bluuebunny/crossref_metadata_2025_split huggingface dataset:
The three-dimensional structure of the magnetic field of a sunspot
Authors: Balthasar Horst , Gömöry Peter 
Published: April 2009

AbstractSpectro-polarimetric observations in several spectral lines allow to determine the height variation of the magnetic field of a small sunspot throughout the solar photosphere. The full Stokes-vector is measured with high spatial resolution. From these data we derive the magnetic field vector. The magnetic field strength decreases with height everywhere in the spot, even in the outer penumbra where some other authors have reported the opposite. The precise value of this decrease depends on the exact position in the spot. Values vary between 0.5 and 2.2 G km−1 when they are determined from an iron and a silicon line in the near infrared. The magnetic field is less inclined in the higher layers where the silicon line is formed. Once the magnetic vector field is known, it is straight forward to determine current densities and helicities. Current densities exhibit a radial structure in the penumbra, although it is still difficult to correlate this with the structure seen in the intensity continuum. In spite of this, current densities have a potential to serve as diagnostic tools to understand the penumbra, at least with the spatial resolution of the upcoming telescopes. The mean infered helicity is negative, as expected for a spot in the northern hemisphere. Nevertheless, there are locations inside the spot with positive helicity.
On the dependence of magnetic line ratios on time and spatial scales
Authors: Demidov Mikhail L. 
Published: April 2009

AbstractComparison of magnetic fields measurements made in different spectral lines and observatories is an important tool for diagnostics of magnetohydrodynamic conditions in the solar atmosphere. But there is a deficit of information about the dependence of results on detailed position on the solar disk, spatial resolution and time. In this study these issues are discussed in application to the solar large-scale and Sun-as-a-star magnetic fields observations.
Turbulent effects in flux-transport solar dynamos
Authors: Guerrero G. A. , de Gouveia Dal Pino E. M. , Dikpati M. 
Published: April 2009

AbstractThe effects of turbulent pumping and η-quenching on Babcock-Leighton dynamo models are explored separately. Turbulent pumping seems to be important to solve several reported problems in these dynamo models related to the magnetic flux transport and to the parity. On the other hand, the suppression of the magnetic diffusivity, η, could help in the formation of long-lived, small and intense structures of toroidal magnetic field.
The role of magnetic fields for planetary formation
Authors: Johansen Anders 
Published: April 2009

AbstractThe role of magnetic fields for the formation of planets is reviewed. Protoplanetary disc turbulence driven by the magnetorotational instability has a huge influence on the early stages of planet formation. Small dust grains are transported both vertically and radially in the disc by turbulent diffusion, counteracting sedimentation to the mid-plane and transporting crystalline material from the hot inner disc to the outer parts. The conclusion from recent efforts to measure the turbulent diffusion coefficient of magnetorotational turbulence is that turbulent diffusion of small particles is much stronger than naively thought. Larger particles – pebbles, rocks and boulders – get trapped in long-lived high pressure regions that arise spontaneously at large scales in the turbulent flow. These gas high pressures, in geostrophic balance with a sub-Keplerian/super-Keplerian zonal flow envelope, are excited by radial fluctuations in the Maxwell stress. The coherence time of the Maxwell stress is only a few orbits, where as the correlation time of the pressure bumps is comparable to the turbulent mixing time-scale, many tens or orbits on scales much greater than one scale height. The particle overdensities contract under the combined gravity of all the particles and condense into gravitationally bound clusters of rocks and boulders. These planetesimals have masses comparable to the dwarf planet Ceres. I conclude with thoughts on future priorities in the field of planet formation in turbulent discs.
Planetary dynamos: from equipartition to asymptopia
Authors: Roberts Paul H. 
Published: April 2009

AbstractThis review focuses on three topics relevant to naturally-occurring dynamos. The first considers how a common belief, that states of equipartition of magnetic and kinetic energy are preferred in nonrotating systems, is modified when Coriolis forces are influential, as in the Earth's core. The second reviews current difficulties faced by planetary and stellar dynamo theories, particularly in representing the sub-grid scales. The third discusses recent attempts to extract scaling laws from numerical integrations of the Boussinesq dynamo equations.
Electromagnetic interaction between Jupiter's ionosphere and the Io plasma torus
Authors: Su Yi-Jiun 
Published: April 2009

