bluuebunny/crossref_metadata_2025_split huggingface dataset:
Formation of Thin Current Sheets in the Solar Atmosphere
Authors: Cheng C.Z. , Choe G.S. 
Published: July 2016

AbstractIn a two-dimensional quadrupolar magnetic field geometry, it is demonstrated that various magnetohydrostatic equilibrium field configurations containing current sheets can be formed either by changes in thermal properties or by footpoint displacement. The shape of current sheets depends on the mass distribution in flux tubes. If the mass per flux tube is maximum at the center of the low-lying bipolar arcades, a current sheet is formed between these two arcades along the separatrix line as the plasma temperature (or the shearing footpoint displacement) is increased above a critical value. If the mass per flux tube is higher in the outer tubes of the bipolar arcades than in the inner ones, a current sheet with a sharp downward-pointed tip hanging at a distance above the bottom boundary is formed when the temperature is decreased below a critical value. Resistive magnetic reconnection in the latter type current sheet results in a magnetic island wrapped in dipped field lines, which is regarded as a plausible prominence field configuration.
The Prominence-Corona Transition Region and the Problem of Prominence Oscillations
Authors: Oliver R. , Ballester J.L. 
Published: July 2016

AbstractRecent theoretical investigations of prominence oscillations indicate that the prominence-corona transition region (PCTR) may have a great influence on the magnetohydrodynamic modes of the whole system. We here consider an isothermal prominence embedded in an isothermal corona with a thin PCTR providing a smooth temperature transition between prominence and corona. The effect of the PCTR on the oscillations of the prominence body are investigated.
A Fibril Structure Model for Stellar Prominences
Authors: Oliver R. , Ballester J.L. 
Published: July 2016

AbstractExtensive observational background exists about the presence of cool material clouds embedded in the stellar coronae of rapidly rotating, late-type stars. Observations of such clouds in HKAqr (Gliese 890) and HD 197890 suggest that the clouds are at heights smaller than the star’s corotation radius and could be looked at as a phenomenon similar to that of solar prominences. Recent observations of solar prominences have reinforced the evidence about their fibril structure made of many long, thin magnetic flux tubes making angles about 25 degrees with the direction of the filament channel, with only the central 10–20% of the magnetic flux tubes filled with cool matter, which can produce a depression at the summits of the flux tubes. Then, assuming a similar structure for the stellar cool clouds, we have looked for the physical characteristics of such stellar prominences, i.e. the size of the flux tube depression, the density, temperature, half-width and supported mass of the cool region, taking into account gravity variation with height and centrifugal acceleration, since such clouds have been detected at great heights within stellar coronae belonging to rapid rotators.
Filament Channels: Essential Ingredients for Filament Formation
Authors: Gaizauskas V. 
Published: July 2016

AbstractChannels share with their filaments in a hemispheric dependance for the orientation of their axial magnetic fields. This property has significant implications for models of filaments. In support of new model-building exercises, this review examines known signatures of channels, presents evidence that emerging flux plays a key role in creating a new filament channel, and compares channels which form in the belts of active latitudes with those formed closer to the poles.
Force-Free Models of Filament Channels
Authors: Longbottom A.W. 
Published: July 2016

AbstractA fast multigrid method to calculate the linear force-free field for a prescribed photospheric flux distribution is outlined. This is used to examine an idealized model of a filament channel. The magnetic fields, for a number of different field strengths and positions, are calculated and the height up to which field lines connect along the channel is examined. This is shown to strongly depend on the value of the helicity of the system. A possible explanation, in terms of the global helicity of the system, is suggested for the dextral/sinistral hemispheric pattern observed in filament channels.
Wave Activity and Prominence Eruption
Authors: Baudin F. , Bocchialini K. , Delannee C. , Koutchmy S. , Stellmacher G. , Shibata K. , Veselovsky I.S. , Panasenko O.A. , Zhukov A.N. 
Published: July 2016

