bluuebunny/crossref_metadata_2025_split huggingface dataset:
Magnetic Viscosity as the Dominant Shear Force in Accretion Disks
Authors: Kundt W. 
Published: July 2016

Matter in accretion disks spirals inward because of angular-momentum losses due to intrinsic shear torques. For galactic disks, the accretion rates can be as high as 1M⊙/yr, corresponding to a value larger than unity of the dimension-less viscosity parameter α. The required torques exceed turbulent ones by at least an order of magnitude; they can, however, be the result of magnetic tensions.In fast-revolving, clumpy disks, magnetic pressures can exceed static gas pressures and approach ram pressures. Suggestive candidates for such magnetic dominance are the inner, near-rigidly rotating parts of galactic disks.
Magnetorotational Mechanism: Supernova Explosions and Ejections
Authors: Ardeljan N. V. , Bisnovatyi-Kogan G. S. , Moiseenko S. G. 
Published: July 2016

We made simulations of the collapse of the rotating protostellar cloud. Differential rotation leads to the amplification of the toroidal component of the magnetic field and subsequent ejection of the matter due to the magnetorotational mechanism.Our results show that at different initial configurations of the magnetic field formation of qualitatively different types of explosion takes place. Magnetic field of the dipole type produces a jet-like explosion. Quadrupole-like magnetic field produces supernova explosion whith ejection presumably near equatorial plane. Quantitative estimations of the ejected mass and energy are given.We have done simulation of the collapse of the white dwarf and formation of a differentially rotating neutron star. After the collapse stage the rotating neutron star was formed. The rotation of the neutron star is strongly differential. The presence of the magnetic field (even the weak one) could produce magnetorotational supernova explosion.For the simulations we have used 2D numerical scheme, based on the specially developed numerical method (conservative, implicit, triangular grid, Lagrangian, grid reconstruction).
The Effects of Field-Aligned Rotation on the Magnetically Channeled Line-Driven Winds
Authors: ud-Doula Asif , Owocki Stanley 
Published: July 2016

There is extensive evidence that the radiatively driven stellar winds of OB-type stars are not the steady, smooth outflows envisioned in classical models, but instead exhibit extensive structure and variability on a range of temporal and spatial scales. We examine the possible role of stellar magnetic fields in forming large-scale wind structure. It is based on numerical magnetohydrodynamic (MHD) simulations of the interaction of a line-driven flow with an assumed stellar dipole field.Unlike previous fixed-field analyses, the MHD simulations here take full account of the dynamical competition between field and flow, and thus apply to a full range of magnetic field strength, and within both closed and open magnetic topologies. A key result is that the overall degree to which the wind is influenced by the field depends largely on a single, dimensionless, ‘wind magnetic confinement parameter’, η∗ (= B2eqR2∗/Ṁv∞), which characterizes the ratio between magnetic field energy density and kinetic energy density of the wind.We extend these MHD simulations to include field-aligned stellar rotation. The results indicate that a combination of the magnetic confinement parameter and the rotation rate as a fraction of the ‘critical’ rotation now determine the global properties of the wind. For models with strong magnetic confinement, rotation can limit the extent of the last closed magnetic loop, and lead to episodic mass ejections that break through the close loop and are carried outward with a slow, dense, equatorial outflow. Our 2-D numerical simulations indicate that the magnetic fields provide excessive amount of angular momentum to the wind preventing the formation of a Keplerian disk.
The Ghosts of Galaxies: Tidal Debris in Clusters
Authors: Gregg Michael D. , West Michael J. 
Published: July 2016

Gravitational interactions in rich clusters can strip material from the outer parts of galaxies or even completely disrupt entire systems, giving rise to large scale, low surface brightness ghostly features stretching across intergalactic space. The nearby Coma and Centaurus clusters both have striking examples of galaxy ghosts, in the form of 100 kpc-long plumes of intergalactic debris. By searching HST archival images, we have found numerous other examples of galaxy ghosts in rich clusters at low redshift, evidence that galaxy destruction and recycling are ubiquitous, important in cluster formation and evolution, and continue to mold clusters at the present epoch. Many ghosts appear in X-ray bright clusters, perhaps signaling a connection with energetic subcluster mergers.The fate of such material has important ramifications for cluster evolution. Our new HST WFPC2 V & I images of a portion of the Centaurus plume reveal that it contains an excess of discrete objects with −12 < MV < −6, consistent with being globular clusters or smaller dwarf galaxies. This tidally liberated material is being recycled directly into the intracluster population of stars, dwarf galaxies, globular clusters, and gas, which may have been built largely from a multitude of similar events over the life of the cluster.
Infrared interferometry of circumstellar envelopes
Authors: Monnier John D. 
Published: July 2016

