bluuebunny/crossref_metadata_2025_split huggingface dataset:
Recent Observations of the Highly Variable Stab DR Tau
Authors: Bertout C. , Krautter J. , Möllenhoff C. , Wolf B. 
Published: July 2016

Simultaneous spectroscopic and photometric observation of the T Tau-like star DR Tau are reported. The spectrograms show a rich emission spectrum. The Balmer lines are strongly variable. They sometimes show a highly complex structure, and inverse P Cygni profiles were observed on two nights. The metallic lines are essentially undisplaced. Simultaneous photometry indicates irregular variations of the blue continuum by as much as 0.5 magnitude on a time-scale of one day and variations of more than two magnitudes on a time-scale of a few days. The observed spectral properties can be explained by assuming that DR Tauri is a protostar of about 1 M⊙ which has not yet reached its hydrostatic equilibrium Hayashi track.A more complete description of our results is in press in Astronomy and Astrophysics.
Stability of Accreting White Dwarfs in Close Binary Systems
Authors: Sienkiewicz R. , Dziembowski W. 
Published: July 2016

AbstractWe present results of thermal and vibrational stability analysis for 1 M⊙ white dwarf models corresponding to various accretion rates ≳ 10−11 M⊙/y. Accretion is assumed to be spherically symmetric and stationary. Thermal instability due to nuclear burning of hydrogen (at lower accretion rates) and helium (at higher rates) was found. At medium rates two growing thermal modes are simultaneously present. Vibrational instability was found for all models except those corresponding to highest accretion rates. The excitation rates for some nonradial g-modes are at least 3 orders of magnitude higher than those for radial pulsations. These rates are also higher than the excitation rates for thermal modes in certain range of accretion rates corresponding to high luminosities and effective temperatures. Among objects in which these instabilities may be important are symbiotic stars and nuclei of planetary nebulae.
2. Asteroid Compositional Types and their Distributions
Authors: Zellner B. , Bowell E. 
Published: July 2016

A sample of 359 minor planets with available colorimetric, spectropnotometric, thermal-radiometric, and/or polarimetrie data are classified into broadly defined compositional types C (carbonaceous), S (silicaceous), M (metal-rich), E (metal-free; enstatite?), O (ordinary-chondritic?), T (Trojan; unidentified composition), and U (unclassifiable; none of the above).The small asteroids in Mars- or earth-crossing orbits are almost invariably of type S or O. For the main belt between 2.2 and 3.5 AU., distributions of the various types over diameter and orbital parameters are derived with corrections for observational selection bias. For 560 main-belt asteroids with diameters >50 km, 76% are of type C, 16% of type S, 5% M, and 3% of other types. The S objects become progressively less common with distance. The shapes of the diammeter-frequency relations for primitive (C) and evolved (S + M) types are statistically indistinguishable, both showing a change of slope at 160-km diameter.While large asteroids avoid the Kirkwood gaps more strongly than small ones, we find no significant gap-related anomalies in the relative frequencies of compositional types. Thus most of the observational basis for models in which primitive and evolved asteroids respond differently to collisional evolution has been removed.
9. Relationship between the Composition of Solid Solar-System Matter and Distance from the Sun
Authors: Wasson J. T. 
Published: July 2016

The chondritic meteorites have “solar” compositions, indicating that they have avoided fractionation in planetary processes such as partial melting or fractional crystallization. Since chondrites contain solar proportions of volatiles having nebular condensation temperatures ≤500 K, it follows that agglomeration of grains and accretion to parent bodies occurred after the nebula had cooled to such low temperatures, and, that models calling for simultaneous condensation and accretion of high-temperature minerals such as Fe-Ni metal or ferromagnesian silicates are implausible.Two independent intergroup fractionations have been recognized in chondritic materials; refractory element abundances (e.g., the Ca/Si ratio) and degree of oxidation measured by the FeO/(FeO + MgO) ratio decrease through the chondrite sequence: carbonaceous-ordinary-IAB-E. Hiatus between groups result from incomplete sampling by the Earth of the original spectrum. A plausible speculation is that this sequence reflects formation at different nebular pressures and temperatures, the fine-grained, oxidized, volatile-rich carbonaceous chondrites forming at lower temperatures >5 AU from the Sun, and the reduced enstatite chrondrites forming at higher temperatures near the Sun. The high fall frequency of the three groups of ordinary chondrites suggests an origin in the asteroid belt. The degree of oxidation of the IAB chondrites appears to be slightly lower than that of the Earth, suggesting an origin near or slightly less than 1 AU from the Sun. The O-isotope compositions of chondrites are consistent with this picture.Asteroids tend to be light or dark, with a hiatus in properties between these two classes. This can be understood if the light materials are ordinary chondrites originally formed at 2.2-3.5 AU, whereas the dark materials originated at >5 AU, and were deposited in the asteroid belt during a later period as a result of a major increase in the influx of cometary materials through the inner sc1ar system associated with the generation of the Oort cloud of comets.
A Thermal Pressure Inside the Local Bubble, as Revealed by CI Fine-Structure Excitation
Authors: Jenkins Edward B. 
Published: July 2016

