bluuebunny/crossref_metadata_2025 huggingface dataset:
Model of Dusty Envelope of ETA Carinae
Authors: Men’shchikov A.B. , Shustov B.M. , Tutukov A.V.
Published: July 2016

Eta Carinae is a well-known example of a star with a massive circumstellar nebulous shell. The shell is regarded as a remnant of a great outburst of the star in the last century. η Car parameters remain to be a matter of scientific debates: most often investigators revise stellar temperature T, mass loss rate , the velocity of outflow V. We have worked out several numerical evolutionary models of η Car envelope (the homunculus) paying attention to the dependence on the mentioned parameters. The dependence on the parameters of grain evolution was also considered. For a representative model we fixed: V=500km/s; M=10−1 M⊙/y during the first 15 years of the outburst and slowly decreasing in time, T=20000K. A coupled two-components hydrodynamical and radiation model transfer problem has been solved.
Turbulence-Driven Atmospheric Instability and Large-Scale Motions in Super- and Hypergiants
Authors: Boer B. , Carpay J. , de Koter A. , de Jager C. , Nieuwenhuijzen H. , Piters A. , Spaan F.
Published: July 2016

Spectral studies of super- and hypergiants show that the (outward directed) turbulent acceleration approaches the value of the gravitational acceleration for the most luminous stars, which makes their atmospheres unstable.
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

Two 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.
Discovery of a Reflection Dust Envelope Around IRC+10216
Authors: Tamura M. , Hasegawa T. , Ukita N. , Gatley I. , McLean I. S. , Burton M. G. , Rayner J. T. , McCaughrean M. J.
Published: July 2016

Infared polarimetric and photometric mapping observations at K(2.2 μm) and H(1.65 μm) have revealed an extended dust envelope around the late-type star IRC+10216. The observations were made on the 3.8-m United Kingdom Infrared Telescope on Mauna Kea, Hawaii, in 1985 December and 1987 January and February. The polarization observations were made by emplying the Kyoto polarimeter (Sato et al. 1987). Great care was taken to check the contamination by stray light in the telescope and instruments as the source on peak was extremely bright (K~0 mag). From the observations of normal stars, we found that the polarized intensity (degree of polarization times the intensity) was a good measure of the envelope, free from contamination by stray light, although the intensity and the degree of polarization suffered from the contamination separately.
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.
Empirical Determination of the Gravity-Darkening Exponent for the Secondary Components Filling the Roche Lobe in Semi-Detached Close Binary Systems
Authors: Kitamura Masatoshi , Nakamura Yasuhisa
Published: July 2016

The ordinary semi-detached close binary system consists of a main-sequence primary and subgiant (or giant) secondary component where the latter fills the Roche lobe. From a quantitative analysis of the observed ellipticity effect, Kitamura and Nakamura (1986) have deduced empirical values of the exponent of gravity-darkening for distorted main-sequence stars in detached systems and found that the empirical values of the exponent for these stars with early-type spectra are close to the unity, indicating that the subsurface layers of early-main sequence stars in close binaries are actually in radiative equilibrium. The exponent of gravity-darkening can be defined by H ∝ gα with H as the bolonetric surface brightness and g as the local gravity on the stellar surface.
Spectroscopic and Photometric Observations of SN1987a Obtained at SAAO
Authors: Catchpole R.M.
Published: July 2016

The dedication of at least 24 local and visiting observers has enabled the SAAO to accumulate a large body of spectroscopic and photometric observations of SN1987a. Broad band photometry has been obtained on every possible photometric night while spectra at 7A(FWHM), including the first spectrum in the world, were obtained every night for the first 14 nights and thereafter on a weekly basis. These data, which have been more fully presented elsewhere (Menzies et al. 1987, Catchpole et al. 1987) are briefly discussed here for the time interval 24 February until 31 August 1987. Throughout this paper we adopt a distance modulus of 18.5 and a reddening of Av=0.6 for SN1987a.
SN 1987a: Theoretical Considerations
Authors: Woosley S. E.
Published: July 2016