This paper will review the technical progress of interferometric infrared observing techniques from the first 2-element interferometer 25 years ago to the 3+ element arrays now coming into service. To date, only the Infrared Spatial Interferometer (ISI) has published separate-element interferometric data on circumstellar dust shells in the infrared and many of these scientific results will be discussed. Speckle interferometry has also evolved significantly over the last few decades as slit-scanning techniques over single-pixel detectors have largely been replaced by fast-readout of large format detector arrays. Important near-infrared and mid-infrared results derived from speckle data will also be reviewed.Until recently, two-dimensional information about circumstellar dust distributions has been sorely lacking, hence detections of dust shell asymmetries have been difficult and uncertain. New speckle observations using modern, 10-m class telescopes have yielded surprising results, demonstrating the importance of accurate closure phase information in interpreting interferometric data. These discoveries hopefully precursor those to be made from closure-phase imaging with the new generation of separate-element, interferometric arrays.
Link between Mass-loss and Variability Type for AGB Stars?
Authors: Ivezić Željko , Knapp Gillian R. 
Published: July 2016

We find that AGB stars separate in the 25–12 vs. 12-K color-color diagram according to their chemistry (O, S vs. C) and variability type (Miras vs. SRb/Lb). While discrimination according to the chemical composition is not surprising, the separation of Miras from SRb/Lb variables is unexpected.We show that “standard” steady-state radiatively driven models provide excellent fits to the color distribution of Miras of all chemical types. However, these models are incapable of explaining the dust emission from O-rich SRb/Lb stars. The models can be altered to fit the data by postulating different optical properties for silicate grains, or by assuming that the dust temperature at the inner envelope radius is significantly lower (300–400 K) than typical condensation temperatures (800–1000 K), a possibility which is also supported by the detailed characteristics of LRS data. While such lower temperatures are required only for O- and S-rich SRb/Lb stars, they are also consistent with the colors of C-rich SRb/Lb stars.The absence of hot dust for SRb/Lb stars can be interpreted as a recent (order of 100 yr) decrease in the mass-loss rate. The distribution of O-rich SRb/Lb stars in the 25–12 vs. K-12 color-color diagram shows that the mass-loss rate probably resumes again, on similar time scales. It cannot be ruled out that the mass-loss rate is changing periodically on such time scales, implying that the stars might oscillate between the Mira and SRb/Lb phases during their AGB evolution as proposed by Kerschbaum et al. (1996). Such a possibility appears to be supported by recent HST images of the Egg Nebula obtained by Sahai et al. (1997), the discovery of multiple CO winds reported by Knapp et al. (1998), and long-term visual light-curve changes detected for some stars by Mattei (1998).
Carbon Stars in the Sagittarius Dwarf Galaxy
Authors: Whitelock Patricia , Menzies John , Irwin Mike , Feast Michael 
Published: July 2016

Coordinates are listed for 26 carbon stars which are radial velocity members of the Sagittarius Dwarf Galaxy (SDG). Their near-infrared properties indicate that most are on an extended Asymptotic Giant Branch (AGB), and the total number of such stars in the SDG is estimated at about 100. At least one-third of the C stars studied are large amplitude, i.e. Mira, variables; these are the first Miras to be found in a dSph galaxy. Surprisingly, the most luminous C star is not a Mira, but a low amplitude variable.A comparison of the properties of C stars in the Bulge and in the SDG indicates that the two groups cannot be drawn from the same population. It is therefore highly unlikely that the Bulge C stars are, contrary to previous suggestions, related in any way to the SDG.
Discovery of 50 000 K shocked gas in NGC 6888
Authors: Dufour Reginald J. , Buckalew Brent 
Published: July 2016

