bluuebunny/crossref_metadata_2025_split huggingface dataset:
Analysis of 30 Wolf-Rayet Stars
Authors: Schmutz W. , Hamann W.-R. , Wessolowski U. 
Published: July 2016

Temperatures, mass-loss rates and luminosities of 30 galactic Wolf-Rayet stars (24 WN, 6 WC) are derived by fitting the observed equivalent widths of He I λ5876 and He II λ5412 and the absolute visual magnitude. A three-dimensional grid (T*-R*-) of model calculations provides the theoretical values.The results are summarized in Figure 1. The different symbols denote the spectral classification of the individual stars, whereas four groups are distinguished: WNEA, WNE-B, WNL and WC stars. The size of the symbols indicates the mass-loss rates. The uncertainties of the results are estimated to be 0.1 dex in T*, 0.4 dex in and 0.5 dex in L. For the ll stars in common with the sample of Abbott et al. (1986) we find our mass-loss rates to be compatible with their radio flux if the correct ionization equilibrium in the radio emitting region is applied (Schmutz and Hamann, 1986). The model calculations show that for all but the WN2 and WN3 stars helium recombines to He+ before the ions enter the radio-emitting region.
Winds from Red Giants
Authors: Fadeyev Yu. A. 
Published: July 2016

AbstractTwo principal mechanisms that may be responsible for mass loss from red giants are considered: shock wave-driven winds and radiatively (dust)-driven winds. Effect of the periodic shocks accompanying nonlinear oscillations of red giants is most prominent in the outer layers of the stellar atmosphere where shocks are able not only to expel gas but also increase gas density so that some molecular components become supersaturated. In O-rich stars the most abundant condensible species are silicon monoxide and iron, whereas in C-rich stars these are carbon, silicon carbide and iron.
The Mixing Length Ratio, Eddy Diffusivity and Acoustic Waves
Authors: Chan Kwing L. , Sofia Sabatino 
Published: July 2016

Many processes in the convection zone of a star affect the evolution and the atmospheric diagnostics. Here, a progress report is given on our numerical study of some convection related phenomena. The numerical results are obtained by solving the Navier Stokes equations in a three dimensional rectangular domain. The units are chosen such that the initial temperature, pressure, density, and the depth of the domain are all normalized to 1.The mixing length theory relates the convective (enthalpy) flux FC to the envelope structure quite well (Chan and Sofia 1987). For efficient convection that occurs in deep convective regions, the numerical results are compatible with a mixing length ratio of 2.1. The mixing length theory fails to address the significance of the flux of kinetic energy FKE (see Figure 1). FKE is negative and has a magnitude comparable to the total flux. These results are qualitatively similar to those of two dimensional computations (Hurlburt et. al. 1984).
Evolution Diagnostics in Interacting Binaries
Authors: Sahade Jorge 
Published: July 2016

Through the analysis and interpretation of observational material, particularly on the part of Otto Struve and his collaborators, the structure of an interacting binary is depicted (cf. Sahade and Wood 1978) as formed by a) the two stellar components; b) a gaseous stream from the less massive and more evolved component of the system towards the companion; c) a circumstellar gaseous envelope –designated as “ring” or “disk” depending on the density of the material; d) a circumbinary gaseous envelope that surrounds the whole system and is normally in expansion, as suggested by the conventional, ground–based observations.The evolution of interacting binaries involves the effect of matter outflow and transfer in at least a stage of rapid mass loss and a stage of slow mass loss. As a result, the evolution of the components appreciably departs from the evolution of single stars and produces very bizarre objects which find no counterparts among non-binaries. Both the observational and the computational results suggest that the amount of mass involved in the process of mass outflow must be a large percentage of the total mass of the evolving component. It seems, therefore, reasonable to expect to find evidence, in evolved systems, for processed material from the interior of the mass-losing component.
Concluding Remarks of the I.A.U. colloquium NO.108
Authors: Kodaira Keiichi 
Published: July 2016