AbstractObservational evidence of 3 and 5 min vertical oscillations of a filament on the disk are recorded. Wave activity was observed before, during and after a filament disappearance, inside and around the filament. Both an Hα brightening and, later, a blowing out of a faint soft X-ray (Yohkoh) loop system occuring in connection with a flare were noticed. The wave activity seems to be a dynamically important ingredient of this erupting prominence.Propagating MHD waves and convective structures bring their energy and momentum from the photosphere towards the chromosphere up to the coronal heights where they are partially reflected and/or dissipated. The transition from the laminar to the turbulent state of the whole prominence enhances the dissipation rate of the external waves inside this system, adding energy to produce the heating and lifting of the plasma. Internal plasma instabilities could trigger this transition in the framework of a prominence disappearance.
Evolution of a Filament/CH/Magnetic Field Complex
Authors: Ioshpa B.A. , Mogilevsky E.I. , Obridko V.N. 
Published: July 2016

AbstractSOHO and YOHKOH images, as well as Hα filtergrams and magnetograms from IZMIRAN have been used to analyze the evolution of the related solar phenomena – filament, active region, and accompanying pair of coronal holes – during six solar rotations, with an emphasis on the events observed during August–September, 1996. The whole complex has been considered against the large–scale magnetic fields calculated under the potential approximation. A peculiar point has been found along the changing filament. It is shown that the phenomena under investigation (filament, active region, and coronal hole) form a single complex connected with the magnetic field structure.
Global Pattern of the Magnetic Field Vector Above Neutral Lines from 1974 to 1982: Pic-du-Midi Observations of Prominences
Authors: Bommier V. , Leroy J.L. 
Published: July 2016

AbstractThe magnetic field vectors derived from the Hanle effect measured in prominences during the ascending phase and maximum of Cycle 21 have been plotted on the corresponding synoptic maps of Meudon Observatory (e.g., Figure 2). Most of these measurements have been performed in a single line, He I D3. Our previous analysis of 2-line measurements have already proven that prominence magnetic fields have Inverse polarity (Bommier et al. 1994) and remain close to the horizontal plane (Bommier et al. 1986, 1994). We also found earlier that the field component along the filament axis shows a remarkable large scale pattern, especially at high latitudes (Leroy et al. 1983). Given the geometry of horizontal Inverse field, we reinvestigate this large scale organization through our complete set of observations (296 prominences). These new maps fully confirm and extend to medium latitudes the law of reversal of the axial field from one prominence band to the adjacent ones (see Figure 2). This behavior, together with the Inverse polarity pattern, gives a picture consistent with a North-South global field distorted by the differential rotation (see Figure 3).
A New Multichannel Fourier Transform Spectrometer
Authors: Nordgren Tyler E. , Hajian Arsen R. 
Published: July 2016

AbstractStellar spectra have been obtained using a multichannel Fourier Transform Spectrometer (FTS) which incorporates components of the Navy Prototype Optical Interferometer. It is well known that a FTS can provide superior wavelength stability as compared to traditional spectrometers. Unfortunately the FTS traditionally suffers from exceptionally poor sensitivity, which until now has limited its uses to sources with high fluxes and/or those with narrow band emission (e.g. the Sun, nebulae, and laboratory samples). We present stellar observations using a new FTS design which overcomes this sensitivity limitation by using a conventional multichannel spectrometer in conjunction with the FTS system. The signal-to-noise ratio of spectra from our test-bed observations are consistent with the theoretical prediction and show that for N channels the sensitivity scales like N, while the signal-to-noise ratio scales like . With this type of an instrument on a 3-m telescope and 9 000 channels we expect to be able to detect and measure such exciting astrophysical phenomenon as gravitational redshifts from single, main sequence stars and extrasolar planets of terrestrial mass.
Using the Radial Velocity Spectrometer at the Dominion Astrophysical Observatory
Authors: Skuljan J. , Hearnshaw J. B. , Cottrell P. L. 
Published: July 2016

AbstractTwo observing runs have been undertaken at the Dominion Astrophysical Observatory (DAO) using the radial velocity spectrometer (RVS). About 800 observations have been performed with both the K and F masks. All cross-correlation profiles have been divided by the corresponding flux monitor records and full gaussian functions have been fitted in order to determine the profile centers. A number of IAU standard radial velocity stars have been observed, with a typical internal precision for a given star of about 100 m s−1, which is a factor of two or three better than the usually adopted value for the RVS. Zero-point offsets between these observations and the standard values were also investigated.
Minimizing Radial-velocity Errors caused by Spectral-type Mismatch in Early-type Stars
Authors: Verschueren Werner , David Marc , Vrancken Myriam 
Published: July 2016

