bluuebunny/crossref_metadata_2025_split huggingface dataset:
NLTE Model Atmospheres for Extremely Hot Compact Stars
Authors: Rauch T. , Deetjen J. L. , Dreizler S. , Werner K. 
Published: July 2016

Present observational techniques provide stellar spectra with high resolution at a high signal-to-noise ratio over the complete wavelength range—from the far infrared to X-rays.The effects of Non-“Local Thermal Equilibrium” (NLTE) are particularly important for hot stars, hence the use of reliable NLTE stellar model atmosphere fluxes is required for an adequate spectral analysis.State-of-the-art NLTE model atmospheres include metal-line blanketing of millions of lines of all elements from hydrogen up to the iron-group elements, and thus permit precise analyses of extremely hot compact stars, e.g., central stars of planetary nebulae, PG 1159 stars, white dwarfs, and neutron stars. Their careful spectroscopic study is of great interest in several branches of modern astrophysics, e.g., stellar and galactic evolution, and interstellar matter.
Lithium in the Old Open Cluster NGC 2243
Authors: Hill V. , Pasquini L. 
Published: July 2016

We report observations of lithium in a sample of 11 stars in the metal-poor open cluster NGC 2243, that were obtained from high-resolution spectroscopy at CASPEC (ESO 3.6m telescope). The targets are located at the turnoff region, plus one red giant star.NGC 2243 is one of the most metal-poor open cluster, almost as deficient as 47 Tuc, but substantially younger (∼4 Gyrs and [Fe/H]=-0.5 dex), which makes it a very interesting case to compare with more metal rich coeval clusters on the one hand, and old metal-rich globular clusters (47 Tuc) on the other hand. The preliminary Lithium abundances obtained are discussed in this framework.
Constraints on Stellar Hydrodynamics from Abundance Anomalies of LiBeB and Metals
Authors: Michaud Georges , Richer Jacques , Richard Olivier 
Published: July 2016

The availability of large atomic data bases has made it possible to calculate stellar evolution models taking into detailed account the atomic diffusion of all important contributors to opacity. The radiative accelerations and the opacity are continuously calculated during evolution taking the abundance changes of 28 species into account. This leads to the first self-consistent stellar evolution models for A and F stars. In A and F stars an iron-peak convection zone appears.The calculated abundance anomalies are very similar to those observed in AmFm stars in open clusters except that they are larger by a factor of about 3. To reduce the calculated anomalies to the observed ones, an additional source of turbulence (or some other hydrodynamical process) must be introduced. The mixed zone must extend about 5 times deeper than the iron convection zone. Detailed comparisons to a few AmFm stars have been carried out.The LiBeB abundances observed in clusters give additional information. The abundances of the 28 species offer considerable constraints on the models. Various potential turbulence models have been introduced in a stellar evolution code and results of evolutionary calculations for Li gap stars are discussed in the light of the constraints offered by the abundances of LiBeB and metals. The radiative accelerations of LiBeB have also been recalculated taking the effect of changing metal abundances into account. This modifies the expected Li gap in the absence of turbulence.
Radio Spectra of Complete Sample of Galactic Supernova Remnants
Authors: Trushkin S.A. 
Published: July 2016

We present radio continuum spectra for nearly 200 Galactic supernova remnants (SNRs) from 220 known and included in Green's (1998) catalog. Spectra plotting is an “on—line” procedure of the CATS database http://cats.sao.ru/ and it could plot quite accurate spectra with the thermal plasma free—free absorption in fitting the spectra accounted for: to indicate the presence of extended ionized medium along the line of sight.We did not find considerable correlation between spectral index and Galactic coordinates l, b of SNRs. An analysis of 190 spectra showed that 78 SNRs (40 %) have clear low-frequency turnover caused, apparently, by absorption in the thermal foreground of the Milky Way. The turnover frequency vτ=1 for these SNRs has mean value near 30 MHz do not correlate with the Galactic coordinates. But the frequency has significant correlation with the distance to SNR, defined from ∑ — D relation.
Low Frequency Science with the Square Kilometre Array
Authors: Taylor A.R. 
Published: July 2016