We report the results of a re-analysis of KPNO-IRS spectroscopy of 30 locations in the prototypical WR shell nebula NGC 6888 (WN6 central star). The spectra covered a 3500–7200 Å wavelength range with R ≃ 360 effective spectral resolution. Electron temperatures for a majority of the locations were derived from the [N II] λλ(6548+6583)/5755 and the [O III] λλ(4959+5007)/4363 emission line ratios. Te([N II]) ≃ 8000 ± 2400 K for eleven locations in the north and west parts of the shell and Te([N II]) ≃ 13 000 ± 4000 K for five locations in the southwest part. By contrast, the [O III] temperatures show great variation with location, ranging from 12000 K to 55000 K, with the highest value being that for an [O III] filament located to the north and outside of the ensemble of knots defining the northern extent of the mass loaded ejecta from the star.Using a combination of spectral ‘arithmetic’, shock- and photoionization models, we demonstrate that NGC 6888 is permeated by a strong hot wind-shocked gaseous medium (which extends outside the mass-loaded ejected material in the north and west) that contaminates the spectrum of any given location and must be removed to determine realistic Te, Ne, and abundances for the ejected material in the observed knots. For some knots, the effects are such that the abundances obtained via naive empirical methods were found to be several times too small (particularly for O/H) than in reality. We present a few illustrative examples and discuss the implications the results have on abundances previously derived for NGC 6888, but more importantly, what these results imply about the origin of temperature fluctuations in other types of nebulae which may be influenced by stellar winds (i.e., planetary nebulae and H II regions).
The Edinburgh/AAO/Strasbourg Catalogue of Galactic Planetary Nebulae
Authors: Parker Q. A. , Hartley M. , Russeil D. , Acker A. , Ochsenbein F. , Morgan D. H. , Beaulieu S. , Morris R. , Phillipps S. , Cohen M. 
Published: July 2016

We report on version 1.0 of the Edinburgh/AAO/Strasbourg catalogue of new and possible Planetary Nebulae (PN) distributed via cdrom at this meeting. We provide accurate positions, designations, images and other descriptive parameters for the PN. In future releases this will be supplemented by inclusion of spectra and related material such as line ratios, velocities etc.The 900+ PN have been discovered solely from visual scrutiny of narrow-band exposures taken for the AAO/UKST H-alpha survey of the Southern Galactic Plane. Most have classic PN-type morphologies (i.e. bi-polar, rings, shells or ovals). SuperCOSMOS data will soon supersede our visual scanning but it proved an effective preliminary technique to identify candidate PN on the basis of morphology, isolation and identification as an H-alpha nebulosity. We already have confirmatory spectroscopy for ~ 700 objects. Much of our new sample are of very low surface brightness, with no obvious central star, and so have remained undetected in previous surveys. They are revealed here due to the excellent depth, resolution, coverage and uniformity of the H-alpha survey. Many PN are also well extended. The average angular size is 51″ with the median of 27″ but examples extend to several arcminutes. This may indicate many are in a highly evolved state where the central star has faded from easy optical detection and the nebula itself is dissipating into the ambient ISM. Large numbers of candidate PN have also been found in the Galactic Bulge region, most of which have been confirmed via UKST FLAIR/6dF MOS spectroscopy (Parker et al, in preparation and these proceedings).By version 2.0 (release in 2002) we will have doubled the number of Galactic PN accrued from all sources over the last 75 years. This new catalogue should have a profound impact on many aspects of PN research.
Bi-abundance Photoionization Models for Planetary Nebulae
Authors: Péquignot D. , Liu X.-W. , Barlow M. J. , Storey P. J. , Morisset C. 
Published: July 2016

A new class of planetary nebulae, in which abundances derived from optical recombination lines (ORL) seem to be much larger than those derived from collisionally excited lines (CEL) by up to one or two orders of magnitude is now well identified (Liu, these proceedings and references therein).Photoionization models including two components, one highly enriched in C, N, O, Ne and deficient in H and the other one of more usual composition, can account for most of the numerous spectroscopic data available from UV to far-IR in two of the best observed, most extreme examples, namely NGC 6153 and M142 (Péquignot et aI., 2002, and in preparation). The few discrepancies left can generally be understood in terms of inaccuracy of the observations (calibration of infrared line fluxes relative to the optical) and some atomic data, particularly in the unusual conditions prevailing in the H-deficient component.
Sulfur, Chlorine & Argon Abundances in Galactic PNe
Authors: Kwitter Karen B. , Henry Richard B.C. 
Published: July 2016