AbstractWe present the first results of a long-term study to increase the external accuracy of (relative and absolute) radial velocities of early-type stars below the 1 km/s level. The accuracy suffers mainly from important systematic errors caused by intrinsic object-template mismatch in cross-correlation. Based on synthetic spectra, the magnitude of these mismatch errors is shown in different spectral regions and for different rotational velocities around spectral-type mid-A. A line selection and combination scheme is proposed which optimizes the accuracy (systematic errors) in view of the precision (random errors) required.
Importance of Precise Radial Velocities for Cepheid Binaries
Authors: Szabados L. 
Published: July 2016

AbstractIn view of the high incidence of duplicity among Cepheids, a large number of binary systems remain to be discovered because of the low amplitude of the orbital effect superimposed on the radial velocity variation due to radial pulsation. This fact is shown by simple statistics. The importance of combining astrometric data with radial velocity measurements is also emphasized. Finally the systematics of the radial velocity amplitude as a function of the pulsation period is considered for classical Cepheids.
Towards a New Set of High-Precision Radial-Velocity Standard Stars
Authors: Udry S. , Mayor M. , Queloz D. 
Published: July 2016

Abstract20 years of CORAVEL photoelectric radial-velocity measurements associated with the high precision achieved by the ELODIE spectrograph on a fair time span allow us to propose a new list of high-precision radial-velocity standards with velocity variations lower than a few times 10 m s−1.
Dust Storms in Space?
Authors: Roosen Robert G. , Berg Otto E. , Farlow Neil H. 
Published: July 2016

Silverberg (1970) has explained the “dust storms” observed by the early satellite-borne microphone detectors by postulating that the orbital planes of short-period, low-inclination comets are filled with micrometeoroids. We report here on three separate approaches to test the validity of this hypothesis.
(1)Optical scans of the Gegenschein brightness can yield no useful information on the nearly isotropic sheets of dust predicted by Silverberg.(2)An attempt to directly collect dust particles during a predicted high flux period by means of a sounding rocket yielded negative results.(3)Over three years of particle impact data from extremely sensitive detectors flown aboard Pioneers 8 and 9 show no observable dust storms.Hence Silverberg’s hypothesis appears untenable.However, we should not rule out the possibility that observable showers of very small particles can be blown directly off the nuclei of some comets passing between the Earth and the Sun.
Accumulation of Chondrules on Asteroids (Abstract)
Authors: Whipple Fred L. 
Published: July 2016

It is shown that aerodynamic forces could have played a significant role in the selective accumulation of chondrules on asteroids that are moving with respect to the gas in a primeval solar nebula. Particles smaller than millimeter chondrules would sweep by an asteroid moving more slowly than a critical velocity, while larger particles could be accumulated by impact. Theory and calculation cover the case of subsonic velocity and asteroidal diameter up to 50 km independent of the nebular density from 10-8 to 10-6 g cm-3 or higher for smaller asteroids, as illustrated in table 1.The calculations are based on a solar mixture of materials at T = 550° K and on Stokes’ law of aerodynamic drag, which fails for nebular densities below about 10-8 g cm-3 for millimeter sized spheres. Below this limit the limiting velocity falls off proportionally to gas density. Note that the accretion rates for small particles can be severely restricted because of aerodynamic flow at small asteroidal velocities with respect to the gas. Near and above the critical velocity the asteroidal gravity does not materially add to the accretion rate regardless of asteroidal mass.
The Morphology of Barred Galaxies
Authors: Buta Ronald 
Published: July 2016

AbstractIn this review, the morphology of barred galaxies at optical wavelengths is discussed, focussing mainly on early to intermediate revised Hubble types in the range S0+ to Sbc. Previous work by Hubble, Sandage, de Vaucouleurs, Vorontsov-Velyaminov, and Kormendy is described. The primary emphasis is on ring phenomena, and morphological results from the Catalog of Southern Ringed Galaxies are summarized.
Star Formation in Barred Galaxies
Authors: Phillips Andrew C. 
Published: July 2016