Over the past several years an international community of scientists and engineers has emerged with a common goal to solve the technical challenge required to construct a giant radio telescope with a collecting area of one square kilometre. The Square Kilometre Array (SKA) will have a hundred times more collecting area than our most powerful existing radio telescopes, providing sensitivity of a few tens of nanoJy in the centimetre/decimetre wavelength continuum. With a spatial resolution better than the Hubble Space Telescope, a field of view larger than the full moon, and the ability to simultaneously image a wide range of red shift, the SKA will be the worlds premier spectroscopic imaging telescope at any waveband.At long wavelengths the SKA will be able to detect emission from atomic hydrogen gas at extreme redshifts, allowing study of the “Dark Ages” of the Universe, before, and during, the transition phase when the initial stars formed and reionization occurred. The combination of sensitivity, wide field of view and high angular resolution, will allow high resolution imaging of the interstellar media and magnetic field of a vast number of galaxies to high redshift. Measurements of atomic hydrogen emission and continuum emission will trace the star formation history of the Universe from primordial galaxies to the present.
The Galactic Dynamo: Modes and Models
Authors: Rüdiger G. , Elstner D. , Schultz M. 
Published: July 2016

The hitherto known essentials of the present-day dynamo theory of the galactic magnetic fields are reported. The vertical stratification of the interstellar turbulence is exclusively considered as the source of the α-effect. New expressions for this tensorial effect and its quenching by the field are applied. Although its anisotropy and the influence of the halo easily yield the excitation of non-axisymmetric magnetic configurations of diverse equatorial symmetry, the galactic differential rotation finally leads to axisymmetric and quadrupolar solutions. The magnetic strength of the dynamo fields slightly exceeds the equipartition value while its angular momentum transport (into the intergalactic space) is very small.For increasingly steep vertical gradients of the turbulence intensity a saturation of the field strength is found since the eddy diffusivity grows with the α-effect. Only beyond the maximum of the field the equatorial symmetry changes to an equatorial antisymmetry,i.e.to dipolar solutions.In order to produce the observed values of the pitch angles one has only to choose correlation times of the turbulence slightly exceeding the usually accepted 107yrs.
Internal Faraday Rotation in Compact Radio Sources (CRS)
Authors: Björnsson Claes-Ingvar 
Published: July 2016

Standard inhomogeneous synchrotron models can explain the polarization properties of CRS if proper account is taken for internal Faraday rotation; neither depolarization due to synchrotron self-absorption nor a tangled magnetic field plays an important rôle. The effects of inhomogeneities are important; the polarization properties of flux escaping from a Faraday thick region are quite different from those coming from a homogeneous source. The observed polarization of strong lined CRS indicates (i) a jet with constant opening angle, (ii) a large scale magnetic field whose main component is along the jet axis and (iii) the density of Faraday rotating electrons scales with radial distance in the same way as does the density of synchrotron emitting ones. Furthermore, the preferred values of both the degree of polarization and the polarization angle, as well as their approximate frequency independence, are consistent with the assumption that the Faraday rotating electrons and the synchrotron emitting electrons belong to the same energy distribution; the former being the non/semi-relativistic end of the latter. The source properties of weak lined CRS (BL Lac objects) are harder to infer, mainly because much of the polarized flux remains unresolved even at the VLBI-scale. The weak polarization structure in BL Lac objects might be due to shocks and no Faraday rotation but it can also be accounted for with a jet structure similar to the one inferred for the strong lined CRS. Although an efficient Fermi-type acceleration mechanism can not be excluded, the polarization characteristics of Faraday thick shocks in a jet make them unlikely as acceleration sites.The polarization properties of the VLBI-core are not those expected from the unresolved inner parts of the jet structure deduced from the resolved emission. Instead it seems likely that the VLBI-core consists of two components; in addition to the inner part of the jet, there is a component which is directly related to the optical blazar emission region. Time variations and the properties of the circular polarization imply that the latter, blazar component, is characterized both by a large scale magnetic field and large Doppler factors. Relativistic streaming in the blazar component is important; for example, the gross features of the initial phases of radio outbursts can be explained in a scenario where the streaming electrons are pitch angle scattered.
Solar X radiation
Authors: de Jager C. 
Published: July 2016