We report first results from our extensive study of S, Cl and Ar abundances in a large sample (>60) of primarily Type II planetary nebulae (PNe) in the Galaxy (see Kwitter & Henry 2001, ApJ, 562, 804). Ratios of S/O, Cl/O, and Ar/O constitute important tests of differential nucleosynthesis of these elements and serve as strict constraints on massive star yield predictions. The use of PNe to measure the original stellar abundances of S, Cl, and Ar is possible because PN progenitors, intermediate-mass stars between 1 and 8M⊙, lack sufficient mass to synthesize these elements. Therefore, their abundances measured in the nebulae at present reflect accurately the original stellar abundances.Here we present results from new ground-based optical spectra of planetary nebulae extending from 3600–9600Å. The observed spectral range includes the strong near-infrared lines of [S III] λλ9069,9532, which allowed us to extensively test their effectiveness as an indicator of S++. We find that the S++ abundances derived from these lines are quite consistent with those derived from the λ6312 line when the latter is well measured. In addition, we find that the atmospheric absorption affecting the near-infrared lines can be minimized by using in the calculation whichever of the pair is less affected: if the observed ratio of λ9532/λ9069 exceeds 2.48, the theoretical value, we use λ9532; if we observe a ratio <2.48, we use λ9069 instead. We also introduce a new, model-tested ionization correction factor for sulfur that includes the latest atomic data and the effects of matter-boundedness.For the northern subsample we find the following averages: S/O=1.2E-2±0.71E-2, Cl/O=3.3E-4±1.6E-4, Ar/O=5.0E-3±1.9E-3.Subsequent papers in this series will present the full analysis and compare our derived abundances of S, Cl, and Ar with nucleosynthesis predictions from massive stars. (A companion paper in this volume by Milingo, Henry & Kwitter discusses the southern subset observed at CTIO; see also Milingo, J.B., Kwitter, K.B., Henry, R.B.C., & Cohen, R.E. 2002, ApJS, in press; Milingo, J.B., Henry, R.B.C., & Kwitter, K.B. 2002, ApJS, 2002, in press.)
Density of Low-ionization Structures in PNe
Authors: Gonçalves Denise R. , Corradi Romano L.M. , Mampaso Antonio , Campos Cristina 
Published: July 2016

We have recently completed a comprehensive analysis of the morphological and kinematical properties of ~ 50 PNe which are known to possess low-ionization (micro)structures (LISs; Gonçalves et al. 2001). This study has shown that density is a crucial parameter for distinguishing among several physical processes responsible for their formation and evolution.We are at present analysing long-slit, medium-resolution optical spectra of 18 PNe in order to determine the physical parameters of the LISs and their environments. These data were obtained with the 1.5 m Danish (ESO) and the 2.5 m Isaac Newton (ORM) telescopes.Here we show the first results of this survey, presenting the density variation of NGC 7009 and comparing it with the derived densities of K 4-47 and Wray 17-1.
There is a Brown Dwarf Desert of Companions Orbiting Stars between 75 and 1000 AU
Authors: McCarthy Chris , Zuckerman B. , Becklin E. E. 
Published: July 2016

We present results of the first large (> 100 stars) infrared coronographic search for substellar companions to nearby stars. The search consisted of two surveys of stars chosen for their youth and proximity to Earth: 1.) a 178 star infrared survey at Steward and Lick Observatories, with optical followup from Keck Observatory, capable of detecting companions with masses greater than 30MJ, orbiting between about 75 and 300 AU, 2.) a 102 star survey using the Keck telescope, capable of detecting extrasolar brown dwarfs and planets typically more massive than 10 MJ, orbiting between about 75 and 300 AU.This research resulted in the discovery of one brown dwarf companion, zero planets and 23 double stars. The frequency of brown dwarf companions (of any mass) to G, K and M stars orbiting between 75 and 300 AU is measured to be 1 ± 1 %. The frequency of massive (> 30 MJ) brown dwarf companions is found to be 0.6 ± 0.6 %. The frequency of giant planets with masses larger than 10 MJ, between 75 and 300 AU, is measured here for the first time to be no more than about 3 %.
Looking for Very Low-Mass Pre-Main Sequence Objects with SDSS
Authors: McGehee Peregrine M. , Hawley Suzanne L. , Covey Kevin R. 
Published: July 2016

