bluuebunny/crossref_metadata_2025_split huggingface dataset:
Fe(acac)3‐bis(imino)pyridine/MAO: A new catalytic system for ethylene polymerization
Authors: Wang Lincai , Ren Hua , Sun Junquan 
Published: December 2007

AbstractNew catalytic systems, composed of bis(imino)pyridine (2,6‐bis[1‐(2,6‐diisopropylphenylimino)ethyl]pyridine (A1), methylene‐bridged bis(imino)pyridine (A2)), iron (III) acetylacetonate (Fe(acac)3), and methylaluminoxane (MAO), are highly active for ethylene polymerization. The performance of this new catalytic system was evaluated with respect to catalytic activity and polymer properties. The highest catalytic activity of Fe(acac)3‐A1/MAO (catA) system and Fe(acac)3‐A2/MAO (catB) system at optimum condition reaches 10.5 × 106 g PE·(mol Fe h)−1 and 17.3 × 106 g PE·(mol Fe h)−1, respectively. The product viscosity‐average molecular weight (Mη) of two catalytic systems range from 0.40 to 8.7 × 105. GPC characterizations of PE obtained show that the PE has bimodal molecular weight distribution (MWD). 13C NMR and DSC analysis of PE sample indicates that the PE is highly linear and crystalline. Analysis of UV–vis absorption spectra, ethylene polymerization results, and prepared polyethylene structures indicate that Fe(acac)3‐bis(imino)pyridine/MAO catalytic system could generate same active species as bis(imino)pyridyliron catalytic systems. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008
Lanthanide Coordination Polymers Constructed from Infinite Rod‐Shaped Secondary Building Units and Flexible Ligands
Authors: Lü Jian , Bi Wen‐Hua , Xiao Fu‐Xian , Batten Stuart R. , Cao Rong 
Published: December 2007

AbstractThree lanthanide coordination polymers constructed from infinite rod‐shaped secondary building units (SBUs), [Nd2(H2O)2(cis‐chdc)2(trans‐chdc)]⋅2H2O (1), Nd2(H2O)4(trans‐chdc)3 (2), and [Sm2(H2O)2(cis‐chdc)(trans‐chdc)2]⋅4H2O (3) (chdcH2=1,4‐cyclohexanedicarboxylic acid), were hydrothermally synthesized and structurally characterized. The structures of 1–3 are modulated by different ratios of the cis and trans configurations of chdc2− ligands, which was achieved by temperature control in the hydrothermal reactions. Crystal‐structure analysis revealed that 1 is a four‐connected pcu‐type rod packing network built from cross‐linking of rod‐shaped neodymium–oxygen SBUs by cis‐ and trans‐chdc2− ligands in a 2:1 ratio, 2 displays a complicated six‐connected hex‐type rod packing structure built by connection of rod‐shaped neodymium–oxygen SBUs and trans‐chdc2− ligands, and 3 features an unprecedented five‐connected rod packing pattern constructed from rod‐shaped samarium–oxygen SBUs and cis‐ and trans‐chdc2− ligands in a 1:2 ratio.
Are solar cycles predictable?
Authors: Schüssler M. 
Published: December 2007

AbstractVarious methods (or recipes) have been proposed to predict future solar activity levels – with mixed success. Among these, some precursor methods based upon quantities determined around or a few years before solar minimum have provided rather high correlations with the strength of the following cycles. Recently, data assimilation with an advection‐dominated (flux‐transport) dynamo model has been proposed as a predictive tool, yielding remarkably high correlation coefficients. After discussing the potential implications of these results and the criticism that has been raised, we study the possible physical origin(s) of the predictive skill provided by precursor and other methods. It is found that the combination of the overlap of solar cycles and their amplitude‐dependent rise time (Waldmeier's rule) introduces correlations in the sunspot number (or area) record, which account for the predictive skill of many precursor methods. This explanation requires no direct physical relation between the precursor quantity and the dynamo mechanism (in the sense of the Babcock‐Leighton scheme or otherwise). (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Changing stellar activity cycles
Authors: Oláh K. , Strassmeier K.G. , Granzer T. , Soon W. , Baliunas S.L. 
Published: December 2007