AbstractUltraviolet absorption lines from carbon atoms within a neutral cloud inside or near the edge of the Local Bubble can reveal the thermal pressure of the gas, since the relative populations of the fine-structure levels are determined by a balance between collisional excitation and radiative decay. Features from a C I multiplet appearing in the uv spectrum of the starδCyg (l= 79°,b= +10°,d= 52 pc) were observed with the echelle spectrograph of the Goddard High Resolution Spectrograph on HST. An interpretation of absorptions detected from the two lowest levels gives a thermal pressure range 102.7<p/k< 103.7cm−3K.
Structure Within the Local Bubble: Properties of Individual Clouds
Authors: Beckman J.E. , Trapero J. , Álamo J.R. , Génova R. , Lundstrom I. 
Published: July 2016

Starting from the casual observation that neighbouring lines of sight to stars at similar distances in the LISM show abruptly varying column densities, we developed a technique for measuring properties of single clouds. Fig.l shows an idealized version of this. The steps (Trapero et al. 1992, 1995) are:
–Correct for extinction the distances to local stars used, via B-V (where available), or our own measured Nal or KI absorptions.–Measure diametral column density towards A from Nal or KI equivalent width and convert to hydrogen: N(H) [cm−2].–Measure cloud diameter, between E and E’.–Estimate the number density, n(H) = N(H)/d [cm−3].–Use a canonical thermal pressure, nT = 3500 K cmThis at least gives the decision cool (<100K) or warm (>5000K).–Estimate the cloud mass by integrating n(H) over the estimated volume.Following up, cool cores within warm clouds, or warm envelopes around cool clouds can be identified using distance and velocity as tags. We have found empirical rules of thumb to guide us:
–The KI resonance at 7699Å is ideal for finding cool clouds; it is too weak to show up in warm clouds.–NaI D doublet lines weaker than 5mÅ typify warm clouds. There are too many of these, by a factor 50, to come from cool cloud edges. This is confirmed by the “low velocity Routly-Spitzer effect” (see below).
Remarkable Details in the Local Bubble’s HI Distribution
Authors: Bystrova Natalija V. 
Published: July 2016

AbstractWe discuss details in the galactic HI distribution, which are relevant for studies of the Local Bubble. All results are based on the 21 cm Sky survey with the Large Pulkovo Radiotelescope. The antenna temperature scans were decomposed in two components and after the subtraction of the broad one two details in the residual are presented in this paper. The first is an estimate of the distance to the Sancisi-van Woerden filament in the Sco-Oph region. The second is the connection of a 21 cm feature in the southern galactic quadrant II with the negative-velocity elliptical brightness distribution around the South Pole of the Galaxy.
Evolution of Evolution of Superbubbles
Authors: Ikeuchi Satoru 
Published: July 2016

AbstractIn the first place, I will summarize the evolution of our picture of the ISM over the past several decades in order to see the importance of superbubbles for ISM structure. In accordance with the progress of observations, the picture of the ISM has dramatically changed from the 1960s. Then, I will review the evolution of theoretical models of superbubbles including the idea of chimney structures of the ISM.
Origin and Evolution of the Cepheus Bubble
Authors: Patel N.A. , Goldsmith P.F. , Heyer M.H. , Snell R. , Pratap P. 
Published: July 2016

