bluuebunny/crossref_metadata_2025_split huggingface dataset:
Scattered OH Maser sources in the direction of W49N
Authors: Mendoza-Torres Jose E. , Goss W. M. , Streb S. , Deshpande A. A. , Ramachandran R. 
Published: April 2009

AbstractOH masers spots are observed with the VLBA at 1612, 1665 and 1667 MHz. The orientation of the ellipses resulting from scattering are all nearly aligned perpendicular to the galactic plane. These ordered orientation could be due to the galactic magnetic field.
Magnetic fields in the Pipe nebula region
Authors: Franco Gabriel A. P. , Alves Felipe O. , Girart J. M. 
Published: April 2009

AbstractMagnetic fields are supposed to play an important role in the formation and support of self-gravitating clouds and the formation and evolution of protostars in such clouds. We used R-band linear polarimetry collected for about 12000 stars in 46 fields with lines of sight toward the Pipe nebula to investigate the properties of the magnetic fields acting on this dark cloud complex.
Radiative magneto-hydrodynamics in massive star formation and accretion disks
Authors: Kuiper Rolf , Flock Mario , Klahr Hubert 
Published: April 2009

AbstractWe briefly overview our newly developed radiation transport module for MHD simulations and two actual applications. The method combines the advantage of the speed of the Flux-Limited Diffusion approximation and the high accuracy obtained in ray-tracing methods.
The magnetic field of the evolved star W43A
Authors: Amiri Nikta , Vlemmings Wouter , van Langevelde Huib Jan 
Published: April 2009

AbstractPlanetary nebulae (PNe) often show large departures from spherical symmetry. The origin and development of these asymmetries is not clearly understood. The most striking structures are the highly collimated jets that are already observed in a number of evolved stars before they enter the PN phase. The aim of this project is to observe the Zeeman splitting of the OH maser of the W43A star and determine the magnetic field strength in the low density region. The 1612 MHz OH masers of W43A were observed with MERLIN to measure the circular polarization due to the Zeeman splitting of 1612 OH masers in the envelope of the evolved star W43A. We measured the circular polarization of the strongest 1612 OH masers of W43A and found a magnetic field strength of ~100μG. The magnetic field measured at the location of W43A OH masers confirms that a large scale magnetic field is present in W43A, which likely plays a role in collimating the jet.
Magnetic braking and ambipolar diffusion in metal-poor protostars
Authors: Zinnecker Hans 
Published: April 2009

AbstractIn this short note, we raise the issue how magnetic braking of prestellar and protostellar condensations depends on the metallicity of the molecular gas cloud. We suggest that the degree of ionization, which determines the timescale for redistribution of magnetic flux (ambipolar diffusion) in the dense cloud core, depends linearly on the heavy element and dust abundance of the gas. This implies that magnetic braking is less efficient in metal-poor condensations, and hence more of the angular momentum problem of star formation must be solved by other means, such as fragmentation into (wider) binary systems. Observations of orbital periods (separations) of binary systems among the metal-poor Galactic halo stars could test this prediction.
Three-dimensional MHD simulations of molecular cloud fragmentation regulated by gravity, ambipolar diffusion, and turbulence
Authors: Kudoh Takahiro , Basu Shantanu 
Published: April 2009

AbstractWe find that the star formation is accelerated by the supersonic turbulence in the magnetically dominated (subcritical) clouds. We employ a fully three-dimensional simulation to study the role of magnetic fields and ion-neutral friction in regulating gravitationally driven fragmentation of molecular clouds. The time-scale of collapsing core formation in subcritical clouds is a few ×107 years when starting with small subsonic perturbations. However, it is shortened to approximately several ×106 years by the supersonic flows in the clouds. We confirm that higher-spacial resolution simulations also show the same result.
Electro-magnetic fields around a drifting Kerr black hole
Authors: Kopáček Ondřej , Karas Vladimír 
Published: April 2009