AbstractWe investigated continuous long‐term photometric datasets of thirteen active stars, Ca II variability of one single mainsequence star, and 10.7 cm radio data of the Sun, with simple Fourier‐ and time‐frequency analysis. The data reflect the strength of the activity manifested in magnetic spots. All studied stars show multiple (2 to 4) cycles of different lengths. The time‐frequency analysis reveals, that in several cases of the sample one or two of the cycles exhibit continuous changes (increase or decrease). For four stars (V711 Tau, IL Hya, HK Lac, HD 100180) and for the Sun we find that the cycle length changes are strong, amounting to 10–50% during the observed time intervals. The cycle lengths are generally longer for stars with longer rotational periods. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Meridional circulation in the radiation zones of rotating stars: Origins, behaviors and consequences on stellar evolution
Authors: Mathis S. , Decressin T. , Palacios A. , Eggenberger P. , Siess L. , Talon S. , Charbonnel C. , Turck‐Chièze S. , Zahn J.‐P. 
Published: December 2007

AbstractStellar radiation zones are the seat of meridional currents. This circulation has a strong impact on the transport of angular momentum and the mixing of chemicals that modify the evolution of stars. First, we recall in details the dynamical processes that are taking place in differentially rotating stellar radiation zones and the assumptions which are adopted for their modelling in stellar evolution. Then, we present our new results of numerical simulations which allow us to follow in 2D the secular hydrodynamics of rotating stars, assuming that anisotropic turbulence enforces a shellular rotation law and taking into account the transport of angular momentum by internal gravity waves. The different behaviors of the meridional circulation in function of the type of stars which is studied are discussed with their physical origin and their consequences on the transport of angular momentum and of chemicals. Finally, we show how this work is leading to a dynamical vision of the evolution of rotating stars from their birth to their death. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Rapid rotation, active nests of convection and global‐scale flows in solar‐likestars
Authors: Brown B.P. , Browning M.K. , Brun A.S. , Miesch M.S. , Toomre J. 
Published: December 2007

AbstractIn the solar convection zone, rotation couples with intensely turbulent convection to build global‐scale flows of differential rotation and meridional circulation. Our sun must have rotated more rapidly in its past, as is suggested by observations of many rapidly rotating young solar‐type stars. Here we explore the effects of more rapid rotation on the patterns of convection in such stars and the global‐scale flows which are self‐consistently established. The convection in these systems is richly time dependent and in our most rapidly rotating suns a striking pattern of spatially localized convection emerges. Convection near the equator in these systems is dominated by one or two patches of locally enhanced convection, with nearly quiescent streaming flow in between at the highest rotation rates. These active nests of convection maintain a strong differential rotation despite their small size. The structure of differential rotation is similar in all of our more rapidly rotating suns, with fast equators and slower poles. We find that the total shear in differential rotation, as measured by latitudinal angular velocity contrast, ΔΩ, increases with more rapid rotation while the relative shear, ΔΩ/Ω, decreases. In contrast, at more rapid rotation the meridional circulations decrease in both energy and peak velocities and break into multiple cells of circulation in both radius and latitude. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Rotating ‘star‐in‐a‐box’ experiments
Authors: Steffen M. , Freytag B. 
Published: December 2007

AbstractUsing the radiation hydrodynamics code CO5BOLD in its ‘star‐in‐a‐box’ setup, we have performed exploratory simulations of global convection in a rotating reference frame. The goal is to study the interaction of convection and rotation by direct numerical simulation. For these first experiments, we chose an idealized configuration (a scaled‐down, fast rotating Sun) whose properties resemble those of red supergiants in some respect. We describe the setup and time evolution of these models, and discuss the particular problems we have encountered. Finally, we derive the resulting differential rotation pattern and meridional flow field by temporal and azimuthal averaging of the simulation data. We find anti‐solar differential rotation for all cases studied so far. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Does the Sun have a face?
Authors: Tuominen I. , Pelt J. , Brooke J. , Korpi M. 
Published: December 2007