X-ray bursts are defined as electromagnetic radiation originating from electronic transitions involving the lowest electron shells; gamma rays are of nuclear origin. Solar gamma rays have not yet been discovered.According to the origin we have :
1.Quasi thermal X-rays, emitted by (a) the quiet corona, (b) the activity centers without flares, and (c) the X-ray flares.2.Non-thermal X-ray bursts; these are always associated with flares.The following subdivision is suggested for flare-associated bursts :
The visible matter – dark matter coupling
Authors: Sancisi Renzo 
Published: July 2016

In the inner parts of spiral galaxies, of high or low surface brightness, there is a close correlation between rotation curve shape and light distribution. For any feature in the luminosity profile there is a corresponding feature in the rotation curve and vice versa. This implies that the gravitational potential is strongly correlated with the distribution of luminosity: either the luminous mass dominates or there is a close coupling between luminous and dark matter. in a similar way, the declining rotation curves observed in the outer parts of high luminosity systems are a clear signature of the stellar disk which either dominates or traces the distribution of mass.The notion that the baryons are dynamically important in the centres of galaxies, including LSBs, undermines the whole controversy over the cusps in CDM halos and the comparison with the observations. If the baryons dominate in the central regions of all spirals, including LSBs, how can the CDM profiles be compared with the observations? Alternatively, if the baryons do not dominate but simply trace the DM distribution, why, in systems of comparable luminosity, are some DM halos cuspy (like the light) and others (also like the light) are not?
The Determination and Interpretation of the Rotation Parameter v sin i
Authors: Collins George W. 
Published: July 2016

In this paper we review the development of the concept of the stellar rotation parameter commonly known as v sin i. We emphasize that the interpretation of the parameter in terms of physical characteristics of the star always depends on comparison with a model that is intended to represent the physical properties of the star. To that end we will trace the development of such models along with the observational means of determining the parameter. Emphasis will be place on the traditional methods involving stellar spectroscopy, but some attention will be place on indirect methods involving direct measurement of the rotation period and recent interferometric determination of stellar oblatness. In addition we will comment on recent techniques involving the simultaneous measurement of many spectral lines and synthetic spectra to improve the accuracy of rotational half-widths.The natural desire for simplicity of such models has often resulted in erroneous values for stellar parameters. This is particularly the case for the most rapidly rotating stars generally of early spectral type, but may also be present in some giants and supergiants where rapid rotation is difficult to detect. Finally, we will comment on the possibilities of improving the quality of both the measurement and interpretation of this important stellar rotation parameter.
Wolf-Rayet stars in the Magellanic Clouds
Authors: Smith Lindsey F. 
Published: July 2016

The most striking feature of the Wolf-Rayet stars in the Magellanic Clouds is their subtype distribution. The range of WC subtypes found in the Large and Small Cloud agrees with evolutionary model predictions at the corresponding metallicities. It follows that the WR subtype distribution in any external galaxy is a clear indicator of the metallicity in that galaxy.The fluxes in the emission lines of WC stars appear to be consistent within the LMC. This can be used (with some precautions) in the Milky Way and other galaxies to determine reddening, distance and the numbers of WR stars in compact regions.
Improved synthetic TP-AGB models
Authors: Marigo P. 
Published: July 2016

We present recent improvements and results of an extensive analysis of the TP-AGB phase performed by means of a synthetic model (Marigo 1998a, b; Marigo et al. 1998a, b). The improvements concern: i) the use of a homogeneous and accurate set of analytical relations (Wagenhuber & Groenewegen 1998); ii) a new treatment of envelope burning in the most massive TP-AGB stars (M > 3.5M⊙) to account for the possible break-down of the core mass-luminosity relation; iii) a better treatment of the third dredge-up to infer if and when the process takes place.Extensive calculations of synthetic TP-AGB models have been carried out over the mass range (0.8M⊙ ÷ 5M⊙) and for three sets of initial metallicity (Z = 0.019, Z = 0.008, Z = 0.004). The formation of carbon stars is investigated addressing the following issues: a) the reproduction of the observed luminosity functions of carbons stars in both Magellanic Clouds, and b) the formation of very bright and optically obscured carbon stars.
Measurements of the changes in angular diameter of Mira variables with pulsation phase
Authors: Young J. S. , Baldwin J. E. , Boysen R. C. , Haniff C. A. , Pearson D. , Rogers J. , St-Jacques D. , Warner P. J. , Wilson D. M. A. 
Published: July 2016