AbstractThe Cepheus bubble is a giant shell with a diameter of about 120pc, discovered by Kun (1987) from IRAS sky flux maps. We have imaged this 10° × 10° region in the CO J=l-0 line, using the FCRAO Quarry system with sample spacing equal to the 45” FWHM beam width. We estimate the total molecular gas mass to be ~ 105 solar masses and a total kinetic energy ~ 1051 ergs. We find 49 O and B0-B2 type stars in the Cepheus bubble region, most belong to the CepOB2 association. 120 IRAS point sources, selected according to color criteria representative of young stellar objects, fall within our map. Many of these sources are associated with the globules and filamentary structures seen in the CO emission, similar to the situation found in IC1396 (Patel et al. 1995). These stars, which are still associated with dense molecular gas plausibly represent the third and youngest generation of stars in this region.
The ORFEUS Far Ultraviolet Spectrum of the LMC Binary Star HDE 269546
Authors: Widmann H. , Krämer G. , Appenzeller I. , Barnstedt J. , Fromm A. , Gölz M. , Grewing M. , Gringel W. , Haas C. , Hopfensitz W. , Kappelmann N. , Krautter J. , Lindenberger A. , Mandel H. 
Published: July 2016

AbstractFar UV high resolution spectra of 3 LMC and SMC stars were obtained with the Echelle spectrograph during the second ORFEUS mission in Dec. 1996. We present the first results from observations of the LMC star HDE 269546. We find definitely components of very hot gas identified as OVI and SVI absorption in the galactic halo of the Milky Way and in the LMC. Additionally, more than 30 ions of the most abundant elements in different stages of ionization can be identified in both our galaxy and the LMC. For the first time we can identify a significant absorption component of molecular hydrogen in the ORFEUS II Echelle spectrum with a redshift of 200 km s−1, doubtlessly to be attributed to the LMC.
ROSAT Observations of NGC 4258
Authors: Vogler Andreas , Pietsch Wolfgang 
Published: July 2016

AbstractWe performed deep follow up observations of the active spiral galaxy NGC 4258 with the ROSAT HRI and PSPC to study the different emission components. The overall luminosity of NGC 4258 is Lx ~ 2 × 1040 erg s−1 in the 0.1 – 2.4 keV ROSAT band. Besides a weak nuclear point source (Lx ≲ 1038 erg s−1) fourteen point sources are detected in the NGC 4258 disk (integral Lx ~ 3 × 1039 erg s− 1). The bulk of the X-ray emission is not resolved. The main contributions are explained by hot interstellar medium along the anomalous spiral arms of NGC 4258 (Lx ~ 1 × 1040, T ~ 0.4 keV) and by interstellar medium escaping from NGC 4258 disk into the approaching halo hemisphere (Lx ~ 4 × 1039 erg s−1, T ~ 0.2 keV)Surface brightness, temperature and absorption profiles have been taken for the X-rays in the regions of the anomalous arms. The highest and lowest absorption was received for the NW and SE tips of the arms, respectively. Our X-ray findings are compared to a model of the anomalous arms which suggests that the bar of the galaxy causes shocks in the interstellar medium and might heat a small part of it to X-ray temperatures.
Rotationally Modulated Winds of O Stars
Authors: Fullerton Alex W. 
Published: July 2016

AbstractThe stellar wind diagnostics of some well-studied O stars exhibit cyclical variations with periods that are probably related to the rotational period of the underlying star. This rotational modulation is usually attributed to large scale, persistent structures in the wind, which are thought to be generated and maintained by photospheric processes that alter the emergence of the wind from different regions of the stellar surface. In this review, three case studies are used to illustrate the patterns of variability that are attributed to rotational modulation and to highlight some open issues connected with this hypothesis. The problems associated with establishing the occurrence of rotational modulation rigorously are also discussed.
Short and Medium Term Variability of Emission Lines in Selected Southern be Stars
Authors: Rivinius Thomas 
Published: July 2016