AbstractA test field solution is constructed, describing the electromagnetic field around a Kerr black hole which is drifting in an arbitrary direction. An asymptotically uniform magnetic field is assumed with a general orientation (with respect to the rotation axis). Lines of force of electric and magnetic intensities are explored, as measured by an observer orbiting around the black hole (above the innermost stable circular orbit, ISCO) or freely falling (below the ISCO) down to the horizon. Magnetic null point is found.
New global 3D MHD simulations of black hole disk accretion and outflows
Authors: Dobbie Peter B. , Kuncic Zdenka , Bicknell Geoffrey V. , Salmeron Raquel 
Published: April 2009

AbstractIt is widely accepted that quasars and other active galactic nuclei (AGN) are powered by accretion of matter onto a central supermassive black hole. While numerical simulations have demonstrated the importance of magnetic fields in generating the turbulence believed necessary for accretion, so far they have not produced the high mass accretion rates required to explain the most powerful sources. We describe new global 3D simulations we are developing to assess the importance of radiation and non-ideal MHD in generating magnetized outflows that can enhance the overall rates of angular momentum transport and mass accretion.
Two-dimensional numerical study for relativistic outflow from strongly magnetized neutron stars
Authors: Matsumoto Jin , Masada Youhei , Asano Eiji , Shibata Kazunari 
Published: April 2009

AbstractUsing special relativistic magnetohydrodynamic simulation, the nonlinear dynamics of the magnetized outflow triggered on the magnetar surface is investigated. It is found that the strong shock propagates in the circumstellar medium in association with the expanding outflow. The shock velocity vsh depends on the strength of the dipole field anchored to the stellar surface Bdipole and is described by a simple scaling relation vsh ∝ Bdipole0.5. In addition, the outflow-driven shock can be accelerated self-similarly to the relativistic velocity when the density profile of the circumstellar medium is steeper than the critical density profile, that is α ≡ d logρ(r)/d log r ≲ αcrit = −5.0, where the density is set as a power law distribution with an index α and r is the cylindrical radius. Our results suggest that the relativistic outflow would be driven by the flaring activity in a circumstellar medium with a steep density profile.
Magnetic fields and dynamics of the Sun's interior
Authors: Kosovichev Alexander G. 
Published: April 2009

AbstractAdvances in helioseismology provide new knowledge about the origin of solar magnetic activity. The key questions addressed by helioseismology are: what is the physical mechanism of the solar dynamo, how deep inside the Sun are the magnetic fields generated, how are they transported to the surface and form sunspots? Direct helioseismic signatures of the internal magnetic fields are weak and difficult to detect. Therefore, most of the information comes from observations of dynamical effects caused by the magnetic fields. I review results of recent helioseismic observations of the magnetohydrodynamics of the solar interior on various scales, including global dynamics associated with the dynamo processes, and formation of sunspots and active regions.
Magnetic pinch-type instability in stellar radiative zones
Authors: Rüdiger Günther , Kitchatinov Leonid L. , Gellert Marcus 
Published: April 2009

AbstractThe solar tachocline is shown as hydrodynamically stable against nonaxisymmetric disturbances if it is true that no cos4θ term exists in its rotation law. We also show that the toroidal field of 200 Gauss amplitude which produces the tachocline in the magnetic theory of Rüdiger & Kitchatinov (1997) is stable against nonaxisymmetric MHD disturbances – but it becomes unstable for rotation periods slightly slower than 25 days. The instability of such weak fields lives from the high thermal diffusivity of stellar radiation zones compared with the magnetic diffusivity. The growth times, however, result as very long (of order of 105rotation times). With estimations of the chemical mixing we find the maximal possible field amplitude to be ~500 Gauss in order to explain the observed lithium abundance of the Sun. Dynamos with such low field amplitudes should not be relevant for the solar activity cycle.With nonlinear simulations of MHD Taylor-Couette flows it is shown that for the rotation-dominated magnetic instability the resulting eddy viscosity is only of the order of the molecular viscosity. The Schmidt number as the ratio of viscosity and chemical diffusion grows to values of ~20. For the majority of the stellar physics applications, the magnetic-dominated Tayler instability will be quenched by the stellar rotation.
Impact of large-scale magnetic fields on stellar structure and evolution
Authors: Duez Vincent , Mathis S. , Brun A. S. , Turck-Chièze S. 
Published: April 2009