The discussions during the colloquium clearly pointed to the recent rapid progress in applying the atmospheric models to the detailed diagnostics of stellar evolution.The high–quality spectroscopic data now available enable us to evaluate the abundances of scarce elements and isotopes in the classical stellar atmospheres. Many of them are found to be useful to sound the mixing and the gravity settling phenomena in the stellar interior. Extensive contributions were presented about the former process in the AGB stars, and about the latter process in the Li–defficient F stars.The evolutionary interactions between the stellar interior and the atmosphere become distinct when the mass-loss processes set in. It was pointed out that high–rate mass loss can influence the internal evolution and mixing on one side, and that on the other side the mixing (dredging–up) can change the abundances of heavy elements in the atmosphere and further modulate the mass–loss rate. Many speakers presented ample of observational data from UV to mm–wave range indicating occurences of substantial mass loss from various kinds of stars. Self consistent models with spherical geometry were proposed for stellar winds which are driven by the radiation pressure acting on ions or dust grains. This involves comprehensive calculation related to the molecular chemistry and the grain physics. So far as the flow is stationary and spherically symmetric, the escape velocity and the mass–loss rate determine the density structure of the wind in radiative equilibrium.
Mass loss from cool stars
Authors: Dupree A.K. 
Published: July 2016

AbstractRecently obtained spectroscopic observations indicating mass loss in cool stars are reviewed with analogies to the solar atmosphere. Spectral diagnostics of mass loss are discussed with new theoretical calculations of chromospheric line profiles. A general picture of mass loss from cool stars is developed and related to chromospheric and coronal emissions measured by IUE and the HEAO-2 Observatory. These winds range in characteristics from the hot (106 K) and fast wind with low mass loss found in the dwarf stars to the warm ( ~ 105 K), moderate speed winds present in hybrid luminous supergiants, and the coolest massive winds emerging from the latest type supergiants exhibiting lowest thermal velocities and circumstellar shells. Evidence for stellar surface inhomogeneity and variability of outflow is briefly discussed.
Can hot star winds be driven by radiation pressure?
Authors: Leroy Marc , Lafon Jean-Pierre J. 
Published: July 2016

AbstractThe mechanisms by which early type (0 and B) stars loose mass is investigated. The phenomena occuring close to the photosphere are crucial for the structure of the wind. It is shown that the effects of radiation are not sufficient to explain the observed winds. A general scheme for energy balance including a corona is proposed.
Evolution of a 30 Mʘ star: the interplay of nuclear burning and mass loss
Authors: Falk H.J. , Mitalas R. 
Published: July 2016

AbstractEvolutionary tracks for a 30 Mʘ star with various mass loss rates (MLR) were evolved to core He exhaustion. The “overluminosity” of mass losing (ML) stars is explained in terms of the well known mass-luminosity (M-L) law. A critical ZAMS MLR above which mass loss leads to evolution to fainter luminosities is derived. Two tracks showed reversals in their direction of evolution across the HR diagram. These have been shown to be a consequence of mass loss dominating over the effects of the shell source. An analytic criterion for this condition has been derived.
On the significance of mass loss for the evolution of massive stars
Authors: Yungelson L.R. , Massevitch A.G. , Tutukov A.V. 
Published: July 2016

It is shown that mass loss by stellar wind with rates observed in O, B-stars cannot change qualitatively their evolution in the core hydrogen-burning stage. The effects, that are usually attributed to the mass loss, can be explained by other causes: e.g., duplicity or enlarged chemically homogeneous stellar cores.The significance of mass loss by stellar wind for the evolution of massive stars was studied extensively by numerous authors (see e.g. Chiosi et al. (1979) and references therein). However, the problem is unclear as yet. There does not exist any satisfactory theory of mass loss by stars. Therefore one is usually forced to assume that mass loss rate depends on some input parameters.
Masses of Magellanic Wolf-Rayet stars: mass loss and evidence for a WR subclass vs. mass relation
Authors: Moffat A.F.J. 
Published: July 2016

AbstractNew spectroscopic observations in the LMC/SMC combined with published data on Galactic WR stars lead to a correlation between mass ratio, MWR/MO, and WR subclass. As a consequence of this and their high mass loss rates, WR stars probably evolve from cool to hot ionization class in a way which depends on the metallicity.
Effect of mass gain on stellar evolution
Authors: Ebert R. , Zinnecker H. 
Published: July 2016