AbstractWe observed a sample of several southern Be stars from 1995 to 1997 typically for several months in each season using our spectrograph HEROS. One of these stars, μ Cen, was found to be in the process of continued gradual recovery of the Hydrogen emitting disk which had been lost from 1977-1989. During the monitoring period numerous line emission outbursts were observed. A generalized pattern of an outburst cycle is derived from observations of different circumstellar lines at times of various levels of emission from the disk. Relative quiescence in which mostly periodic varaibiltiy is seen, rapid decreases of emission (precursor), outburst, and subsequent relaxation can be distinguished as the main constituing phases, even though there are distinct differences between different groups of spectral lines. Based on this empirical phenomenology, a schematic picture of the associated ejection of matter into a near-stellar orbit is sketched and similarities between μ Cen and other stars will be outlined.
Non-isotropic Outflows in the Infrared: ISO Imaging of LBVs
Authors: Trams Norman R. , van Tuyll C.I. , Voors Robert H.M. , de Koter Alex , Waters Laurens B.F.M. , Morris Patrick W. 
Published: July 2016

AbstractIn this review we present the ISO imaging of nebulae around Luminous Blue Variables. Three LBVs have been imaged with ISO: HR Car, AG Car and the LBV candidate G79.29+0.46. The ISOCAM instrument did not resolve the nebula around HR Car. However some nebular emission lines are seen in the spectral energy distribution of this source. A proper deconvolution of the images may resolve the nebula. For AG Car and G79.29+0.46 the nebula is clearly resolved. The structure and intensity distribution of the nebular emission is dependent on the wavelength, indicating a separation between the ionised matter and the dust in the nebulae. Some model calculations are presented for the G79.29+0.46 nebula.
Long-Term Behaviour of the Variable Wind of P Cygni
Authors: Kolka Indrek 
Published: July 2016

P Cygni has been for a long time an often-used target of observations, and a test object to prove several modelling approaches (cf. e.g. Najarro et al. 1997, and references therein). However, the origin of it’s stable variability pattern with moderate amplitudes is poorly known yet. In this contribution I will concentrate on the spectroscopic variability. A more elaborated version of this paper considering many kinds of variability data on P Cygni will be published elsewhere (Kolka 1998).The striking phenomenon in the spectra of P Cygni is the cyclic drift of subsequent individual components through the absorption part of the P Cyg-type profile. This is demonstrated in Fig.1a in the case of three Balmer lines in 1982 (the data adopted from Markova, Kolka 1989). Every new drift-cycle starts when the component with smaller Doppler-shift becomes deeper than it’s neighbour.
Pulsation Hydrodynamics of Luminous Blue Variables and Pulsation-Driven Winds
Authors: Guzik Joyce A. , Cox Arthur N. , Despain Kate M. , Soukup Michael S. 
Published: July 2016

AbstractMany physical factors, including radial and nonradial pulsation, rotation, radiation pressure, convection, magnetic fields, or dynamical instabilities may play important roles in the hydrodynamics of Luminous Blue Variables. We review the current status of hydrodynamic modeling of LBV envelopes, and describe results of our models using the one-dimensional nonlinear hydrodynamics code of Ostlie and Cox. We find that the models pulsate in several simultaneous radial modes, driven by the helium and Fe ionization к effect. The pulsations have quasi-periods between 5 and 80 days, with radial velocity amplitudes of 50-200 km/sec, and may be identified with the LBV microvariations. In some cases, depending on luminosity-to-mass ratio and helium abundance, deep layers in the model can periodically exceed the Eddington luminosity limit. The key to exceeding LE is the inclusion of the time dependence of convection: Near the regions of opacity peaks produced by Fe and helium ionization, convection is turning on and off during each pulsation cycle. If convection cannot turn on rapidly enough to transport the required luminosity through the region, the Eddington limit is exceeded. If this region of the star is sufficiently adiabatic, an “outburst” may occur. In the hydrodynamic models, an outburst is indicated by the photospheric radial velocity suddenly becoming very large, and the photospheric radius increasing monotonically over several pulsation cycles. Such pulsation-triggered outbursts may be responsible for the driving of variable, nonspherical winds. If large and infrequent enough, these outbursts may be identified with the classic LBV eruptions accompanied by episodic mass loss.
Instabilities in LBVs and WR Stars
Authors: Ødegaard Knut Jørgen Røed 
Published: July 2016

AbstractUsing a dynamical stellar code, detailed evolutionary models have been computed that follow the evolution and nucleosynthesis of massive stars from the pre-MS (accretion phase) up to the onset of O-burning. Various instabilities occur during the evolution. As an example, the evolution of a star with maximum mass 120 Mʘ is discussed.
Chaos in Relativity and Cosmology
Authors: Contopoulos G. , Voglis N. , Efthymiopoulos C. 
Published: July 2016