AbstractThe electromagnetic interaction between Jupiter and Io has been studied extensively since the discovery of Io-controlled decametric radio emissions (DAMs). A variety of mechanisms for electromagnetic disturbances have been considered including a unipolar inductor, the excitation of large-amplitude Alfvén waves, the generation of electrostatic electric fields parallel to the ambient magnetic field, and etc. Recently, three auroral acceleration regions categorized by terrestrial physicists have been applied to the Jupiter-Io coupling system: the Alfvénic acceleration region is associated with bright emissions at Io's magnetic footprint, whereas the quasi-static system of anti-planetward and planetward currents set up at the inner and outer edges of the torus in the downstream region of Io's wake. This review paper summarizes the current understanding of the coupling mechanisms between Jupiter's ionosphere and the Io plasma torus, as well as the electron acceleration mechanism necessary to excite Io-associated emissions.
Effects of magnetic braking and tidal friction on hot Jupiters
Authors: Barker Adrian J. , Ogilvie Gordon I. 
Published: April 2009

AbstractTidal friction is thought to be important in determining the long-term spin-orbit evolution of short-period extrasolar planetary systems. Using a simple model of the orbit-averaged effects of tidal friction (Eggleton et al. 1998), we analyse the effects of the inclusion of stellar magnetic braking on the evolution of such systems. A phase-plane analysis of a simplified system of equations, including only the stellar tide together with a model of the braking torque proposed by Verbunt & Zwaan (1981), is presented. The inclusion of stellar magnetic braking is found to be extremely important in determining the secular evolution of such systems, and its neglect results in a very different orbital history. We then show the results of numerical integrations of the full tidal evolution equations, using the misaligned spin and orbit of the XO-3 system as an example, to study the accuracy of simple timescale estimates of tidal evolution. We find that it is essential to consider coupled evolution of the orbit and the stellar spin in order to model the behaviour accurately. In addition, we find that for typical Hot Jupiters the stellar spin-orbit alignment timescale is of the same order as the inspiral time, which tells us that if a planet is observed to be aligned, then it probably formed coplanar. This reinforces the importance of Rossiter-McLaughlin effect observations in determining the degree of spin-orbit alignment in transiting systems.
The basic role of magnetic fields in stellar evolution
Authors: Maeder André , Meynet Georges , Georgy Cyril , Ekström Sylvia 
Published: April 2009

AbstractMagnetic field is playing an important role at all stages of star evolution from star formation to the endpoints. The main effects are briefly reviewed. We also show that O–type stars have large convective envelopes, where convective dynamo could work. There, fields in magnetostatic balance have intensities of the order of 100 G.A few OB stars with strong polar fields (Henrichs et al. 2003a) show large N–enhancements indicating a strong internal mixing. We suggest that the meridional circulation enhanced by an internal rotation law close to uniform in these magnetic stars is responsible for the observed mixing. Thus, it is not the magnetic field itself which makes the mixing, but the strong thermal instability associated to solid body rotation.A critical question for evolution is whether a dynamo is at work in radiative zones of rotating stars. The Tayler-Spruit (TS) dynamo is the best candidate. We derive some basic relations for dynamos in radiative layers. Evolutionary models with TS dynamo show important effects: internal rotation coupling and enhanced mixing, all model outputs being affected.
The MiMeS project: magnetism in massive stars
Authors: Wade Gregg A. , Alecian E. , Bohlender D. A. , Bouret J.-C. , Grunhut J. H. , Henrichs H. , Neiner C. , Petit V. , Louis N. St. , Aurière M. , Kochukhov O. , Silvester J. , ud-Doula A. ,   the MiMeS Collaboration
Published: April 2009

AbstractThe Magnetism in Massive Stars (MiMeS) Project is a consensus collaboration among the foremost international researchers of the physics of hot, massive stars, with the basic aim of understanding the origin, evolution and impact of magnetic fields in these objects. The cornerstone of the project is the MiMeS Large Program at the Canada-France-Hawaii Telescope, which represents a dedication of 640 hours of telescope time from 2008-2012. The MiMeS Large Program will exploit the unique capabilities of the ESPaDOnS spectropolarimeter to obtain critical missing information about the poorly-studied magnetic properties of these important stars, to confront current models and to guide theory.
Measuring T Tauri star magnetic fields
Authors: Johns–Krull Christopher M. 
Published: April 2009