We report the direct detection of cyclic diameter variations in the Mira variable χ Cygni. Interferometric observations made between 1997 July and 1998 September, using the Cambridge Optical Aperture Synthesis Telescope (COAST) indicate periodic changes in the apparent angular diameter with amplitude 45 per-cent of the smallest value.The measurements were made in a 50 nm bandpass centred on 905 nm, which is only moderately contaminated by molecular absorption features. To assess the effects of atmospheric stratification on the apparent diameter measured in this band, we have also measured near-infrared diameters for a sample of five Miras, in both the J-band (1.3 μm) and Wing's (1971) 1.04 μm band, which is expected to isolate essentially pure continuum emission. We present J-band visibility curves which indicate that the intensity profiles of the stars in the sample differ greatly from each other.
MACHO observations of LMC red giants: Mira and semi-regular pulsators, and contact and semi-detached binaries
Authors: Wood P.R. , Alcock C. , Allsman R.A. , Alves D. , Axelrod T.S. , Becker A.C. , Bennett D.P. , Cook K.H. , Drake A.J. , Freeman K.C. , Griest K. , King L.J. , Lehner M.J. , Marshall S.L. , Minniti D. , Peterson B.A. , Pratt M.R. , Quinn P.J. , Stubbs C.W. , Sutherland W. , Tomaney A. , Vandehei T. , Welch D.L. 
Published: July 2016

The MACHO data base has been used to examine light curves of all red giant stars brighter than Mbol ∼ −2 in a 0.5° × 0.5° area of the LMC bar. Periods, often multiple, have been searched for in all stars found to be variable. Five distinct period-luminosity sequences have been found on the low mass (M ≲ 2.25M⊙) giant branch. Comparison of observed periods, luminosities and period ratios with theoretical models identifies Miras unambiguously as radial fundamental mode pulsators, while semi-regular variables can be pulsating in the 1st, 2nd or 3rd overtone, or even the fundamental. All these variables lie on just 3 of the 5 distinct sequences, and they all appear to be on the AGB.The fourth sequence contains red giants on the first giant branch (FGB) or at the red end of the core-helium burning loops of intermediate mass stars (M ≳ 2.25M⊙). The light curves of these stars strongly suggest that they are contact binaries, and they make up ∼0.5% of stars within 1 mag. of the FGB tip. Stars on the fifth sequence show semi-regular, eclipse-like light curves. The light curves and periods of these stars suggest that they are in semi-detached binaries, transfering mass to an invisible companion via a stellar wind or Roche lobe overflow. They make up ∼25% of AGB stars. If the existence of these red giant contact and semi-detached binaries is confirmed, then extant theories of binary star evolution will require substantial modification.
Long-term evolution of AGB wind envelopes: Insights from hydrodynamical models
Authors: Steffen M. , Schönberner D. , Szczerba R. 
Published: July 2016

Up to now, hydrodynamical models of dust-driven AGB winds do not generally take into account the ‘long-term’ changes of the stellar parameters (on stellar evolution time scales of 103 to 105 yrs), although it is well known that the luminosity and (very likely) the mass loss rate undergo significant variations when so called ‘thermal pulses’ occur on the upper AGB. In this review we demonstrate that time-dependent radiation hydrodynamics calculations are needed to understand the formation, structure, and spectral energy distribution of detached dust shells detected by IRAS and ISO. Combined with appropriate models, these observations can reveal part of the previous mass loss history on the AGB and allow an empirical check of presently adopted mass loss laws.Based on insights from hydrodynamical simulations, we discuss the two competing scenarios that have been put forward to explain the origin of the very thin molecular shells recently discovered around some carbon stars. We find that the signature of a short mass loss ‘eruption’ broadens quickly with time due to the related velocity gradient across the shell. Hence, this scenario is not considered a likely explanation of detached CO shells. On the other hand, the alternative mechanism, interaction of winds, is shown to be capable of producing very thin shells of greatly enhanced gas density in the dusty outflows from AGB stars by sweeping up matter at the interface between both type of winds.
Orbits versus Star Formation Histories: A Progress Report
Authors: Grebel Eva K. , Kolatt Tsafrir , Brandner Wolfgang 
Published: July 2016