AbstractWe study the impact on the stellar structure of a large-scale magnetic field in stellar radiation zones. The field is in magneto-hydrostatic (MHS) equilibrium and has a non force-free character, which allows us to study its influence both on the mechanical and and on the energetic balances. This approach is illustrated in the case of an Ap star where the magnetic field matches at the surface with an external potential one. Perturbations of the stellar structure are semi-analytically computed. The relative importance of the magnetic physical quantities is discussed and a hierarchy, aiming at distinguishing various refinement degrees in the implementation of a large-scale magnetic field in a stellar evolution code, is established. This treatment also allows us to deduce the gravitational multipolar moments and the change in effective temperature associated with the presence of a magnetic field.
Isolated quasi-axisymmetric sunspots
Authors: Koutchmy Serge , Le Piouffle Vincent 
Published: April 2009

AbstractWe briefly review the question of the origin, during a sunspot cycle, of well isolated sunspots. This includes big sunspots like the one observed in Nov. 2006. An overall axi-symmetric morphology is not perfectly observed when the morphological details of both the umbra and of the penumbra are considered. This is especially the case of umbral dots always present inside the core of a sunspot and also of penumbral filaments with non radial parts. However, the distribution of the surrounding fields, including deep layers, the occurrence of persistent coherent running penumbral waves, the magnetic moat behavior, the bright ring phenomena, etc. seem to justify a revival of the naive former but revised (converging motions are considered) Larmor model of a sunspot (as suggested by Lorrainet al. 2006). To discuss the “emergence” of single isolated sunspots from deep layers we performed a quasi-statistical analysis limited to cycle 23. It is based on MDI data taken in the continuum, using the accompanying magnetograms to check our assertion. Surprisingly, single sunspots are definitely and preferably found to occur at low latitude and during the descending branch of the cycle. To explain our observations we speculate about the behavior of the deeply seated magnetic loop, following the original idea of H. Alfven (with whirl rings which follow the global dipolar field when approaching the surface). It could lead to a closed loop approximately orthogonal to the local radius, similar to “smoke rings” arriving at the surface of the Sun and sometimes also called a plasmoid. The ring will only very weakly feel the destabilizing Coriolis force, when emerging at very low latitudes, which seems consistent with our observations.
The Sun as a magnetic star: on the manifestation of different surface structures in disk-integrated observations
Authors: Demidov Mikhail L. 
Published: April 2009

AbstractA reality of solar global magnetic fields Stokes-meter observations with polarimetric accuracy as high as 10−5 – 10−6, and the dependence of Stokes V profiles on distribution of surface magnetic fields in the central zone of the disk are demonstrated. A possibility to use solar disk-integrated and spatially resolved magnetic fields measurements for testing of magnetic fields mapping methods on Sun-like stars is suggested.
Prediction of solar magnetic cycles by a data assimilation method
Authors: Kitiashvili Irina N. , Kosovichev Alexander G. 
Published: April 2009

AbstractWe consider solar magnetic activity in the context of sunspot number variations, as a result of a non-linear oscillatory dynamo process. The apparent chaotic behavior of the 11-year sunspot cycles and undefined errors of observations create uncertainties for predicting the strength and duration of the cycles. Uncertainties in dynamo model parameters create additional difficulties for the forecasting. Modern data assimilation methods allow us to assimilate the observational data into the models for possible efficient and accurate estimations of the physical properties, which cannot be observed directly, such as the internal magnetic fields and helicity. We apply the Ensemble Kalman Filter method to a low-order non-linear dynamo model, which takes into account variations of the turbulent magnetic helicity and reproduces basic characteristics of the solar cycles. We investigate the predictive capabilities of this approach, and present test results for prediction of the previous cycles and a forecast of the next solar cycle 24.
MHD remote numerical simulations: evolution of coronal mass ejections
Authors: Hernández-Cervantes Liliana , Santillán A. , González-Ponce A. R. 
Published: April 2009

