for binary separation measurements in the case of α Aur.What has been observed both from the technological and the astrophysical point of view will be discussed.
Interferometric Measurements of Flattend Circumstellar Envelopes
Authors: McCarthy D. W. 
Published: June 2019

AbstractAn incoherent spatial interferometer has been used to measure the shapes of circumsteliar dust envelopes at infrared wavelengths. The objects IRC +10216, VY CMa, and NML Cyg exhibit a large variation of fringe visibility with position angle Their circumstellar envelopes are probably flattened disks. For IRC +10216, visibility measurements from 5.0 to 20µm are discussed in detail to illustrate the physical information available from interferometric work. In VY CMa and NML Cyg, disk orientation is strongly correlated with polarimetric position angles and with the distribution of OH maser sources. Interferometric measurements are shown to be relevant to studies of star/planet formation and to studies of mass loss in long-period variables
Status and Needs of Astrometry
Authors: Hughes James A. 
Published: June 2019

AbstractA review will be presented of the status of the fundamental reference system. Mention will be made of the problems limiting the accuracy of transit circles and photographic astrometry and attempts at possible improvments in the near future. The progress in the preparation of the FK5 will be briefly mentioned, as will be the various methods used to approximate an inertial reference frame. Needs for higher accuracy in Astronomy fall in the area of better modeling of the Earth's motion (rotation, nutation, precession, the ecliptic, etc.) as well as a parallax and proper motion determination for galactic structure studies, stellar dynamics and stellar evolution. It will be assumed that interferometric binary star studies will be discussed elsewhere.
Les Etoiles Doubles Faibles
Authors: Couteau P. 
Published: June 2019

La notion d'étoiles faibles concerne deux catégories d'objets: une étoile peut être de faible éclat parce qu'elle est loin, ou bien parce qu'elle est de faible luminosité. D'ailleurs ces deux facteurs se combinent entr'eux, selon les distances au soleil, de sorte que beaucoup d'étoiles géantes sont des étoiles faibles.Il est évident qu'une étoile double faible n'est intéressante que dans la mesure où son mouvement est appréciable, ce qui élimine les géantes. Notre rôle est done de dissocier parmi les étoiles faibles, celles qui le sont intrinsèquement.Avant d'entamer le vif du sujet, il est bon de rappeler que si nous étudions les étoiles doubles depuis trois siècles, ce n'est que depuis trente ans que nous avons réellement des connaissances sur les étoiles à faible luminosité. Mais faut-il encore définir ce que nous entendons par là.
Observational Procedures for Visual Double-Star Work
Authors: Franz Otto G. 
Published: June 2019

When IAU Symposium No. 17, the most recent international conference on visual double stars to take place in the United States, was held in Berkeley in 1961, there was little discussion on observational techniques and no introductory remarks were presented on the subject. The reasons for this are obvious. The classical techniques of double-star observation, those that provided virtually all the data available on visual double stars, were too well known to require introduction or discussions; other methods, with the possible exception of early image tube and electronic camera experiments, did not exist.Today, more than ten years later, most double-star work continues to be carried out by the same classical techniques. However, several methods have come into use or have recently been developed that are capable of contributing importantly to visual double-star research.
The N IV λ5820 Multiplet in WN Stars
Authors: Nussbaumer H. 
Published: June 2019

AbstractEvaluating transition probabilities of N IV 2p3d 3 P ° to 2s4s 3S and 2p3p 3P and an emission feature at λ7410 it is shown that a disputed emission at λ5810 in WN stars may not be attributed to N IV.
Line Formation in Moving Atmospheres
Authors: Kalkofen W. 
Published: June 2019

AbstractWe discuss an integral equation method that permits the calculation of the line source functions and of the emergent profiles in finite and semiinfinite atmospheres with macroscopic motion normal to the surface. Solutions are presented for a semiinfinite atmosphere with a temperature rise in the outward direction and with a flow that decays with increasing depth. The computed profiles have the form of P Cygni lines.
Coherent Line Formation with Depth-Dependent Parameters
Authors: Carson T. R. 
Published: June 2019

AbstractA discrete ordinate method is developed for solving the equation of coherent line formation, for arbitrary given variations with depth in an atmosphere of the temperature (or Planck function, assuming local thermodynamic equilibrium) and the line absorption and scattering coefficients. The direct solution thus obtained can then be used as the starting point of an iterative procedure. Results obtained for an exactly soluble case indicate the utility of the method.
Circumstellar CA II K Lines in G, K and M Giants and Supergiants
Authors: Vaughan A. H. , Skumanich A. 
Published: June 2019

AbstractTentative evidence, based on photoelectric observations of the Ca+ emission core, is presented for the existence of circumstellar envelopes in several G and K type “giants.” A significant emission asymmetry in α Tau may imply a chromospheric rather than a circumstellar source.