There is now general agreement that the presupernova star for 1987a was indeed SK-69-202, a star that had visual magnitude 12.36 (Walborn et al. 1987), bolometric correction 1.15m (Humphreys and McElroy 1984), and extinction 0.5m (Woosley et al. 1987). The distance modulus to the LMC is presently controversial, but most astronomers prefer a value near 18.5 (e.g., Walker 1985; Chiosi and Pigatto 1986) with an uncertainty of perhaps 0.2m. Putting it together, one arrives at a bolometric magnitude for SK-69-202 of −7.8 with a probable range −7.5 to −8.2, or a luminosity of 4 × 1038 erg s−1 with a range of 3 to 6 × 1038 erg s−1. At the time of the supernova the hydrogen burning shell contributes negligible energy generation so the critical quantity determining the luminosity is the helium core mass. Inspection of a variety of current stellar models indicates that SK-69-202 had a helium core mass of 6 ± 1 M⊙ from which we may infer that, on the main sequence, it had a mass of 19 ± 3 M⊙. A B3-I supergiant has a surface temperature of ∼16,000 K (Humphreys and McElroy 1984). For the above range of luminosities and assuming a temperature in the range 15,000 to 18,000 K one obtains a radius for SK-202-69 of 3 ± 1 × 1012 cm.
Quasi-Static and Steady-State Pictures for Collapsing Core of Type II Supernova
Authors: Sugimoto D. , Sasaki A. , Ebisuzaki T.
Published: July 2016

Explosion of type II supernova is, in principle, a difficult process: The presupernova star was in gravitationally bound state with negative energy but it has to be divided into two parts, the collapsed core of still lower (negative) energy and the ejected envelope of positive energy. This process is against nature in the sense most of the phenomena in nature proceed towards equipartition of energies. Thus, some finely tuned mechanism should be necessary for successful explosion. Two different mechanisms have been proposed; one is the prompt explosion where the gravitational energy release by the core collapse is transferred to the mantle by a shock wave, and the other is delayed explosion where it is transferred slowly by neutrinos diffusing out of the core. In what follows we shall concentrate in the case of the delayed explosion.
Neutrino Emission Processes in the Weinberg-Salam Theory
Authors: Itoh Naoki
Published: July 2016

The neutrino emission processes play essential roles in stellar evolution as expemplified by the observations of the neutrinos from SN 1987a by the KAMIOKANDE-II and IMB experiments. Recently a very extensive study of the various neutrino emission processes based on the Weinberg-Salam theory has been completed by the present author and his collaborators. The neutrino emission processes calculated by the author’s group include pair, photo-, plasma, and bremsstrahlung neutrino processes. The neutrino energy loss rates due to pair, photo-, and plasma processes in the framework of the Weinberg-Salam theory are found to be substantially lower than the result obtained by Beaudet, Petrosian, and Salpeter. The reduction factor α is in the range 0.35 < α < 0.88 depending on the neutrino masses, density, and temperature. The ionic correlation effects play important roles in the bremsstrahlung neutrino process. The present author and his collaborators recently calculated the bremsstrahlung neutrino energy loss rate taking into account the ionic correlation effects in the crystalline lattice state as well as in the liquid metal state. They found that the ionic correlation effects suppress the bremsstrhlung neutrino energy loss typically by a factor 2-20. The present findings will bear great importance in neutrino astronomy.
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

Recently 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.
The theory of winds in early type stars
Authors: Hearn A.G.
Published: July 2016

I assume that the purpose of this review of the theory of winds from early type stars is to summarize the way in which the mass loss rate of a star may be included in a calculation of stellar evolution. Let me summarize my conclusions. It is not possible. One can only use estimates of mass loss rates obtained from the observations. Even these give a large uncertainty. The observed mass loss rates for different stars of the same spectral type vary. Further the mass loss rates obtained by different methods for the same star differ. An extreme example of this is 9 Sgr. The mass loss rate derived from the radio observations is forty times greater than that derived from the U.V. and optical measurements (Abbott et al. 1980).
Possible links between supersonic stellar winds and the origin of cosmic rays
Authors: Montmerle T. , Cassé M. , Paul J.
Published: July 2016

Cosmic rays (CR) are fast or relativistic protons and electrons (and nuclei), detected on the top of the terrestrial atmosphere. Except at the highest energies (>1017eV) the Larmor radii of the particles in the galactic magnetic field are much smaller than the scale height of the interstellar gas, and the arriving particles have completely lost memory of their sources. Therefore, their origin is unknown, at least directly. But recent advances in γ-ray astronomy, and in shock acceleration mechanisms, have shed a radically new light on a old problem.
Empirical wind models from detail ed UV-line fits: Tau Scorpii
Authors: Hamann W.-R.
Published: July 2016

The main-sequence BO star Tau Sco has been studied previously by Lamers and Rogerson (1978). However, their line fit was restricted to the blue wings of the UV resonance lines because the line formation was calculated in Sobolev approximation. We now repeat this investigation by means of the comoving-frame (CMF) method, which we have extended to the treatment of overlapping doublets. From a systematic comparison (Hamann 1980b) we know that the results may deviate considerably from those of the Sobolev method. Our method has been applied previously to the other “prototype” of mass loosing early-type stars, the Of star Zeta Puppis (Hamann 1980a).
Can hot star winds be driven by radiation pressure?
Authors: Leroy Marc , Lafon Jean-Pierre J.
Published: July 2016