AbstractCoronal mass ejections (CMEs) are solar eruptions into interplanetary space of as much as a few billion tons of plasma, with embedded magnetic fields from the Sun's corona. These perturbations play a very important role in solar–terrestrial relations, in particular in the spaceweather. In this work we present some preliminary results of the software development at the Universidad Nacional Autónoma de México to performe Remote MHD Numerical Simulations. This is done to study the evolution of the CMEs in the interplanetary medium through a Web–based interface and the results are store into a database. The new astrophysical computational tool is called the Mexican Virtual Solar Observatory (MVSO) and is aimed to create theoretical models that may be helpful in the interpretation of observational solar data.
Magnetic and tidal interactions in spin evolution of exoplanets
Authors: Kitiashvili Irina N. 
Published: April 2009

AbstractThe axis-rotational evolution of exoplanets on close orbits strongly depends on their magnetic and tidal interactions with the parent stars. Impulsive perturbations from a star created by periodical activity may accumulate with time and lead to significant long-term perturbations of the planet spin evolution. I consider the spin evolution for different conditions of gravitational, magnetic and tidal perturbations, orbit eccentricity and different angles between the planetary orbit plane and the reference frame of a parent star. In this report I present a summary of analytical and numerical calculations of the spin evolution, and discuss the problem of the star-planet magnetic interaction.
3D structures of steady flow of ideal compressible fluid in MHD: a new model of primordial solar nebula
Authors: Salmin Vladimir V. 
Published: April 2009

AbstractStereographic projection of Hopf field on the 3-sphere into Euclidean 3-space is used as a model of 3D steady flow of ideal compressible fluid in MHD. In such case, flow lines are Villarceau circles lying on tori corresponding to the levels of Bernoulli function. Existence of an optimal torus with minimal relative surface free energy is shown. Optimal tori are considered as precursors of planetary orbits.
Magnetic field observations of low-mass stars
Authors: Reiners Ansgar 
Published: April 2009

AbstractDirect measurements of magnetic fields in low-mass stars of spectral class M have become available during the last years. This contribution summarizes the data available on direct magnetic measurements in M dwarfs from Zeeman analysis in integrated and polarized light. Strong magnetic fields at kilo-Gauss strength are found throughout the whole M spectral range, and so far all field M dwarfs of spectral type M6 and later show strong magnetic fields. Zeeman Doppler images from polarized light find weaker fields, which may carry important information on magnetic field generation in partially and fully convective stars.
Magnetic fields in O-type stars measured with FORS 1 at the VLT
Authors: Hubrig Swetlana , Schöller M. , Schnerr R. S. , Ilyin I. , Henrichs H. F. , Ignace R. , González J. F. 
Published: April 2009

AbstractThe presence of magnetic fields in O-type stars has been suspected for a long time. The discovery of such fields would explain a wide range of well documented enigmatic phenomena in massive stars, in particular cyclical wind variability, Hα emission variations, chemical peculiarity, narrow X-ray emission lines and non-thermal radio/X-ray emission. Here we present the results of our studies of magnetic fields in O-type stars, carried out over the last years.
The magnetic field of the B3V star 16 Pegasi
Authors: Henrichs Huib F. , Neiner C. , Schnerr R. S. , Verdugo E. , Alecian A. , Catala C. , Cochard F. , Gutiérrez J. , Huat A.-L. , Silvester J. , Thizy O. 
Published: April 2009

AbstractThe slowly pulsating B3V star 16 Pegasi was discovered by Hubrig et al. (2006) to be magnetic, based on low-resolution spectropolarimetric observations with FORS1 at the VLT. We have confirmed the presence of a magnetic field with new measurements with the spectropolarimeters Narval at TBL, France and Espadons at CFHT, Hawaii during 2007. The most likely period is about 1.44 d for the modulation of the field, but this could not be firmly established with the available data set. No variability has been found in the UV stellar wind lines. Although the star was reported once to show Hα in emission, there exists at present no confirmation that the star is a Be star.
Analysis of magnetic pressure effects in atmospheres of CP stars
Authors: Shulyak Denis , Valyavin G. , Kochukhov O. , Burlakova T. 
Published: April 2009