AbstractIn this paper we present a fully hydrodynamical treatment of the stationary isothermal accretion problem onto a moving gravitating point mass. The derivation is purely analytical. We find that the accretion rate is more than a factor of 50 higher than the accretion rate derived from the partially non-hydrodynamical treatment by Hoyle and Lyttleton (1939) or Bondi and Hoyle (1944). This result may have some bearing on the evolutionary tracks of young pre-Main Sequence stars still embedded in their parent protocluster cloud. We discuss the work by Federova (1979) who investigated the pre-Main Sequence evolution of degenerate low mass ‘stars’ with strong accretion of protocluster cloud material. We suggest that the stars which lie below the Main Sequence in young clusters could strongly accrete matter at the pre-Main Sequence stage.
The influence of mass loss on the evolution of binaries
Authors: de Loore C. 
Published: July 2016

Mass loss can affect the evolution of binaries in various ways, during different stages of the evolution.1. For massive stars stellar wind mass loss will change the masses of the components during their main sequence evolution.2. During the Roche lobe overflow phase (or tidal interaction phase) matter can leave the system.3. For low mass stars matter can leave the system during the mass exchange phase and can be stored in envelopes, disks or rings.4. Sufficiently massive stars(>8-15M0) undergo at the end of their life a supernova explosion, where most of the matter is blown away and a compact object, a neutron star or a black hole can be left.5. For intermediate stars one of the components can evolve into a degenerate He or CO dwarf; a reverse mass transfer can dump matter on this degenerate dwarf. If the conditions are favorable the white dwarf can explode with loss of matter, and a neutron star can be the result.6. The chemical abundances in the outer layers change.
Ionization effects in stellar winds of massive X-ray binaries
Authors: Hammerschlag-Hensberge G. 
Published: July 2016

AbstractHigh resolution ultraviolet spectroscopy of the early-type X-ray binary Vela X-l/HD 77581 provides a unique example to study the ionization structure of the stellar wind which varies under influence of the X-ray source. New results for other early-type X-ray binaries will be presented and compared with the Vela X-1 model. These results show that the observed variations strongly depend on the intensity of the X-ray source and on the density structure of the stellar wind.
The nature of V861 Sco (=HD 152667)
Authors: Howarth Ian D. , Wilson R. 
Published: July 2016

AbstractThe physical parameters of the components of V861 Sco are derived from light-curve analysis and published spectroscopy. Good agreement with evolutionary models is obtained. The stellar wind is investigated using IUE data; the results include no large phase dependence of the mass loss rate and insensitivity of the velocity of the wind (measured with respect to interstellar lines) to changes in the photospheric velocity, even near the base of the wind. However, small random changes in velocity near the base of the wind are amplified to larger changes further out.
Solar Oscillations Observed in the Total Irradiance
Authors: Woodard M. , Hudson H. 
Published: July 2016

AbstractThe total solar irradiance measurements obtained by the active-cavity radiometer on board the Solar Maximum Mission have been analyzed for evidence of global oscillations. We find that the most energetic low-degree p-mode oscillations in the five-minute band have amplitudes of a few parts per million of the total irradiance, and we positively detect modes with l = 0, 1, and 2. The distribution in l differs from that of the velocity spectrum, with relatively more power at lower l values. The individual modes have narrow line widths, corresponding to values of Q greater than a few thousand, or lifetimes of at least a week. We do not detect the 160-min oscillation in the power spectrum, and place an upper limit of 5 parts per million (99.9% confidence) on its amplitude.
Recent Observations of High-Degree Solar p-Mode Oscillations at the Kitt Peak National Observatory
Authors: Rhodes Edward J. , Harvey John W. , Duvall Thomas L. 
Published: July 2016