AbstractStar formation in barred spirals is discussed, including star formation rates, distribution of H ɪ regions, and H ɪ region luminosity functions. In general, global star formation rates and other properties are consistent with non-barred spirals of comparable Hubble class. In barred spirals of intermediate class (SBb-SBc), the effects of the bar on star formation is clearly seen in the distribution of star forming sites. Two patterns emerge, one that shows star formation concentrated in ring-like zones corresponding to inner and nuclear rings (earlier types) and one that shows star formation concentrated in the bar itself (later types). These properties appear to be well-correlated with both Hubble stage and bar type.
Properties of H ɪɪ Regions Along Galactic Bars
Authors: Martin P. 
Published: July 2016

AbstractA study of general properties of H ɪɪ regions along the bars of 11 spiral galaxies is presented. From Hα imaging, distributions, morphology and star formation rates are established. Physical conditions derived from spectrophotometry reveal that strong shocks are absent in H ɪɪ regions along bars and electron densities appear to be normal. Abundance distributions indicate that mixing of the ISM is taking place along bars.
The Effect of Bars on the Fueling of Star Formation and Nonstellar Activity in Galaxy Nuclei
Authors: Ho Luis C. , Filippenko Alexei V. , Sargent Wallace L. W. 
Published: July 2016

AbstractTheoretical studies suggest that large-scale stellar bars can be highly effective in delivering gas to the central few hundred parsecs of a spiral galaxy, which may then initiate rapid star formation. Further instabilities may lead to additional inflow to physical scales relevant for active galactic nuclei. We test these predictions in light of recent observations. Compared to unbarred spirals, barred galaxies of type S0-Sbc have a higher probability of exhibiting nuclear star formation, as well as a higher formation rate of massive stars; neither effect is present in spirals of later morphological type. Bars, on the other hand, do not have an obvious influence on active nuclei. We discuss the implications of these findings for the fueling of central star formation and active nuclei.
Pattern Speeds in Barred Galaxies
Authors: Elmegreen B. 
Published: July 2016

AbstractThe ratio R of the corotation radius to the bar radius ranges from 1 to 1.8 in theoretical models that fit the observed dynamics of gas or stars in the bar and spiral regions of galaxies. Most models cluster around a ratio R = 1.2 for early type galaxies. This ratio confirms the bar-aligned orbit theory long held by Contopoulos and others, which requires that R < 1, but it also suggests that bars do not end exactly at corotation, as is often assumed, but significantly inside. The difference between R = 1 in the old interpretation and R = 1.2 or 1.4 in the fitted models is critical for our interpretation of bar-spiral morphologies. It appears that the bar-to-spiral transition in early Hubble types occurs between the inner 4:1 resonance and corotation, in the range corresponding to R = 1.55 to 1 for a flat rotation curve, because of orbit stochasticity where the 2m: 1 orbits in the x1 family cluster together in space. Corotation then occurs relatively far out in the arms, at an angle from the bar end that may exceed 60° for tightly wrapped spirals. This position sometimes corresponds to the crossing of a dust lane from inside the arms to outside (NGC 1300, NGC 1365) or to a spiral arm bifurcation as in non-barred galaxies. The relatively high value of R is also consistent with the identification of inner rings with the inner 4:1 resonance, outer rings with the outer Lindblad resonance, and offset bar dust lanes and nuclear rings with the inner Lindblad resonance. In contrast to this clear interpretation for early type galaxies, there is little consensus on the location of corotation in late type galaxies, which usually lack an inner Lindblad resonance interior to the bar. It is also not known whether late type spirals corotate with their bars, although this appears to be the case for early type galaxies based on the ubiquity and location of outer spirals and rings.
The presence of a Bar—Like Structure in the Central Regions of Seyfert Galaxies
Authors: Afanasiev V. L. , Shapovalova A. I. 
Published: July 2016

AbstractThe central (8″ × 8″) regions of the Seyfert galaxies Mrk 744 and NGC 4151 were studied by the 3D-spectroscopy method. There is evidence that a mini-bar (0.5–2 kpc) is present in the central regions of these galaxies, which provides transportation of gas of the disk to fuel the nucleus.
Box/Peanut Galaxies in the Near-IR
Authors: Baggett S. M. , MacKenty J. W. 
Published: July 2016