AbstractSeveral dynamical processes may induce considerable Lorentz forces in atmospheres of mCP stars, thus modifying the hydrostatic structure of their atmospheres. This modification can be seen as characteristic rotational variability of certain spectral features such as hydrogen Balmer lines. In this work we present the first results of modeling the magnetic pressure effects in atmospheres of mCP stars in the framework of model atmosphere analysis with accurate treatment of microscopic properties of atmospheric plasma. We show that at least part of the rotational variability of hydrogen lines seen in high-resolution spectra of mCP stars could be attributed to the non-zero electrical currents flowing along stellar surfaces.
Simulations of magneto-hydrodynamic waves in atmospheres of roAp stars
Authors: Khomenko Elena , Kochukhov Oleg 
Published: April 2009

AbstractWe report 2D time-dependent non-linear magneto-hydrodynamical simulations of waves in the atmospheres of roAp stars. We explore a grid of simulations in a wide parameter space. The aim of our study is to understand the influence of the atmosphere and the magnetic field on the propagation and reflection properties of magneto-acoustic waves, formation of shocks and node layers.
Gradient of the stellar magnetic field in measurements of hydrogen line cores
Authors: Kudryavtsev Dimitry O. , Romanyuk Iosif I. 
Published: April 2009

AbstractWe report the observed systematic differences in longitudinal magnetic field values, obtained from measurements of metal lines and the core of the Hβ line for a number of Ap stars, having strong global magnetic fields. In overwhelming majority of cases the magnetic field values, obtained from measurements of hydrogen lines cores, is smaller then the ones obtained from metal lines. We discuss some possible explanations of this effect, the most probable of which is the existence of the gradient of the magnetic field in stellar atmospheres.
Angular momentum loss and stellar spin-down in magnetic massive stars
Authors: ud-Doula Asif , Owocki Stanley P. , Townsend Richard H.D. 
Published: April 2009

AbstractWe examine the angular momentum loss and associated rotational spin-down for magnetic hot stars with a line-driven stellar wind and a rotation-aligned dipole magnetic field. Our analysis here is based on our previous 2-D numerical MHD simulation study that examines the interplay among wind, field, and rotation as a function of two dimensionless parameters, W(=Vrot/Vorb) and ‘wind magnetic confinement’, η∗ defined below. We compare and contrast the 2-D, time variable angular momentum loss of this dipole model of a hot-star wind with the classical 1-D steady-state analysis by Weber and Davis (WD), who used an idealized monopole field to model the angular momentum loss in the solar wind. Despite the differences, we find that the total angular momentum loss averaged over both solid angle and time follows closely the general WD scaling ~ ṀΩR2A. The key distinction is that for a dipole field Alfvèn radius RA is significantly smaller than for the monopole field WD used in their analyses. This leads to a slower stellar spin-down for the dipole field with typical spin-down times of order 1 Myr for several known magnetic massive stars.
Polarized hydrogen emission lines in Mira stars: a mystery behind the shock
Authors: Fabas Nicolas , Lèbre Agnès , Gillet Denis 
Published: April 2009

AbstractWe present a full spectropolarimetric study (in the Stokes parameters I, Q, U and V) on omicron Ceti (the prototype of Mira stars), and focus on the strong polarization detected in Balmer emission lines. This study is made with observations from NARVAL instrument at TBL (Telescope Bernard Lyot) in Pic du Midi Observatory, France.
Dynamo-generated magnetic fields in fast rotating single giants
Authors: Konstantinova-Antova Renada , Aurière Michel , Schröder Klaus-Peter , Petit Pascal 
Published: April 2009

AbstractRed giants offer a good opportunity to study the interplay of magnetic fields and stellar evolution. Using the spectro-polarimeter NARVAL of the Telescope Bernard Lyot (TBL), Pic du Midi, France and the LSD technique we began a survey of magnetic fields in single G-K-M giants. Early results include 6 MF-detections with fast rotating giants, and for the first time a magnetic field was detected directly in an evolved M-giant: EK Boo. Our results could be explained in the terms of α–ω dynamo operating in these giants.