AbstractStellar magnetic fields including a strong dipole component are believed to play a critical role in the early evolution of newly formed stars and their circumstellar accretion disks. It is currently believed that the stellar magnetic field truncates the accretion disk several stellar radii above the star. This action forces accreting material to flow along the field lines and accrete onto the star preferentially at high stellar latitudes. It is also thought that the stellar rotation rate becomes locked to the Keplerian velocity near the radius where the disk is truncated. This paper reviews recent efforts to measure the magnetic field properties of low mass pre-main sequence stars, focussing on how the observations compare with the theoretical expectations. A picture is emerging indicating that quite strong fields do indeed cover the majority of the surface on these stars; however, the dipole component of the field appears to be alarmingly small. The current measurements also suggest that given their strong magnetic fields, T Tauri stars are somewhat faint in X-rays relative to what is expected from simple main sequence star scaling laws.
Magnetic coronae of active main-sequence stars
Authors: Jardine Moira , Donati Jean-Francois 
Published: April 2009

AbstractThe coronal structure of main sequence stars continues to puzzle us. While the solar corona is relatively well understood, it has become clear that even stars of the same mass as the Sun can display very non-solar coronal behaviour, particularly if they are rapid rotators or in a binary system. At masses greater than and also less than that of the Sun, the non-solar internal structure appears to affect both the geometry and dynamics of the stellar corona and the nature of the X-ray and radio emission. In this talk I will describe some recent advances in our understanding of the structure of the coronae of some of the most active (and interesting) main sequence stars.
Analysis of the hydrogen-rich magnetic White Dwarfs in the SDSS
Authors: Külebi Baybars , Jordan Stefan , Euchner Fabian , Hirsch Heiko 
Published: April 2009

AbstractWe have calculated optical spectra of hydrogen-rich (DA) white dwarfs with magnetic field strengths between 1 MG and 1000 MG for temperatures between 7000 K and 50000 K. Through a least-squares minimization scheme, we have analyzed the spectra of 114 magnetic DAs from the Sloan Digital Survey (SDSS; 95 previously published plus 14 newly discovered within SDSS).
The MiMeS project: first results
Authors: Grunhut Jason H. , Alecian E. , Bohlender D. A. , Bouret J.-C. , Henrichs H. , Neiner C. , Petit V. , Louis N. St. , Aurière M. , Kochukhov O. , Silvester J. , Wade G. A. , ud-Doula A. ,   the MiMeS Collaboration
Published: April 2009

AbstractMassive stars are those stars with initial masses above about 8 times that of the sun, eventually leading to catastrophic explosions in the form of supernovae. These represent the most massive and luminous stellar component of the Universe, and are the crucibles in which the lion's share of the chemical elements are forged. These rapidly-evolving stars drive the chemistry, structure and evolution of galaxies, dominating the ecology of the Universe - not only as supernovae, but also during their entire lifetimes - with far-reaching consequences. Although the existence of magnetic fields in massive stars is no longer in question, our knowledge of the basic statistical properties of massive star magnetic fields is seriously incomplete. The Magnetism in Massive Stars (MiMeS) Project represents a comprehensive, multidisciplinary strategy by an international team of recognized researchers to address the “big questions” related to the complex and puzzling magnetism of massive stars. This paper present the first results of the MiMeS Large Program at the Canada-France-Hawaii Telescope.
New magnetic field measurements of β Cephei stars and slowly pulsating B stars
Authors: Hubrig Swetlana , Briquet M. , De Cat P. , Schöller M. , Morel T. , Ilyin I. 
Published: April 2009

AbstractWe present the results of the continuation of our magnetic survey with FORS 1 at the VLT of a sample of B-type stars consisting of confirmed or candidate β Cephei stars and Slowly Pulsating B stars. Roughly one third of the studied β Cephei stars have detected magnetic fields. The fraction of magnetic Slowly Pulsating B and candidate Slowly Pulsating B stars is found to be higher, up to 50%. We find that the domains of magnetic and non-magnetic pulsating stars in the H-R diagram largely overlap, and no clear picture emerges as to the possible evolution of the magnetic field across the main sequence.
Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars
Authors: Hubrig Swetlana , Grady C. , Schöller M. , Schütz O. , Stelzer B. , Pogodin M. , Cure M. , Yudin R. 
Published: April 2009