Determining the process(es) by which brown dwarfs form is key to understanding their intrinsic nature. If their origins are within circumstellar disks they are akin to giant planets. If, on the other hand, they coalesce from molecular cloud cores, then they share a common lineage with low mass stars. These two mechanisms can be distinguished by investigation of young (< 10 Myr) substellar objects. If brown dwarfs are small failed stars, we expect to find very low mass analogs of the Classical T Tauris, with primordial magnetic fields from the molecular cloud collapse. Accretion onto these objects leads to characteristic magnetic activity signatures such as chromospheric and coronal emission resulting in an ultraviolet excess continuum.The Orion OB1b association (m-M = 7.9, 2 Myr) provides a laboratory for following the strength and occurrence of the accretion process as a function of mass. Studies of the substellar mass function within the sigma Orionis cluster at the southern end of the association indicate that brown dwarfs are common. Based on model isochrones and the SDSS M dwarf sequence we expect the 95% completeness limit of the “Orion” scans to correspond to 0.1 and 0.03 solar masses for the u and g bands.
The New Opacities and B-Star Pulsations
Authors: Dziembowski W.A. 
Published: July 2016

Over thirty years ago Baker & Kippenhahn (1962) demonstrated that an instability driven by the opacity mechanism is the cause of Cepheid pulsations. Recently it has been shown that the same mechanism is responsible for oscillations observed in β Cephei, SPB and perhaps in other variable B-type stars. The search for the driving mechanism in hot stars began in the late sixties with no success until the opacities calculated with the OPAL code (Iglesias, Rogers and Wilson 1990, 1992) became available. The crucial new feature in the opacity is the local maximum at T ≈ 2 × 105 K caused by iron lines which was ignored in earlier calculations. Recently, stellar opacity data from an independent project (OP) became available (Seaton et al., 1993). The agreement between the two opacity data is satisfactory.In B stars the opacity mechanism drives two distinct categories of normal modes. The one relevant to β Cep stars encompasses low order p- and g-modes with periods 0.1–0.3 d. The other includes high-order g-modes with periods ranging up above 4 d. Excitation of such modes may explain most of the slow variability observed in B stars. The theoretical instability domain in the H-R diagram is very sensitive to metal abundance. For the standard value of Z = 0.02, the total instability domain in the main sequence band extends from spectral type O9.5 to B9. In types later than B2 only high-order g-modes are unstable.
Energy distribution of Be stars
Authors: Kilambi Gopal C. 
Published: July 2016

Be stars are defined to be non-supergiant early-type stars of spectral type B showing at times Balmer emission lines in their spectra. These stars often develop strong stellar winds considered to be variable in nature (Slettebak 1988) and have high rotational velocities compared to normal stars of similar spectral types. They also tend to show an excess amount of energy in the near- and far-infrared region compared to normal stars which is presumed to be due the surrounding material around the central star. Thus, the observed energy is a combination of that due to the stellar source and the surrounding material. Various attempts have been made to disentangle the stellar energy component from that of the circumstellar component in order to understand the nature, size and temperature of the envelope. These include:a) Radius determination based on IR excess (Gehrz et al. 1974, Dachs and Hanuschik 1984; Waters et al. 1987),b) Radius estimates from polarization and spectrophotometric data (Jones 1979),c) Envelope dimensions derived from the width of shell absorption cores (Kogure 1969; Hirata and Kogure 1977),d) Dachs et al. (1992) attempted to understand the physical properties, flow patterns and density distribution of the gas by a comparison of synthetic emission line profiles and empirical profiles measured for real Be stars.
What is the Maximun IR Luminosity of a Single Spiral Galaxy?
Authors: Mickaelian A.M. , Véron-Cetty M.-P. , Véron P. 
Published: July 2016

A new sample of IRAS galaxies at high galactic latitudes has been built by optically identifying IRAS point sources.In no more than about 100 cases, an apparently interacting system of galaxies is located near the IR source. The large majority are unambiguously identified with a single, relatively isolated galaxy. This confirms that, although galaxy interaction may trigger starburst activity, the IR luminosity in most objects is due to normal star formation in the disc.