AbstractWe analyse the Greenwich sunspot data with methods using kinematic frames, which allow to detect and filter off any systematic motion, such as differential rotation, of the longitudinal activity traces. The aim is to check the recent claim of the existence of century‐scale persistent solar active longitudes exhibiting antisolar differential rotation. As a result, we find no evidence for such features. Nevertheless, as is well known, the sunspot distribution is highly clustered in longitude (activity nests); the simple cell‐counting statistics allows us to estimate the coherence time of these features, giving roughly10 Carrington rotations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Nonlinear simulations of magnetic instabilities in stellar radiation zones: The role of rotation and shear
Authors: Brun A.S. 
Published: December 2007

AbstractUsing the 3‐dimensional ASH code, we have studied numerically the instabilities that occur in stellar radiation zones in presence of large‐scale magnetic fields, rotation and large‐scale shear. We confirm that some configurations are linearly unstable, as predicted by Tayler and collaborators, and we determine the saturation level of the instability. We find that rotation modifies the peak of the most unstable wave number of the poloidal instability but not its growth rate as much as in the case of the m = 1 toroidal instability for which it is changed to σ = $ \sigma^2_{\rm A} $/Ω. Further in the case with rotation and shear, we found no sign of the dynamo mechanism suggested recently by Spruit even though we possess the essential ingredients (Tayler's m = 1 instability and a large scale shear) supposedly at work. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Non‐kinematic flux‐transport dynamos with variable meridional flow
Authors: Rempel M. 
Published: December 2007

AbstractA single counter clockwise flow cell is the assumption underlying most flux‐transport dynamo models to date. On the other hand, global 3D simulations of the solar convection zone byMiesch et al. indicate that the meridional flow is strongly variable and shows at a given time a multi‐cellular flow structure, with only the long term average reflecting a more regular flow field. We investigate the influence of such a highly time variable meridional flow on a flux‐transport dynamo model. In our model the differential rotation and meridional flow are driven self‐consistently through a parameterization of the Reynolds‐stress (Λ‐effect) and also macroscopic Lorentz‐force feedback is considered. We achieve the time variable flow by adding random fluctuations with a given correlation time and length scale to both components of the turbulent angular momentum flux. We find that a significant amount of random fluctuations can be tolerated before the dynamo loses its coherence, provided that the correlation time scale of the random component is significantly shorter than the cycle length. Stronger constraints on the amplitude of random fluctuations come from helioseismic constraints on the variability of differential rotation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Modelling the differential rotation of F stars
Authors: Küker M. , Rüdiger G. 
Published: December 2007

AbstractWe model stellar differential rotation based on the mean‐field theory of fluid dynamics. DR is mainly driven by Reynolds stress, which is anisotropic and has a non‐diffusive component because the Coriolis force affects the convection pattern. Likewise, the convective heat transport is not strictly radial but slightly tilted towards the rotation axis, causing the polar caps to be slightly warmer than the equator. This drives a flow opposite to that caused by differential rotation and so allows the system to avoid the Taylor‐Proudman state. Our model reproduces the rotation pattern in the solar convection zone and allows predictions for other stars with outer convection zones. The surface shear turns out to depend mainly on the spectral type and only weakly on the rotation rate. We present results for stars of spectral type F which show signs of very strong differential rotation in some cases. Stars just below the mass limit for outer convection zones have shallow convection zones with short convective turnover times. We find solar‐type rotation and meridional flow patterns at much shorter rotation periods and horizontal shear much larger than on the solar surface, in agreement with recent observations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Simulation of turbulent convection in a slowly rotating red giant star
Authors: Palacios A. , Brun A.S. 
Published: December 2007