AbstractA brief summary is given of a program which is currently being carried out with the McMath telescope of the Kitt Peak National Observatory in order to study high-degree (l ≳ 150) solar p-mode oscillations. This program uses a 244 x 248 pixel CID camera and the main spectrograph of the McMath telescope to obtain velocity-time maps of the oscillations which can be converted into two-dimensional (kh – ω) power spectra of the oscillations. Several different regions of the solar spectrum have been used in order to study the oscillations at different elevations in the solar atmosphere. The program concentrates on eastward- and westward-propagating sectoral harmonic waves so that measurements can be made of the absolute rotational velocities of the solar photospheric and shallow sub-photospheric layers. Some preliminary results from this program are now available. First, we have been unable to confirm the existence of a radial gradient in the equatorial rotational velocity as was previously suggested. Second, we have indeed been able to confirm the presence of p-mode waves in the solar chromosphere as was first suggested by Rhodes et al. (1977). Third, we have been able to demonstrate differences in photospheric and chromospheric power spectra.
Kolmogorov Unstable Stellar Oscillations
Authors: Perdang J. 
Published: July 2016

AbstractWe survey the mathematics of non-linear Hamiltonian oscillations with emphasis being laid on the more recently discovered Kolmogorov instability. In the context of radial adiabatic oscillations of stars this formalism predicts a Kolmogorov instability even at low oscillation energies, provided that sufficiently high linear asymptotic modes have been excited.Numerical analysis confirms the occurrence of this instability. It is found to show up already among the lowest order modes, although high surface amplitudes are then required (ǀδrǀ/R ~ 0.5 for an unstable fundamental mode – first harmonic coupling). On the basis of numerical evidence we conjecture that in the Kolmogorov unstable regime the enhanced coupling due to internal resonance effects leads to an equipartition of energy over all interacting degrees of freedom. We also indicate that the power spectrum of such oscillations is expected to display two components: A very broad band of overlapping pseudo-linear frequency peaks spread out over the asymptotic range, and a strictly non-linear 1/f-noise type component close to the frequency origin.It is finally argued that the Kolmogorov instability is likely to occur among non-linearly coupled non-radial stellar modes at a surface amplitude much lower than in the radial case. This lends support to the view that this instability might be operative among the solar oscillations.
A Convenient Method to Obtain Stellar Eigenfrequencies
Authors: Knölker M. , Stix M. 
Published: July 2016

AbstractThe differential equations describing stellar oscillations are transformed into an algebraic eigenvalue problem. Frequencies of adiabatic oscillations are obtained as the eigenvalues of a banded real symmetric matrix. We employ the Cowling-approximation, i.e. neglect the Eulerian perturbation of the gravitational potential, and, in order to preserve selfadjointness, require that the Eulerian pressure perturbation vanishes at the outer boundary. For a solar model, comparison of first results with results obtained from a Henyey method shows that the matrix method is convenient, accurate, and fast.
On Waves in Non-Isothermal, Compressible, Ionized and Viscous Atmospheres
Authors: Campos L. M. B. C. 
Published: July 2016

AbstractA review is given of the properties of waves in atmospheres, with particular emphasis on (Section 1) the variation of amplitude and phase with altitude for propagating waves (Figures 1 to 4) and the waveforms of standing modes (Figure 5). The cases dealt with concern waves under the combined influences of gravity and compressibility, and examine the effects of: (Section 2) temperature gradients in a non-isothermal atmospheric model; (Section 3) external magnetic field, either vertical or horizontal; (Section 4) dissipation by viscosity and electrical resistance. The results are relevant to (Section 5) the assessment of atmospheric wave growth and shock formation, and to the calculation of heating functions describing the deposition of wave energy.
Seismology of Sunspot Atmospheres
Authors: Žugžda Y.D. , Locāns V. , Staude J. 
Published: July 2016

AbstractThe present work deals with the theory of oscillations with periods of about 3 min observed in the chromosphere above sunspot umbrae. The model of these oscillations (slow mode magneto-acoustic waves trapped in a chromospheric resonant cavity) provides an independent method of checking empirical models of the chromosphere above sunspots.Making use of this method, we investigate sunspot models which have been derived from spectroscopic data; the calculated periods of the oscillations fit well the observed periods.
Nonlinear Anelastic Modal Theory for Solar Convection
Authors: Latour Jean , Toomre Juri , Zahn Jean-Paul 
Published: July 2016