AbstractChaos appears in various problems of Relativity and Cosmology. Here we discuss (a) the Mixmaster Universe model, and (b) the motions around two fixed black holes, (a) The Mixmaster equations have a general solution (i.e. a solution depending on 6 arbitrary constants) of Painlevé type, but there is a second general solution which is not Painlevé. Thus the system does not pass the Painlevé test, and cannot be integrable. The Mixmaster model is not ergodic and does not have any periodic orbits. This is due to the fact that the sum of the three variables of the system (α + β + γ) has only one maximum for τ = τm and decreases continuously for larger and for smaller τ. The various Kasner periods increase exponentially for large τ. Thus the Lyapunov Characteristic Number (LCN) is zero. The “finite time LCN” is positive for finite τ and tends to zero when τ → ∞. Chaos is introduced mainly near the maximum of (α + β + γ). No appreciable chaos is introduced at the successive Kasner periods, or eras. We conclude that in the Belinskii-Khalatnikov time, τ, the Mixmaster model has the basic characteristics of a chaotic scattering problem, (b) In the case of two fixed black holes M1 and M2 the orbits of photons are separated into three types: orbits falling into M1 (type I), or M2 (type II), or escaping to infinity (type III). Chaos appears because between any two orbits of different types there are orbits of the third type. This is a typical chaotic scattering problem. The various types of orbits are separated by orbits asymptotic to 3 simple unstable orbits. In the case of particles of nonzero rest mass we have intervals where some periodic orbits are stable. Near such orbits we have order. The transition from order to chaos is made through an infinite sequence of period doubling bifurcations. The bifurcation ratio is the same as in classical conservative systems.
A Symplectic Mapping Model as a Tool to Understand The Dynamics of 2/1 Resonant Asteroid Motion
Authors: Hadjidemetriou John D. 
Published: July 2016

AbstractWe present a 3-D symplectic mapping model that is valid at the 2:1 mean motion resonance in the asteroid motion, in the Sun-Jupiter-asteroid model. This model is used to study the dynamics inside this resonance and several features of the system have been made clear. The introduction of the third dimension, through the inclination of the asteroid orbit, plays an important role in the evolution of the asteroid and the appearance of chaotic motion. Also, the existence of the secondary resonances is clearly shown and their role in the appearance of chaotic motion and the slow diffusion of the elements of the orbit is demonstrated.
Escape from a Crisis in Fokker-Planck Models
Authors: Portegies Zwart Simon F. , Takahashi Koji 
Published: July 2016

AbstractRecent N-body simulations have shown that there is a serious discrepancy between the results of N-body simulations and the results of Fokker-Planck simulations for the evolution of globular and rich open clusters under the influence of the galactic tidal field. In some cases, the lifetime obtained from Fokker-Planck calculations is more than an order of magnitude smaller than those from N-body simulations. In this paper we show that the principal cause for this discrepancy is an oversimplified treatment of the tidal field used in previous Fokker-Planck simulations. We performed new Fokker-Planck calculations using a more appropriate implementation for the boundary condition of the tidal field. The implementation is only possible with anisotropic Fokker-Planck models, while all previous Fokker-Planck calculations rely on the assumption of isotropy. Our new Fokker-Planck results agree well with N-body results. Comparison of the two types of simulations gives a better understanding of the evolution of such clusters.
Conditional Entropy: A Tool to Explore the Phase Space
Authors: Cincotta P. , Simó C. 
Published: July 2016

AbstractIn this paper we show that the Conditional Entropy of nearby orbits may be a useful tool to explore the phase space associated to a given Hamiltonian. The arc length parameter along the orbits, instead of the time, is used as a random variable to compute the entropy. In the first part of this work we summarise the main analytical results to support this tool while, in the second part, we present numerical evidence that this technique is able to localise (stable) periodic and quasiperiodic orbits, ‘aperiodic’ orbits (chaotic motion) and unstable periodic orbits (the ‘source’ of chaotic motion). Besides, we show that this technique provides a measure of chaos which is similar to that given by the largest Lyapunov Characteristic Number. It is important to remark that this method is very simple to compute and does not require long time integrations, just realistic physical times.