AbstractThe first 3‐D non‐linear hydrodynamical simulation of the inner convective envelope of a rotating low mass red giant star is presented. This simulation, computed with the ASH code, aims at understanding the redistribution of angular momentum and heat in extended convection zones. The convection patterns achieved in the simulation consist of few broad and warm upflows surrounded by a network of cool downflows. This asymmetry between up and downflows leads to a strong downward kinetic energy flux, that must be compensated by an overluminous enthalpy flux in order to carry outward the total luminosity of the star. The influence of rotation on turbulent convection results in the establishment of largescale mean flows: a strong radial differential rotation and a single cell poleward meridional circulation per hemisphere. A detailed analysis of angular momentum redistribution reveals that the meridional circulation transports angular momentum outward in the radial direction and poleward in the latitudinal direction, with the Reynolds stresses acting in the opposite direction. This simulation indicates that the classical hypothesis of mixing length theory and solid‐body rotation in the envelope of red giants assumed in 1‐D stellar evolution models are unlikely to be realized and thus should be reconsidered. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Force‐free magnetic fields in the solar atmosphere
Authors: Seehafer N. , Fuhrmann M. , Valori G. , Kliem B. 
Published: December 2007

AbstractReliable measurements of the solar magnetic field are restricted to the level of the photosphere. For about half a century attempts have been made to calculate the field in the layers above the photosphere, i.e. in the chromosphere and in the corona, from the measured photospheric field. The procedure is known as magnetic field extrapolation. In the superphotospheric parts of active regions the magnetic field is approximately force‐free, i.e. electric currents are aligned with the magnetic field. The practical application to solar active regions has been largely confined to constant‐αor linear force‐free fields, with a spatially constant ratio,α, between the electric current and the magnetic field. We review results obtained from extrapolations with constant‐αforce‐free fields, in particular on magnetic topologies favourable for flares and on magnetic and current helicities. Presently, different methods are being developed to calculate non‐constant‐αor nonlinear force‐free fields from photospheric vector magnetograms. We also briefly discuss these methods and present a comparison of a linear and a nonlinear force‐free magnetic field extrapolation applied to the same photospheric boundary data. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Stability of toroidal magnetic fields in the solar tachocline and beneath
Authors: Kitchatinov L.L. , Rüdiger G. 
Published: December 2007

AbstractStability of toroidal magnetic field in a stellar radiation zone is considered for the cases of uniform and differential rotation. In the rigidly rotating radiative core shortly below the tachocline, the critical magnetic field for instability is about 600 G. The unstable disturbances for slightly supercritical fields have short radial scales ∼1 Mm. Radial mixing produced by the instability is estimated to conclude that the internal field of the sun can exceed the critical value of 600 G only marginally. Otherwise, the mixing is too strong and not compatible with the observed lithium abundance. Analysis of joint instability of differential rotation and toroidal field leads to the conclusion that axisymmetric models of the laminar solar tachocline are stable to nonaxisymmetric disturbances. The question of whether sun‐like stars can posses tachoclines is addressed with positive answer for stars with rotation periods shorter than about two months. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Analytical theory of tachocline transport at mid latitudes
Authors: Leprovost N. , Kim E.‐J. 
Published: December 2007

AbstractWe provide a theory of magnetic diffusion, momentum transport, and mixing in the solar tachocline by considering magnetohydrodynamics (MHD) turbulence on a β plane subject to a large scale shear (provided by the latitudinal differential rotation). In the strong magnetic field regime, we find that the turbulent viscosity and diffusivity are reduced by magnetic fields only, similarly to the two‐dimensional MHD case (without Rossby waves). In the weak magnetic field regime, we find a crossover scale (LR) from a Alfvén dominated regime (on small scales) to a Rossby dominated regime (on large scales). For parameter values typical of the tachocline, LR is larger than the solar radius so that Rossby waves are unlikely to play an important role in the transport of magnetic field and angular momentum. This is mainly due to the enhancement of magnetic back‐reaction by shearing which efficiently generates small scales, thus strong currents. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Energy distribution in nonaxisymmetric magnetic Taylor‐Couette flow
Authors: Gellert M. , Rüdiger G. , Fournier A. 
Published: December 2007