The 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

Evolutionary 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

New 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.
The hydrogen/helium ratio on the surface of Wolf-Rayet stars
Authors: Conti Peter S. , Massey Philip
Published: July 2016

As yet there have been few detailed analyses of Wolf-Rayet (WR) emission line spectra. Such analyses must certainly take account of line transfer in a spherically extended, expanding medium, a problem not yet completely addressed in the complexity of WR envelopes. However, visual examination of optical spectra suggests real differences in line strengths among the Balmer and Pickering series lines in WN stars. If hydrogen is present, the Balmer lines will be stronger than the adjacent Pickering lines of ionized helium; if hydrogen is weak or absent, then there will be a smooth transition in intensities along the Pickering series with no additional contribution from the Balmer lines at alternatively blending wavelengths (Underhill 1967).
Theoretical evidence of mass loss from globular cluster stars
Authors: Castellani V. , Tornambè A.
Published: July 2016

The total amount of mass lost during the first giant phase of Population II red giants is derived from Horizontal Branch properties: it is found to be proportional to the metal content.
Effect of mass gain on stellar evolution
Authors: Ebert R. , Zinnecker H.
Published: July 2016

In 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

High 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

The 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.
Concluding Remarks
Authors: Dallaporta N.
Published: July 2016

Since I am here, I have been asking myself why on earth have I been chosen by the Organizing Committee to this final and quite uneasy job to formulate some concluding remarks to the enormous complexity of this conference. And the only reason I can find, is that I am the least competent person to do so among all the people gathered in this room, as I have never directly put my hands, nor very deeply speculated, on any of the items which have been here discussed. I feel however somewhat encouraged by the belief that the secret wisdom which has dictated my choice was precisely to select an outsider whose function should essentially consist to look at things from a certain distance, with no a priori opinions related to so many unsettled questions. And therefore I shall mostly try to keep myself in such a role asking in advance for forgiveness for all the shortcomings to which my lack of specific differentiated knowledge may have induced me to commit.
Solar Oscillations Observed in the Total Irradiance
Authors: Woodard M. , Hudson H.
Published: July 2016

The 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.
Helioseismology with High Degree p-Modes
Authors: Deubner F.-L.
Published: July 2016

The value of p-modes of high degree l as a diagnostic for the structure and dynamics of the solar envelope is reviewed.
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

A 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.
Short Period Oscillations
Authors: Deubner F.-L. , Laufer J.
Published: July 2016

Short period oscillations (with periods less than 150 s) are shown to be non-uniformly distributed on the solar surface, and in time. Rather, they appear concentrated in short bursts which preferentially occur in regions with strong instantaneous downflow motion throughout the observed extent of the solar atmosphere.
Kolmogorov Unstable Stellar Oscillations
Authors: Perdang J.
Published: July 2016

We 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

The 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

A 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

The 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

Preliminary 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

The 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

I 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).
Observations of Hβ and HE II λ 4686 Lines in the Spectra of Flares of UV Cet-Type Stars
Authors: Chugainov P.F. , Petrov P.P. , Shcherbakov A.G.
Published: July 2016

We have carried out 45.4 hours of continuous spectroscopic and photoelectric B-band observations of AD Leo, DT Vir and YZ CMi. The spectroscopic resolution was about 1 A in wavelength and 5-10 min in time. 6 flares were recorded. Light curves and line profiles are to be presented in the Izv.Crim. Astrophys. Obs., Vol.69. In this paper only the main results are discussed.
The Timescales of Variations in Continuum and Hydrogen Lines During Stellar Flares
Authors: Pettersen B.R.
Published: July 2016

Light curves of major stellar flares have been used to study the behavior of U-B, B-V, and V-R. The majority of the flux transmitted through these filters is continuum radiation, but U and B are affected by emission lines. The variability of Ha and H$ emission lines were monitored through narrow band filters. The timescales of emission line variability are considerably longer than those for the continuum, and the emission line flare peak occurs a few minutes after the continuum flare maximum. No variability in lines at a timescale of seconds is detected in our data.
Flares on the Sun: Selected Results from HINOTORI
Authors: Tanaka Katsuo
Published: July 2016

