Field Geometry (field + geometry)

Distribution by Scientific Domains

Kinds of Field Geometry

  • magnetic field geometry


  • Selected Abstracts


    Discovery of 17 new sharp-lined Ap stars with magnetically resolved lines,

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
    L. M. Freyhammer
    ABSTRACT Chemically peculiar A stars (Ap) are extreme examples of the interaction of atomic element diffusion processes with magnetic fields in stellar atmospheres. The rapidly oscillating Ap stars provide a means for studying these processes in three dimensions and are at the same time important for studying the pulsation excitation mechanism in A stars. As part of the first comprehensive, uniform, high-resolution spectroscopic survey of Ap stars, which we are conducting in the Southern hemisphere with the Michigan Spectral Catalogues as the basis of target selection, we report here the discovery of 17 new magnetic Ap stars having spectroscopically resolved Zeeman components from which we derive magnetic field moduli in the range 3,30 kG. Among these are (1) the current second strongest known magnetic A star, (2) a double-lined Ap binary with a magnetic component and (3) an A star with particularly peculiar and variable abundances. Polarimetry of these stars is needed to constrain their field geometries and to determine their rotation periods. We have also obtained an additional measurement of the magnetic field of the Ap star HD 92499. [source]


    Lattice Boltzmann simulation on flow fields connected with multiple side-channels

    HEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 2 2007
    Shohji Tsushima
    Abstract For this study, using the lattice Boltzmann method (LBM), we conducted flow-field analyses in which two straight channels were mutually connected with multiple side-channels. Results showed that calculated flow fields can be categorized into three types of flow pattern depending on flow field geometry and flow conditions. The following typical flow patterns were identified: case 1, the incoming flow passes uniformly through the side channels; case 2, the flow passes preferentially through the side channel in the inlet and the outlet; and case 3, the flow passes mainly through the side channel of the outlet side. Results also indicate that these flow patterns depend on two dimensionless parameters: the ratio of permeability of the side channels to the channel width, and the Reynolds number. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(2): 96,104, 2007; Published online in Wiley InterScience (www. interscience.wiley.com). DOI 10.1002/htj.20143 [source]


    The determination of the rotation period and magnetic field geometry of the strongly magnetic roAp star HD 154708,

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
    S. Hubrig
    ABSTRACT We obtained 13 spectropolarimetric observations of the strongly magnetic rapidly oscillating Ap star HD 154708 over 3 months with the multimode instrument FORS 1, installed at the 8-m Kueyen telescope of the Very Large Telescope. These observations have been used for the determination of the rotation period of P= 5.3666 ± 0.0007 d. Using stellar fundamental parameters and the longitudinal magnetic field phase curve, we briefly discuss the magnetic field geometry. The star is observed nearly pole-on and the magnetic field geometry can be described by a centred dipole with a surface polar magnetic field strength Bd between 26.1 and 28.8 kG and an inclination of the magnetic axis to the rotation axis in the range to . [source]


    Magnetic fields and chemical peculiarities of the very young intermediate-mass binary system HD 72106

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008
    C. P. Folsom
    ABSTRACT The recently discovered magnetic Herbig Ae and Be stars may provide qualitatively new information about the formation and evolution of magnetic Ap and Bp stars. We have performed a detailed investigation of one particularly interesting binary system with a Herbig Ae secondary and a late B-type primary possessing a strong, globally ordered magnetic field. 20 high-resolution Stokes V spectra of the system were obtained with the ESPaDOnS instrument mounted on the Canada,France,Hawaii Telescope. In these observations we see clear evidence for a magnetic field in the primary, but no evidence for a magnetic field in the secondary. A detailed abundance analysis was performed for both stars, revealing strong chemical peculiarities in the primary and normal chemical abundances in the secondary. The primary is strongly overabundant in Si, Cr and other iron-peak elements, as well as Nd, and underabundant in He. The primary therefore appears to be a very young Bp star. In this context, line profile variations of the primary suggest non-uniform lateral distributions of surface abundances. Interpreting the 0.639 95 ± 0.000 09 d variation period of the Stokes I and V profiles as the rotational period of the star, we have modelled the magnetic field geometry and the surface abundance distributions of Si, Ti, Cr and Fe using magnetic Doppler imaging. We derive a dipolar geometry of the surface magnetic field, with a polar strength Bd= 1230 G and an obliquity ,= 57°. The distributions Ti, Cr and Fe are all qualitatively similar, with an elongated patch of enhanced abundance situated near the positive magnetic pole. The Si distribution is somewhat different, and its relationship to the magnetic field geometry less clear. [source]


    Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph,

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
    J.-F. Donati
    ABSTRACT From observations collected with the ESPaDOnS spectropolarimeter, we report the discovery of magnetic fields at the surface of the mildly accreting classical T Tauri star (cTTS) V2129 Oph. Zeeman signatures are detected, both in photospheric lines and in the emission lines formed at the base of the accretion funnels linking the disc to the protostar, and monitored over the whole rotation cycle of V2129 Oph. We observe that rotational modulation dominates the temporal variations of both unpolarized and circularly polarized line profiles. We reconstruct the large-scale magnetic topology at the surface of V2129 Oph from both sets of Zeeman signatures simultaneously. We find it to be rather complex, with a dominant octupolar component and a weak dipole of strengths 1.2 and 0.35 kG, respectively, both slightly tilted with respect to the rotation axis. The large-scale field is anchored in a pair of 2-kG unipolar radial field spots located at high latitudes and coinciding with cool dark polar spots at photospheric level. This large-scale field geometry is unusually complex compared to those of non-accreting cool active subgiants with moderate rotation rates. As an illustration, we provide a first attempt at modelling the magnetospheric topology and accretion funnels of V2129 Oph using field extrapolation. We find that the magnetosphere of V2129 Oph must extend to about 7R, to ensure that the footpoints of accretion funnels coincide with the high-latitude accretion spots on the stellar surface. It suggests that the stellar magnetic field succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of V2129 Oph. The magnetospheric geometry we derive qualitatively reproduces the modulation of Balmer lines and produces X-ray coronal fluxes typical of those observed in cTTSs. [source]


    An upper limit to polarized submillimetre emission in Arp 220

    MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007
    Michael Seiffert
    ABSTRACT We report the results of pointed observations of the prototypical ultraluminous infrared galaxy (ULIRG) Arp 220 at 850 ,m using the polarimeter on the Submillimetre Common User Bolometer Array instrument on the James Clerk Maxwell Telescope. We find a Bayesian 99 per cent confidence upper limit on the polarized emission for Arp 220 of 1.54 per cent, averaged over the 15-arcsec beam-size. Arp 220 can serve as a proxy for other, more distant such galaxies. This upper limit constrains the magnetic field geometry in Arp 220 and also provides evidence that polarized ULIRGs will not be a major contaminant for next-generation cosmic microwave background polarization measurements. [source]


    Tayler instability of toroidal magnetic fields in MHD Taylor-Couette flows

    ASTRONOMISCHE NACHRICHTEN, Issue 1 2010
    G. Rüdiger
    Abstract The nonaxisymmetric Tayler instability (TI) of toroidal magnetic fields due to axial electric currents is studied for conducting incompressible fluids between two infinitely long corotating cylinders. For given Reynolds number of rotation the magnetic Prandtl number Pm of the liquid conductor and the ratio of the cylinder's rotation rates are the free parameters. It is shown that for resting cylinders the critical Hartmann number for instability does not depend on Pm hence the TI also exists in the limit Pm , 0. By rigid rotation the instability is suppressed where for Pm = 1 the rotational quenching takes its maximum. Rotation laws with negative shear (i.e. d, /dR < 0) strongly destabilize the toroidal field if the rotation is not too fast. In galaxies with their quadrupolar magnetic field geometry this effect could have drastic implications. For sufficiently high Reynolds numbers of rotation, however, the TI completely disappears. For the considered magnetic constellation the superrotation laws support the rotational stabilization. The angular momentum transport of the instability is anticorrelated with the shear so that an eddy viscosity can be defined which proves to be positive. We have also shown the possibility of laboratory TI experiments with a wide-gap container filled with fluid metals like sodium or gallium. Even the effect of the rotational stabilization can be reproduced in the laboratory with electric currents of only a few kA (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]