AbstractPreliminary solar envelope models have been computed using the single-mode anelastic equations as a description of turbulent convection. This approach provides estimates for the variation with depth of the largest convective cellular flows, akin to giant cells, with horizontal sizes comparable to the total depth of the convection zone. These modal nonlinear treatments are capable of describing compressible motions occurring over many density scale heights. Single-mode anelastic solutions have been constructed for a solar envelope whose mean stratification is nearly adiabatic over most of its vertical extent because of the enthalpy (or convective) flux explicitly carried by the big cell; a sub-grid scale representation of turbulent heat transport is incorporated into the treatment near the surface. The single-mode equations admit two solutions for the same horizontal wavelength, and these are distinguished by the sense of the vertical velocity at the center of the three-dimensional cell. It is striking that the upward directed flows experience large pressure effects when they penetrate into regions where the vertical scale height has become small compared to their horizontal scale. The fluctuating pressure can modify the density fluctuations so that the sense of the buoyancy force is changed, with buoyancy braking actually achieved near the top of the convection zone. The pressure and buoyancy work in the shallow but unstable H+ and He+ ionization regions can serve to decelerate the vertical motions and deflect them laterally, leading to strong horizontal shearing motions. It appears that such dynamical processes may explain why the amplitudes of flows related to the largest scales of convection are so feeble in the solar atmosphere.
160 m Pulsations in the Magnetosphere of the Earth Possibly Caused by Oscillations of the Sun
Authors: Vladimirsky B.M. , Bobova V.P. , Bondarenko N.M. , Veretennikova V.K. 
Published: July 2016

AbstractThe measurements of the amplitudes envelope of Pc 3–4 geomagnetic micropulsations obtained at the Borok Geophysical Observatory were analysed by the cosinor method to search for magnetospheric pulsations with a period of about 160 m. 216 days of observations in 1974–1978 were used. It was found that Pc3–4 amplitudes are modulated by the period 160.010 m with a stable phase. The maximum of the Pc3–4 amplitudes follows approximately 20 m after the maximum of the solar expansion velocity (for the center of the disk) in the optical observations of Severny et al. This modulation of the Pc3–4 amplitudes could be caused by the presence of an oscillating component in solar UV radiation over the wavelength range 100-900 Å. The amplitude of the UV flux variation may be as large as 2–4%.
The Quiescent Chromospheres and Transition Regions of Active Dwarf Stars: What Are We Learning from Recent Observations and Models?
Authors: Linsky Jeffrey L. 
Published: July 2016

ABSTRACTI will review the rapid progress in our understanding of active dwarf stars, which has been stimulated by recent IUE, Einstein, and ground-based observations, by asking a series of questions. The most fundamental question is the extent to which magnetic fields control nonflare phenomena in these stars. There are a number of aspects to this question:(1) What is the evidence for large scale magnetic structures similar to solar plages in these stars and how does a plage system differ from a quiescent spectrum?(2) Can the enhanced heating in these stars be explained by solar-like magnetic flux tubes?(3) What roles do systematic flows play in active dwarf atmospheres?(4) What Is the relation between heating rates in different layers of these stars?(5) By what mechanisms are active dwarf chromospheres and transition regions heated?(6) What are semiempirical models telling us about active dwarf stars?Recent observations are permitting us to begin to answer these questions.
Two-Spot Modeling of Synoptic Light Curves of II PEG
Authors: Fauci G. La , Rodonò M. 
Published: July 2016

The light curve (LC) of the single-line spectroscopic binary system II Peg (K2-3, IV-III) generally shows an asymmetrical and highly variable shape (Fig.l, left). Therefore, single circular spot models can be successfully applied only to symmetric LCs, as the 1977 one by Vogt (1981). When asymmetrical or almost flat LCs develop, as appears to be the rule for II Peg, at leasts two-spot modeling is required (Bopp and Noah 1980, Dorren and Guinan 1982).From classical single-spot models (Torres and Ferraz Mello 1973, Friedman and Gurtler 1975, Bopp and Evans 1973) we have developed a computer code including two separate circular spots of different size which are allowed to assume any relative location on the stellar surface. The model is symmetric with respect to the equator. The spots are assumed to radiate as back bodies with temperature ranging from 100 to 2000 degree lower than the temperature of the unspotted photosphere (4500°K).