The X-ray observations of solar flares by Japanese astronomy satellite Hinotori (ASTRO-A) are reviewed. Detailed results from the X-ray telescope and the soft X-ray spectrometers are given. The hard X-ray images in the 17-40 keV range show a wide variety of source structures. Examples of compact single source, stationary coronal source, and double sources are presented. High resolution spectra in the range 1.7-2.0A indicate strong turbulence and blue-shifted components in the beginning of flares. From the FeXXV and FeXXVI spectra two kinds of thermal plasma are shown to appear. The cooler component of Te=15-20xl06K, which increases in the impulsive phase of the hard X-ray burst, is suggested to be produced by the dissipation of the electron beams. The hotter component of Te=30-50xl06K and Ne=1011-1012 increases towards the flare maximum. This component is also responsible for emitting 17-40keV continuum, and emitted from a very compact source. The results are discussed in comparison with flare loop models. It is shown that evolutionary changes in the loop density realize various source structures in the hard X-ray under the unitary electron beam hypothesis.
Doppler Imaging of Starspots
Authors: Vogt Steven S. , Penrod G. Donald
Published: July 2016

We discuss a newly-developed technique for spatially resolving starspots on some of the more rapidly rotating RS CVn stars. Basically, the method uses high resolution, very high signal-to-noise spectral line profiles and exploits the Doppler velocity correspondence between position across the stellar disk and wavelength position across a rotationally broadened line profile to synthesize an image of the star, showing the location, sizes, and shapes of its starspots. Though still in a developmental stage, the technique is already yielding information about the structure and general appearance of starspots. Examples of Doppler Imaging observations of HR 1099 will be presented, along with a movie showing the behavior of synthetic line profiles generated from a computer spot model.
Features of the Wave-Like Distortion in Some Rs CVn Binaries
Authors: Blanco C. , Bodo G. , Catalano S. , Cellino A. , Marilli E. , Pazzani V. , Rodonò M. , Scaltriti F.
Published: July 2016

One of the outstanding features of RS CVn-type binaries is the changing shape of the so called “wave-like distortion” or “photometric wave” (PW) and its phase migration on the light curve (LC). Here we present a preliminary analysis of the observations of the following objects: CQ Aur, RU Cnc, VV Mon and SZ Psc. For each system we have derived the amplitude and phase of the PW maximum and its migration rate.
Contact Binary Stars
Authors: Dupree A.K.
Published: July 2016

Ultraviolet and X-ray surveys of the W Ursae Majoris type stars are reviewed. These systems exhibit extended coronas and transition regions that are confined close to the optically determined surfaces. Correlations of X-ray activity with period or rotational velocity indicate a turn-over or saturation of emission at the short periods or high velocities found in the W UMa-type systems. For a number of systems, ultraviolet emission appears to be anti-correlated with the strength of X-ray emission. These observations are discussed in terms of solar structures, activity, and evolution.
Rotation-Activity Connections in Main Sequence Binaries
Authors: Vilhu O. , Rucinski S.M.
Published: July 2016

We have studied transition-region (NV + SilV + CIV) and chromospheric (MgII) emission observed with IUE (Vilhu and Rucinski, 1982; Rucinski and Vilhu, 1982; Ayres et al., 1981) together with coronal (soft X-ray) emission observed with the Einstein satellite (Cruddace and Dupree, 1982; Walter, 1982) for contact (W UMa) and detached main sequence binaries. The components are main sequence stars (or near ms) and rapid rotators due to spin-orbit coupling. They can thus be expected to give information of dynamo-processes in rapid rotators, although the binary effects clearly produce extra complications in this discussion.
Periodic Light Variability in Four Late-Type Pre-Main-Sequence Stars
Authors: Vrba F.J. , Rydgren A.E. , Schmelz J.T.
Published: July 2016

While the T Tauri stars are the best known of the late-type pre-main-sequence (PMS) stars, there are also some late-type PMS stars with only weak line emission in their visible spectra. Several years ago we noted that the weak-emission PMS stars have B-V colors too blue for their V-I colors and suggested that their surfaces might have regions of differing temperature. During October 1981 we used the USNO 40-inch and Kitt Peak National Observatory No. 4 16-inch telescopes to monitor, over a 7 night interval with UBVRI photometry, four of these weak-emission PMS stars: HD 283447, V410 Tau, and X-ray stars 1 and 2 of Feigelson and Kriss (1981). The PMS nature of these stars is established from (1) their membership in the Taurus dark cloud T-association and (2) their location within the T Tauri band region of the H-R diagram.
Chromospheric Properties of T Tauri Stars Determined from EUV Spectra
Authors: Brown A. , Jordan C.
Published: July 2016