AbstractWe present the results of a new magnetic field survey of Herbig Ae/Be and A debris disk stars. They are used to determine whether magnetic field properties in these stars are correlated with the mass-accretion rate, disk inclinations, companion(s), Silicates, PAHs, or show a more general correlation with age and X-ray emission as expected for the decay of a remnant dynamo.
Magnetic fields in classical Be stars
Authors: Yudin Ruslan , Hubrig S. , Pogodin M. , Savanov I. , Schöller M. , Peters G. , Cure M. 
Published: April 2009

AbstractWe report the results of our study of magnetic fields in a sample of Be stars using spectropolarimetric data obtained at the European Southern Observatory with the multi-mode instrument FORS 1 installed at the 8m Kueyen telescope. The detected magnetic fields are rather weak, not stronger than ~150G. A few classical Be stars display cyclic variability of the magnetic field with periods of tens of minutes.
First magnetic stars
Authors: Romanyuk Iosif I. , Kudryavtsev Dimitry O. 
Published: April 2009

AbstractThis contribution dedicated to the analysis of the magnetism of chemically peculiar (CP) stars of the upper Main Sequence. We use our own measurements and published data to compile a catalog of magnetic CP stars containing a total of 326 objects with confidently detected magnetic fields and 29 stars which are very likely to possess magnetic field. Our analysis shows that the number of magnetic CP stars decreases with increasing field strength in accordance with exponential law, hotter and faster rotating stars have stronger fields. Intensity of depressions in the continua correlates with the magnetic field strength.
Cartography of the magnetic fields and chemical spots of Ap stars
Authors: Silvester James , Kochukhov O. , Wade G. A. , Piskunov N. , Landstreet J. D. , Bagnulo S. 
Published: April 2009

AbstractWe will introduce a project using Magnetic Doppler Imaging (MDI) to create assumption-free vector magnetic field maps and chemical surface structure maps of chemically peculiar A and B type (or Ap) stars. We are exploiting the latest generation of spectropolarimeters (NARVAL at the Pic du Midi observatory, and ESPaDOnS at the Canada-France-Hawaii telescope), to obtain high-resolution time series of Stokes IQUV spectra of a selection of Ap stars. The spectra have superior signal-to-noise ratio, resolution and wavelength coverage to those used previously. This combined with the ground-breaking inversion techniques introduced by Kochukhov et al. (2002) results in maps which represent the state-of-the-art in the field of stellar cartography. These maps will allow us to better understand the links between the magnetic field and the physical processes leading to the formation of chemical structures in the photosphere and allow us to address questions surrounding the detailed magnetic field geometry of Ap stars.
Advanced model atmospheres with magnetic field effects included
Authors: Shulyak Denis , Khan S. , Kochukhov O. 
Published: April 2009

AbstractThe atmospheres of magnetic chemically peculiar (mCP) stars display the presence of magnetic fields of different geometry and strength, ranging from a few hundred G up to tens of kG. Except several very approximate attempts there were no detailed studies of magnetic field effects on model atmospheres structure, possibly leading to errors in the stellar parameter determination and abundance analysis routines. We present the magnetic model atmospheres based on LLmodels code which accounts for the detailed treatment of anomalous Zeeman splitting and polarized radiative transfer.
Probing the size of a magnetosphere of a young solar-like star
Authors: Hamilton Catrina M. , Johns-Krull C. M. , Mundt R. , Herbst W. , Winn J. N. 
Published: April 2009

AbstractWe have obtained high resolution spectra of the pre-main sequence binary system KH 15D (V582 Mon) while the star is fully visible, fully occulted, and during several ingress and egress events over the course of five contiguous observing seasons. The Hα line profile is a standard probe of the magnetospheric accretion flows on young stars such as KH 15D. We use these time series data to map out the size of the magnetosphere and find that it changes size from one observing season to the next.
Numerical simulations of magnetized winds of solar-like stars
Authors: Vidotto Aline A. , Opher M. , Jatenco-Pereira V. , Gombosi T. I. 
Published: April 2009