Milky Way dwarf companions and satellites of distant spirals seem to show a preference for polar orbits. We suggest that five out of six M31 dwarf spheroidal companions as well as two dwarf irregulars may also be located near a polar plane.We briefly discuss preliminary results from a statistical approach to study correlations between star formation histories and orbits of Local Group dwarf galaxies, such as a possible correlation between star formation episodes and galaxy separations.
HST-STIS spectroscopy of the Bubble Nebula, NGC 7635
Authors: Buckalew Brent , Dufour Reginald J. , Ghavamian Parviz , Hartigan Patrick , Walter Don K. , Hester Jeff J. , Scowen Paul A. 
Published: July 2016

We report the results of long-slit spectroscopy of the wind-blown bubble and photo-evaporating knots around the O6.5IIIf star BD+60°2522 made with the HST-Space Telescope Imaging Spectrograph. The Of star is the primary ionizing source for the H II region NGC 7635, located in the Perseus Arm. The spectra were taken through a 0″.2 x 52″ slit with low and medium resolution gratings covering the wavelength range 2900-6870 Å. Observations with two slit orientations were made; one across the line of embedded knots to the west of the Of star and the second running from the Of star across the bubble to the NE.The 2D-STIS spectra permit us to subtract the surrounding H II region's diffuse emission from that of the knots and the bubble, and to study the spatial variations in various emission lines in these features to a resolution of ∼ 0″.2, an order of magnitude improvement over the best ground-based spectra of this object in the literature. We present high spatial-resolution emission-line and line-ratio profiles across the bubble and knots, and compare them with the predicted variations from photo-ionization, photo-evaporation, and wind-shock models. We also present an analysis of temperatures, densities, and abundances in the features from higher S/N spectra extracted over selected lengths of the slit.
Evolution of Galaxies in Clusters
Authors: Funato Yoko 
Published: July 2016

In this paper I show recent results of our studies on the evolution of an isolated cluster of galaxies using N-body simulations. In Sensui, Funato & Makino (1999) only one initial cluster model are investigated. In the present study, we varied the initial model of galaxies and clusters, and studied the dependence of evolution on initial conditions. We found that the mass of galactic halos are stripped and a common halo develops for any cluster models. Using result of scattering experiments of two halo-halo encounters, we show that the growth rate of the common halo (and complimentarily decrease rate of average mass of galactic halos) can be explained as a result of stripping due to cumulative encounters between two galactic halos.We also found that the galaxies evolved so as to satisfy the relation between the masses of galaxies mgx and their velocity dispersion σgx expressed as mgx ∝ σ3∼4gx for all galaxy models as a consequence of their dynamical evolution through galaxy-galaxy interactions. We discuss the relation between our result and the observed Faber-Jackson relation.
On The Dynamics of Non-Stationary Stellar Cluster in Galaxy
Authors: Kozhanov T.S. , Omarkulov K.A. 
Published: July 2016

We followed Wielen's (1974) and Osipkov's, Kozhanov's (1986) procedure to derive the equation of motion of a cluster in the solar neighborhood in taking into account of a galactic tidal field, which is assume to have axial symmetry, and gravitational influence of spherical stellar clouds.On the basic of this equations and using the linear approximation for the galactic field (Wielen 1974) one can derive virial equation of Jacobi for variable masses of stellar clusters. The non-stability by Lagrange condition, describing behavior of this system is obtained.
Fossil magnetic fields and rotation of early-type stars
Authors: Dudorov A.E. 
Published: July 2016

Observational data of the last 10 years allow two main conclusions:a) Main sequence stars can be separated in two classes: - magnetic (Bp) stars with surface strengths of a dipole or quadrupole magnetic field of Bs ≈ n · (102 − 103) G, n = 2,3,4…7, and - normal main sequence stars (F-O) with magnetic fields Bs ≈ 1 − 100 G (< 300 G);b) Typical star formation takes place in interstellar molecular clouds with magnetic field strengths B ≈ 10-5 G (See Dudorov 1990).