AbstractAzimuthal magnetorotational instability is a mechanism that generates nonaxisymmetric field pattern. Nonlinear simulations in an infinite Taylor‐Couette system with current‐free external field show, that not only the linearly unstable mode m = 1 appears, but also an inverse cascade transporting energy into the axisymmetric field is possible. By varying the Reynolds number of the flow and the Hartmann number for the magnetic field, we find that the ratio between axisymmetric (m = 0) and dominating nonaxisymmetric mode (m = 1) can be nearly free chosen. On the surface of the outer cylinder this mode distribution appears similarly, but with weaker axisymmetric fields.We do not find significant differences in the case that a constant current within the flow is added. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Rotation of the magnetic elements in polar regions on the Sun
Authors: Benevolenskaya E.E. 
Published: December 2007

AbstractUsing the Michelson Doppler Imager (MDI) data from Solar and Heliospheric Observatory (SOHO), the rotation rate of the unipolar magnetic regions in North high‐latitude regions of the Sun is estimated by tracking individual magnetic elements. The analysis reveals a strong spin down near the pole, which is greater than the Doppler and magnetic rotation rates estimated by Snodgrass & Ulrich (1990), and rotation rate inferred from helioseismology (Birch & Kosovichev 1998), and is probably related to variation of velocity gradient in the subsurface shear layer. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Looking inside the butterfly diagram
Authors: Ternullo M. 
Published: December 2007

AbstractThe suitability of Maunder's butterfly diagram to give a realistic picture of the photospheric magnetic flux large scale distribution is discussed. The evolution of the sunspot zone in cycle 20 through 23 is described. To reduce the noise which covers any structure in the diagram, a smoothing algorithm has been applied to the sunspot data. This operation has eliminated any short period fluctuation, and given visibility to long duration phenomena. One of these phenomena is the fact that the equatorward drift of the spot zone center of mass results from the alternation of several prograde (namely, equatorward) segments with other stationary or poleward segments. The long duration of the stationary/retrograde phases as well as the similarities among the spot zone alternating paths in the cycles under examination prevent us from considering these features as meaningless fluctuations, randomly superimposed on the continuous equatorward migration. On the contrary, these features should be considered physically meaningful phenomena, requiring adequate explanations. Moreover, even the smoothed spotted area markedly oscillates. The compared examination of area and spot zone evolution allows us to infer details about the spotted area distribution inside the butterfly diagram. Links between the changing structure of the spot zone and the tachocline rotation rate oscillations are proposed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Differential rotation of giant stars
Authors: Weber M. 
Published: December 2007

AbstractFrom a set of high‐resolution spectral observations of late type giant stars we used Doppler imaging to derive time‐series temperature maps of the stellar surfaces. Using these temperature maps, it is possible to track the temporal changes of the spot features and derive estimates of the strength and sign of the differential surface rotation of these stars. Looking into the latitudinal changes of the surface maps, it is also possible to derive meridional flows on these stars. But due to the lower accuracy of the latitudes of the reconstructed spot features, the data requirements are higher than for the detection of differential rotation. Nevertheless, a correlation between the differential rotation and meridional flow estimates is suggested. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Rapid differential rotation in intermediate mass stars
Authors: Toque N. , Deupree R.G. , Gillich A. , Lovekin C.C. 
Published: December 2007

AbstractZAMS models with internal differential rotation are computed for 6.5 M⊙ and 10 M⊙ models with the 2D evolution code ROTORC. Two applications of these models, their pulsational frequencies and deduced locations in the HR diagram, are discussed in order to show how the internal differential rotation could be investigated. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Ultra‐cool and extra‐vigorous: Rotation and activity in Mand L dwarfs
Authors: Reiners A. 
Published: December 2007

AbstractThe study of rotation and activity in low‐mass stars or brown dwarfs of spectral classes M and L has seen enormous progress during the last years. I summarize the results from different works that measured activity, rotation, and sometimes magnetic fields. The generation of magnetic activity seems to be unchanged at the threshold to completely convective stars, i.e. no change in the efficiency of the magnetic dynamos is observed. On the other hand, a sudden change in the strength of rotational braking appears at the threshold mass to full convection, and strong evidence exists for rotational braking weakening with lower mass. A probable explanation is that the field topology changes from dipolar to small scale structure as the objects become fully convective. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)