IUE spectra of six pre-main-sequence (PMS3 stars are analysed and the resultant emission measure distributions compared with that of T Tau for which a chromospheric model has been calculated. The general shape and absolute level of the mean emission measure distributions are remarkably similar, indicating the relevance of the T Tau chromospheric model to other PMS stars. Evidence for the influence of large scale motions and/or stellar winds on the transition region and coronal emission measures is found. The relative importance of different energy balance terms is discussed.
Quasi-Periodic Variations of Balmer Line Profiles in Spectra of the T Tau-Type Star RW Aurigae
Authors: Grinin V.P. , Petrov P.P. , Shakhovskaya N.I.
Published: July 2016

RW Aur is one of the most active T Tau-type stars with essentially variable emission line spectrum. In order to study the nature of this variability,patrol spectroscopic observations of the star were carried outfrom November 1979 to March 1982 at the 2.6-meter telescope of the Crimean Astrophysical Observatory. The spectral region λλ38OO-67OO A has been observed with a dispersion of about 40 A/mm. All nights were supplied with simultaneous spectrophotometry of the star.
The Interaction of Fast Particles with Frozen Gases in T Tau Nebulae: The Physical Background
Authors: Pirronello V. , Strazzulla G. , Foti G.
Published: July 2016

The interaction of energetic particles with frozen gas layers plays a relevant role in a variety of astrophysical scenarii and also in T Tau nebulae because of the large fluxes of particles ejected by these stars during their continuous flaring activity and the observed presence of ice grains at least around the very young star KL Tau (Cohen, 1975).
Photospheric Sources of Magnetic Field Aligned Currents
Authors: Nordlund Ake
Published: July 2016

Possible (small-scale) photospheric sources of coronal magnetic field aligned currents are discussed. Such currents are equivalent to local (small-scale) twists of the coronal magnetic field, and may cause field topologies that are (MHD or resistively) unstable, and thus contribute to the (small-scale) coronal activity.

One electro-motive force associated with photospheric magnetic fields is due to the asymmetry between ions and electrons as agents of conductivity and as agents of momentum transfer to the (dominant) neutral gas component. In the solar photosphere, where only some of the easily ionized heavier elements (Mg, Si, Fe, ...) are significantly ionized, the ratio of number density of electrons and ions to neutrals is very small, of the order of the total abundance of these elements, which is of the order 10-4, by number. Due to the larger electron than ion mobility, electric currents are mainly carried by the electron component of the plasma whereas, because of the ions greater momentum transfer to the neutrals in collisions, the friction between the charged and the neutral components of the plasma is mainly due to the ions. Thus, the Lorenz force i x B acts mainly on the electron component of the plasma, whereas the balancing gas pressure gradient and gravity terms act mainly on the neutral component.
Turbulent Sound Generation in Red Dwarf Stars
Authors: Bonn H.U.
Published: July 2016

Since the acoustic heating theory (c.f. Ulmschneider 1979) has been proven successful for the solar chromosphere, it was common practice to extend this concept to other stars. However, as it appeares from observed chromospheric and coronal emissions, the usual theoretical acoustic fluxes for red dwarf star, particularly, are too small to account for the heating of chromospheres and coronae (e.g. Blanco et al 1974; Vaiana et al, 198l) . It is therefore the intention of this paper to discuss improvements on the current model calculations for turbulent sound generation from outer convection zones.
Flares on Red Dwarf Stars as a Result of the Dynamical Response of the Chromosphere to the Heating
Authors: Katsova M.M. , Livshits M.A.
Published: July 2016

The impulsive hard X-ray, EUV and microwave radio bursts always start at or after the onset of a soft X-ray increase - the main phase of a solar flare - before the Hα flare maximum. This hard phase of a solar flare lasts ~ 100 s and consists of several elementary bursts of 10 s duration. In this time electrons are accelerated up to energy 20-100 keV, then they are injected vertically downwards into denser chromospheric layers. The electron heating as well as the thermal conductivity heating leads to the appearance of the secondary process, in particular, to gasdynamical motions (Kostyuk and Pikel'ner, 1940.
Analysis of a Simple Mechanism to Deplete the Kirkwood Gaps
Authors: Lemaitre Anne
Published: July 2016

In most applications resonance problems are implicitely or explicitely modelized by the Fundamental Model of Resonance i.e. the pendulum, characterized by its Hamiltonian function:
Asymmetric Periodic Orbits in the Three-Body Problem and Their Stability
Authors: Tsouroplis A. , Zagouras C.G.
Published: July 2016

An algorithm for the numerical determination of asymmetric periodic solutions of the planar general three body problem is described. The elements of the “variational” matrix which are used in this algorithm are computed by numerical integration of the corresponding “variational” equations. These elements are also used in the study of the linear isoenergetic stability. A number of asymmetric periodic orbits are presented and their stability parameters are given.