Galaxies with box/peanut-shaped (b/p) bulges have been known for some time (e.g., NGC 128 and NGC 7332 (Sandage 1961)). Observationally, b/p features are detected in edge-on systems, and are visible in images, contour maps, and brightness profiles as isophote sections that near minor axis are relatively flat and parallel to major axis (see Figure 1). Peanut-type bulges have isophotes indented at the intersection with the minor axis.There have been several morphological surveys to date. The two most recent studies have found that about 20% of early-type galaxies and nearly 45% of all disk galaxies are b/p galaxies (see Shaw 1987, Dettmar 1989, and references therein) and that the b/p galaxy properties appear similar to normal spirals in the optical, radio, and infrared. A few kinematical studies have been done: b/p bulges are found to rotate more like disks than bulges (e.g., 150 km s−1up to 6 kpc above the plane in NGC 128 (Jarvis 1990) and the velocities remain constant up to large z, implying cylindrical rotation (e.g., NGC 3079 has v constant out to 1.6 kpc (Shaw et al. 1993)).
Neutral Hydrogen in the Bar of NGC 4151
Authors: Mundell C. G. , Pedlar A. , Shone D. L. , Asif M. W. , Robinson A. , Axon D. J. 
Published: July 2016

Bars are thought to play a crucial role in the fueling of AGN (see e.g., Mundell et al., in these proceedings and references therein), and as part of a project to investigate this, we have studied the neutral hydrogen structure in the bar of the Seyfert 1.5 galaxy NGC 4151.High-sensitivity VLA observations have enabled us to image the neutral hydrogen emission from the bar of NGC 4151 in unprecedented detail, and is a continuation of the work by Pedlar et al. (1992).Figure 1 shows images of integrated neutral hydrogen emission. The large scale emission extends over approximately 20 kpc and the outer part consists of a well-defined two-armed spiral which originates from a fat bar at a radius of about 5 kpc. The bar shows complex structure, including two sharp features which are reminiscent of the shocks seen in many simulations, e.g., Athanassoula (1992). These features appear to join the central 1-kpc ring discovered by Vila-Vilaró et al. (1994), and for which there appears a partial counterpart in neutral hydrogen emission.
The Role of Chaos in Barred Galaxies
Authors: Contopoulos G. , Voglis N. 
Published: July 2016

AbstractOrdered orbits in barred galaxies appear along the bar and between the −4/1 and −2/1 resonances of the outer spiral. Chaotic orbits appear mainly near corotation. Such orbits support the bar and the spiral for long times and they are important for self-consistency. There are three main mechanisms for transition from order to chaos: (a) infinite bifurcations, (b) infinite gaps, and (c) infinite spirals. The Lyapunov characteristic number is zero for ordered orbits and positive for chaotic orbits. But much more information is provided by the distribution of the stretching numbers (one-period Lyapunov characteristic numbers). The spectrum of stretching numbers is invariant with respect to initial conditions in a connected chaotic domain. We provide examples of such spectra for 2-D maps, plane galactic orbits, 2-D dissipative systems, 3-D systems (represented by 4-D maps), and systems depending periodically on the time.
A Multiwavelength Study of the Barred Spiral Galaxy NGC 7479
Authors: Laine S. , Gottesman S. T. 
Published: July 2016

AbstractThe detailed investigation of the activity and dynamics in NGC 7479 confirms the importance of understanding individual galaxies. We have discovered an S-shaped emission structure in 21 cm continuum. This detection together with strong nuclear 10 μm and Hα emission may imply the existence of a central massive black hole. We also address the origin of the asymmetric spiral structure in NGC 7479. The lack of any visible companion and the unperturbed H I velocity field suggest that the asymmetry has an internal origin.
First Analysis of Results of the OH Survey of the Inner Galaxy: Evidence for a Stellar Bar
Authors: Sevenster M. N. 
Published: July 2016

AbstractPart of a large survey of the inner galactic Plane (|ℓ| < 45° and |b| < 3° total) in the OH 1612MHz line in search for OH/IR stars is analyzed. We find strong evidence for a central m=2 distortion based on geometrical considerations. The observed deviation from axisymmetry cannot be explained by lopsidedness and agrees with other recent models of the galactic Bar on length, inclination and axis ratio.