AbstractWe investigate magnetized solar-like stellar winds by means of self-consistent three-dimensional (3D) magnetohydrodynamics (MHD) numerical simulations. We analyze winds with different magnetic field intensities and densities as to explore the dependence on the plasma-β parameter. By solving the fully ideal 3D MHD equations, we show that the plasma-β parameter is the crucial parameter in the configuration of the steady-state wind. Therefore, there is a group of magnetized flows that would present the same terminal velocity despite of its thermal and magnetic energy densities, as long as the plasma-β parameter is the same.
On MHD rotational transport, instabilities and dynamo action in stellar radiation zones
Authors: Mathis Stéphane , Brun A.-S. , Zahn J.-P. 
Published: April 2009

AbstractMagnetic field and their related dynamical effects are thought to be important in stellar radiation zones. For instance, it has been suggested that a dynamo, sustained by a m = 1 MHD instability of toroidal magnetic fields (discovered by Tayler in 1973), could lead to a strong transport of angular momentum and of chemicals in such stable regions. We wish here to recall the different magnetic transport processes present in radiative zone and show how the dynamo can operate by recalling the conditions required to close the dynamo loop (BPol → BTor → BPol). Helped by high-resolution 3D MHD simulations using the ASH code in the solar case, we confirm the existence of the m = 1 instability, study its non-linear saturation, but we do not detect, up to a magnetic Reylnods number of 105, any dynamo action.
HD 232862: a magnetic and Lithium-rich giant field star
Authors: Lèbre Agnès , Palacios A. , do Nascimento J. D. , De Medeiros J. R. , Konstantinova-Antova R. , Kolev D. , Aurière M. 
Published: April 2009

AbstractFrom spectropolarimetric data and spectral synthesis analysis, we report the serendipitous discovery of an unusually high lithium content field giant. HD 232862, classified as a G8II star, appears to be the first Lithium-rich field giant star hosting a surface magnetic field.
Zeeman-Doppler imaging of II Peg
Authors: Carroll Thorsten A. , Kopf Markus , Strassmeier Klaus G. , Ilyin Ilya , Tuominen Ilkka 
Published: April 2009

AbstractWe present Zeeman-Doppler images of the active K2 star II Peg for the years 2004 and 2007. The surface magnetic field was reconstructed with our new ZDI code iMap which provides a full polarized radiative transfer driven inversion to simultaneously reconstruct the surface temperature and magnetic vector field distribution. II Peg shows a remarkable large scale magnetic field structure for both years. The magnetic field is predominantly located at high latitudes and is arranged in active longitudes. A dramatic evolution in the magnetic field structure is visible for the two years, where a dominant and largely unipolar field in 2004 has changed into two distinct and large scale bipolar structures in 2007.
Magnetic Doppler imaging of II Peg
Authors: Kochukhov Oleg , Piskunov N. , Ilyin I. , Tuominen I. 
Published: April 2009

AbstractRotational modulation of the intensity and polarization spectra of magnetic stars offers a unique possibility to reconstruct the structure of surface magnetic fields and to investigate their relation to cool starspots. We have developed a new magnetic Doppler imaging code which aims at self-consistent temperature and magnetic mapping of cool active stars. Here we present magnetic Doppler imaging analysis of high-resolution circular polarization observations of the active star II Peg. We demonstrate that a self-consistent approach to magnetic inversion unveils stronger magnetic fields than found previously through disjoint analyses of polarization and intensity observations of active stars.
Discovery of fossil magnetic fields in the intermediate-mass pre-main sequence stars
Authors: Alecian Evelyne , Wade Gregg A. , Catala Claude 
Published: April 2009

AbstractIt is now well-known that the surface magnetic fields observed in cool, lower-mass stars on the main sequence (MS) are generated by dynamos operating in their convective envelopes. However, higher-mass stars (above 1.5 M⊙) pass their MS lives with a small convective core and a largely radiative envelope. Remarkably, notwithstanding the absence of energetically-important envelope convection, we observe very strong (from 300 G to 30 kG) and organised (mainly dipolar) magnetic fields in a few percent of the A and B-type stars on the MS, the origin of which is not well understood. In this poster we propose that these magnetic fields could be of fossil origin, and we present very strong observational results in favour of this proposal.